Matter is anything that has mass and takes up space. Depending on how tightly its particles are packed and how they move, matter exists in different states.
🔍 The three common states
In a solid, particles are packed tightly in fixed positions and only vibrate in place. In a liquid, particles stay close together but can slide past each other. In a gas, particles are spread far apart and move freely in every direction.
🌡️ Changes of state
Matter can change state without becoming a new substance — this is a physical change. Melting (solid → liquid), freezing (liquid → solid), evaporation/boiling (liquid → gas), and condensation (gas → liquid) are the most common changes.
Water boils at 100°C and freezes at 0°C at normal atmospheric pressure.
Two less common changes skip the liquid state entirely: sublimation is solid → gas directly (like dry ice turning straight into vapor), and deposition is gas → solid directly. A vacuum is a space containing no matter or particles at all. And heating matter generally causes slight thermal expansion — the particles move more and spread out a little, so the substance takes up a bit more space.
⚖️ Density
Density tells us how much mass is packed into a given volume. It helps explain why some objects float and others sink.
Density = mass ÷ volume
mass = 50 g, volume = 10 cm³
Density = 50 ÷ 10 = 5 g/cm³
📝 Worked examples
Solid: fixed shape, fixed volume, particles vibrate in place
Liquid: fixed volume, takes the shape of its container
Gas: no fixed shape or volume, particles far apart
density = mass ÷ volume
40 g ÷ 8 cm³ = 5 g/cm³
During a change of state the temperature stays constant. Ice melts at 0 °C and the temperature does not rise until all the ice has melted.
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Particles in a solid
In a solid, the particles are:
A solid keeps its shape, so its particles cannot swap places.
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An object has a mass of 50 g and a volume of 10 cm³.
What is its density, in g/cm³?
Enter an exact answer, without rounding.
g/cm³
Density = mass ÷ volume: 50 ÷ 10.
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Melting
Melting is the change of state from:
Melting happens when ice warms up.
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An object has a mass of 100 g and a volume of 20 cm³.
What is its density, in g/cm³?
Enter an exact answer, without rounding.
g/cm³
Density = mass ÷ volume: 100 ÷ 20.
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Particles in a gas
Gas particles are spread out and:
A gas spreads out to fill any container.
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Water is heated at normal atmospheric pressure.
What temperature, in °C, does water boil at?
Enter an exact answer, without rounding.
°C
It is a round number on the Celsius scale.
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Particles
In which state are particles furthest apart?
Gases fill their container.
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Freezing
Freezing is the change from:
Water becoming ice.
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A sample has a mass of 40 g and a volume of 8 cm³.
What is its density?
Enter an exact answer, without rounding.
g/cm³
Divide mass by volume.
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Water boils at sea level.
At what temperature in °C?
Enter an exact answer, without rounding.
°C
It is the top fixed point of the Celsius scale.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Sublimation is the change from:
Question 2 of 5
Condensation is:
Question 3 of 5
Which state of matter is easiest to compress?
Question 4 of 5
100 g of a material occupies 20 cm³. What is its density?
Question 5 of 5
Evaporation differs from boiling because it:
Lesson 2 · Matter
🏗️ Elements, Compounds & Mixtures
All matter can be sorted into just a few basic categories — understanding these categories is the foundation for everything else in chemistry.
🔍 Sorting matter
Matter is either a pure substance or a mixture. A pure substance is either an element (only one type of atom, cannot be broken down chemically — like oxygen or gold) or a compound (two or more elements chemically combined in a fixed ratio, like water or salt — can only be separated by chemical means). A mixture is two or more substances physically combined with no fixed ratio, and can be separated by physical means (like sand and water, or a fruit salad). Alloys are a real-world example of a mixture of metals — steel is a mixture of iron and carbon, and brass is a mixture of copper and zinc.
📝 Worked examples
Gold (Au) → element
Water (H₂O) → compound (2 H + 1 O, bonded)
CO₂ → compound
Salad → mixture (physically combined)
Air → mixture of gases
C₆H₁₂O₆ → 6 + 12 + 6 = 24 atoms in one molecule
Compounds have a fixed ratio of elements and different properties from the elements that make them. Mixtures can vary in composition and can be separated physically.
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Elements
Matter that contains only one type of atom and cannot be broken down chemically is a(n):
Gold, oxygen and carbon are examples. Each is just one kind of atom.
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Compounds
Two or more elements chemically combined in a fixed ratio form a:
Water and salt are examples: elements joined by chemical bonds.
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Mixtures
A mixture is best described as:
Think of sand and salt stirred together: nothing is bonded.
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Think about the very top of the classification of matter.
How many main categories does matter split into at the top level (pure substance and mixture)?
Enter an exact answer, without rounding.
Pure substance and mixture.
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Pure substances split further into two types.
How many types of pure substances are there (element and compound)?
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Element and compound.
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Water (H₂O) is a compound made from hydrogen and oxygen.
How many different elements combine to form water?
Enter an exact answer, without rounding.
H stands for one element and O for another.
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Compound
Which of these is a compound?
Look for more than one element in the formula.
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Mixture
What is true about a mixture?
Think of sand and salt.
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A water molecule has the formula H₂O.
How many atoms are in one molecule?
Enter an exact answer, without rounding.
Add the subscripts (the missing one is 1).
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Glucose is C₆H₁₂O₆.
How many atoms are in one molecule?
Enter an exact answer, without rounding.
Add up all the subscripts.
📝 Lesson quiz · 5 questions
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Question 1 of 5
An element is a substance that:
Question 2 of 5
Air is a:
Question 3 of 5
Sugar (C₁₂H₂₂O₁₁) is a:
Question 4 of 5
The properties of a compound are:
Question 5 of 5
How many different elements are in H₂SO₄?
Lesson 3 · Atoms
⚛ Atoms & Elements
Every substance around you is built from atoms — incredibly tiny particles made of even smaller pieces. An element is a pure substance made of only one type of atom.
🔍 Inside an atom
An atom has a tiny, dense nucleus at its centre, made of protons (positive charge) and neutrons (no charge). Tiny, negatively charged electrons zoom around the nucleus in shells.
Proton (+)Neutron (neutral)Electron (−)
🔢 Atomic number & mass number
The atomic number is the number of protons in an atom — it's what makes an element unique. The mass number is the total number of protons plus neutrons.
Mass number = protons + neutrons
protons = 6, neutrons = 6
Mass number = 6 + 6 = 12
In a neutral atom, the number of electrons always equals the number of protons.
📝 Worked examples
Carbon: atomic number 6, mass number 12
protons 6 · electrons 6 · neutrons 12 − 6 = 6
Sodium: atomic number 11, mass number 23
neutrons = 23 − 11 = 12
electron arrangement 2, 8, 1
The nucleus contains protons (+) and neutrons (no charge). Electrons (−) orbit in shells. Each element has a one- or two-letter symbol: C, O, Na (sodium), K (potassium), Fe (iron).
The atomic number is the number of protons, and it identifies the element.
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Charge of a proton
Which particle has a positive charge?
P for proton, P for positive.
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An atom has 6 protons.
What is its atomic number?
Enter an exact answer, without rounding.
The atomic number is the number of protons.
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Electrons
Electrons are:
Electrons are tiny and move around outside the nucleus.
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An atom has 6 protons and 6 neutrons.
What is its mass number?
Enter an exact answer, without rounding.
Mass number = protons + neutrons.
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Charge of a neutron
What is the charge of a neutron?
Neutron sounds like neutral.
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An atom has 8 protons and 8 neutrons.
What is its mass number?
Enter an exact answer, without rounding.
Mass number = protons + neutrons.
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Nucleus
The nucleus of an atom contains:
The nucleus is positively charged.
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Atomic number
The atomic number of an atom equals its number of:
It is also the number of electrons in a neutral atom.
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An oxygen atom has atomic number 8 and mass number 16.
How many neutrons does it have?
Enter an exact answer, without rounding.
Mass number minus atomic number.
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A neutral aluminium atom has atomic number 13.
How many electrons does it have?
Enter an exact answer, without rounding.
Charges must balance.
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Question 1 of 5
The smallest unit of an element is the:
Question 2 of 5
What is the chemical symbol for sodium?
Question 3 of 5
What is the chemical symbol for potassium?
Question 4 of 5
An atom has 6 protons and a mass number of 14. How many neutrons does it have?
Question 5 of 5
Which particle has a negative charge?
Lesson 4 · Elements
📊 The Periodic Table
The periodic table organises every known element by its atomic number, arranging them so elements with similar properties line up in the same column.
📑 Reading an element box
Each box shows the element's atomic number, its chemical symbol, its name, and its atomic mass.
Anatomy of an element box
A slice of the table
📈 Rows and columns
The horizontal rows are called periods. The vertical columns are called groups (or families) — elements in the same group have similar chemical properties because they have the same number of outer electrons.
The standard periodic table has 18 groups and 7 periods.
📝 Worked examples
Rows are PERIODS → number of electron shells
Columns are GROUPS → similar properties
Chlorine: Group 17, Period 3
→ 3 shells and 7 valence electrons
Magnesium: Group 2, Period 3
→ 3 shells and 2 valence electrons
Dmitri Mendeleev arranged the elements by atomic mass and left gaps for undiscovered ones. Today the table is arranged by atomic number. Metals are on the left and centre, non-metals on the right, and metalloids along the stair-step line.
For Groups 1 and 2 the group number equals the number of valence electrons.
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How elements are arranged
Elements in the periodic table are arranged by:
Each element has one more proton than the one before it.
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An element has an atomic number of 11.
How many protons does it have?
Enter an exact answer, without rounding.
The atomic number is the number of protons.
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Groups
The vertical columns of the periodic table are called:
Columns go up and down; rows are called periods.
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Think about the standard periodic table.
How many groups does it have?
Enter an exact answer, without rounding.
Count the columns across the full table.
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Noble gases
Which of these is a noble gas?
Noble gases are in the far right column and hardly react.
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Oxygen has an atomic number of 8.
How many electrons does a neutral oxygen atom have?
Enter an exact answer, without rounding.
A neutral atom has the same number of electrons as protons.
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Rows
The horizontal rows of the periodic table are called:
Columns are groups.
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Similar behaviour
Elements in the same group have similar properties because they have the same number of:
Outer electrons decide reactivity.
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An element is in Group 2.
How many valence electrons does it have?
Enter an exact answer, without rounding.
Use the group number.
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Period 1 holds only hydrogen and helium.
How many elements are in Period 1?
Enter an exact answer, without rounding.
Count the elements in the first row.
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Question 1 of 5
The vertical columns are called:
Question 2 of 5
Who created the first widely accepted periodic table?
Question 3 of 5
Metalloids are found:
Question 4 of 5
Elements in Period 3 have how many electron shells?
Question 5 of 5
The modern table is arranged by increasing:
Lesson 5 · Elements
📈 Families of the Periodic Table
Elements in the same column of the periodic table, called a group, often share a family name and similar chemical behaviour.
🔍 Three important families
Alkali metals (Group 1, e.g. lithium, sodium) are very reactive metals with 1 outer (valence) electron. Halogens (Group 17, e.g. fluorine, chlorine) are very reactive non-metals with 7 valence electrons. Noble gases (Group 18, e.g. helium, neon) are mostly unreactive ("inert") because their outer shell is already full.
📝 Worked examples
Group 1 alkali metals: Li, Na, K, Rb, Cs — soft, very reactive
Group 2 alkaline earth metals: Be, Mg, Ca
Group 17 halogens: F, Cl, Br, I — reactive non-metals
Group 18 noble gases: He, Ne, Ar — unreactive
Sodium + chlorine → sodium chloride (NaCl)
Alkali metals get more reactive going down the group. Halogens get less reactive going down. Noble gases already have full outer shells.
Alkali metals form 1+ ions. Halogens form 1− ions.
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Alkali metals
Alkali metals are found in which group of the periodic table?
They are in the far left column.
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Halogens
Halogens are known for being:
Fluorine and chlorine react easily and are not metals.
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Noble gases
Noble gases are known for being:
Their outer shell is already full.
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Alkali metals are in Group 1 of the periodic table.
How many valence (outer) electrons do alkali metals have?
Enter an exact answer, without rounding.
The group number tells you the number of outer electrons for Group 1.
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Halogens are in Group 17 of the periodic table.
How many valence (outer) electrons do halogens have?
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For groups 13–18, take 10 away from the group number.
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Noble gases have a full outer electron shell.
Which group number are noble gases found in?
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They sit in the last column of the table.
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Noble gases
Why are noble gases so unreactive?
Their shells are already complete.
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Most reactive
Which alkali metal is the most reactive?
Look lower in the group.
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Halogens are in Group 17.
How many valence electrons does a halogen have?
Enter an exact answer, without rounding.
Group number minus 10.
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Argon is a noble gas in Period 3.
How many valence electrons does argon have?
Enter an exact answer, without rounding.
Noble gases have full outer shells.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which is a halogen?
Question 2 of 5
Which is an alkali metal?
Question 3 of 5
Helium is used in balloons because it is:
Question 4 of 5
Alkali metals form ions with a charge of:
Question 5 of 5
Halogens form ions with a charge of:
Lesson 6 · Bonding
🔗 Molecules & Compounds
Atoms rarely stay alone — they bond together to form molecules. When the bonded atoms come from different elements, the result is called a compound.
🔬 Molecules vs. compounds
A molecule is two or more atoms bonded together (like O₂, two oxygen atoms). A compound is a substance made of two or more different elements chemically bonded together (like H₂O, or CO₂).
Water (H₂O)
Carbon Dioxide (CO₂)
📝 Reading chemical formulas
A subscript number shows how many atoms of that element are in one molecule. H₂O means 2 hydrogen atoms and 1 oxygen atom — 3 atoms total.
O₂ (oxygen gas) is a molecule made of only one element, so it's not a compound — a compound always needs at least two different elements.
📝 Worked examples
H₂O → 2 H + 1 O = 3 atoms
CO₂ → 1 C + 2 O = 3 atoms
Ca(OH)₂ → 1 Ca + 2 O + 2 H = 5 atoms
3H₂O → 3 molecules × 3 atoms = 9 atoms in total
A molecule is two or more atoms bonded together. Some molecules contain only one element (O₂, H₂), so they are not compounds. A compound contains two or more different elements.
Subscript vs. coefficient: the small number after a symbol counts atoms in one molecule. The big number in front counts whole molecules.
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Reading H₂O
The formula H₂O tells you a water molecule has:
The small 2 belongs to the H in front of it. No number means one.
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Look at the formula H₂O.
How many total atoms are in one water molecule?
Enter an exact answer, without rounding.
Add the 2 hydrogens and the 1 oxygen.
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What makes a compound
A compound is defined as a substance made of:
A compound needs at least two kinds of element.
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Look at the formula CO₂.
How many total atoms are in one carbon dioxide molecule?
Enter an exact answer, without rounding.
Add the 1 carbon and the 2 oxygens.
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Is O₂ a compound?
Oxygen gas, O₂, is:
O₂ has only one kind of element.
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You have 3 separate molecules of H₂O.
How many oxygen atoms are there in total?
Enter an exact answer, without rounding.
Each H₂O has one oxygen atom. Multiply by 3.
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Table salt
What is the formula of table salt?
Sodium and chlorine.
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Molecule vs compound
Which is a molecule but NOT a compound?
A compound needs different elements.
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Ethane has the formula C₂H₆.
How many atoms are in one molecule?
Enter an exact answer, without rounding.
Add the subscripts.
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Calcium hydroxide is Ca(OH)₂.
How many atoms are in one formula unit?
Enter an exact answer, without rounding.
The 2 outside the bracket multiplies both O and H.
📝 Lesson quiz · 5 questions
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Question 1 of 5
H₂ is:
Question 2 of 5
How many atoms are in one molecule of CH₄?
Question 3 of 5
O₂ is an example of:
Question 4 of 5
How many atoms in total are in 3H₂O?
Question 5 of 5
The formula CO₂ tells us a molecule has:
Lesson 7 · Bonding
🤝 Chemical Bonding
Atoms bond together in two main ways — by transferring electrons or by sharing them — and the type of bond shapes how a substance behaves.
🔍 Ionic vs. covalent bonds
Ionic bonds form when electrons are transferred from a metal to a non-metal, creating oppositely charged ions that attract each other (e.g. table salt, NaCl: sodium loses 1 electron to become Na+, chlorine gains 1 electron to become Cl−). Covalent bonds form when two non-metal atoms share electrons instead of transferring them (e.g. water, H₂O, where oxygen shares electrons with two hydrogen atoms).
Ionic bond (NaCl)
Covalent bond (H₂O)
📝 Worked examples
Ionic: Na (1 valence e⁻) gives an e⁻ to Cl (7 valence e⁻)
→ Na⁺ and Cl⁻ attract → NaCl
Covalent: O shares 2 electron pairs, one with each H → H₂O
Metallic: metal atoms share a "sea" of free electrons
Atoms bond to get a full outer shell. Ionic compounds usually have high melting points and conduct electricity when dissolved or molten.
Group 1 loses 1 electron (1+). Group 2 loses 2 (2+). Group 17 gains 1 (1−). Group 16 gains 2 (2−).
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Ionic bonds
Ionic bonds form when atoms:
In ionic bonding one atom gives and the other takes.
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Covalent bonds
Covalent bonds form when atoms:
Co- means together: the atoms share.
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Table salt bonding
Table salt (NaCl) is held together by which type of bond?
Sodium is a metal and chlorine is a non-metal.
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In forming NaCl, sodium transfers electrons to chlorine.
How many electrons does sodium lose in forming NaCl?
Enter an exact answer, without rounding.
Sodium is in Group 1: it has one outer electron to give away.
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In forming NaCl, chlorine gains the electron sodium loses.
How many electrons does chlorine gain in forming NaCl?
Enter an exact answer, without rounding.
Chlorine gains what sodium loses.
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In a water molecule (H₂O), oxygen forms covalent bonds by sharing electrons with hydrogen atoms.
How many hydrogen atoms does the oxygen atom share electrons with?
Enter an exact answer, without rounding.
Look at the formula: how many H atoms?
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Ionic
Which compound has ionic bonding?
Look for a metal combined with a non-metal.
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Covalent
In a covalent bond, electrons are:
Co- means together.
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A single covalent bond is one shared pair of electrons.
How many electrons are shared in a single bond?
Enter an exact answer, without rounding.
A pair is two.
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Magnesium (Group 2) loses its two valence electrons.
What is the charge on the magnesium ion? (Enter the number, e.g. 2 for 2+)
Enter an exact answer, without rounding.
Lose 2 electrons, gain +2.
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A metal bonding with a non-metal forms which bond?
Question 2 of 5
Atoms of non-metals bonded together form:
Question 3 of 5
Ionic compounds conduct electricity when:
Question 4 of 5
What is the charge on a chloride ion?
Question 5 of 5
Oxygen gas (O₂) has what kind of bond between its atoms?
Lesson 8 · Mixtures
💧 Mixtures & Solutions
A mixture is two or more substances physically combined — not chemically bonded — so each substance keeps its own properties and can be separated again.
⚖️ Solutions: solute + solvent
A solution is a special mixture where one substance (the solute) dissolves completely into another (the solvent). Salt water is a solution: salt is the solute, water is the solvent.
Concentration = mass of solute ÷ volume of solution
solute = 20 g of salt, solvent = 4 L of water
Concentration = 20 ÷ 4 = 5 g/L
📁 Separating mixtures
Filtration separates an insoluble solid from a liquid by passing the mixture through a filter paper — the solid (residue) stays behind, and the liquid (filtrate) passes through.
Insoluble solid (caught by the filter)Liquid (passes through)
📝 Worked examples
Solution = solute + solvent
25 g sugar + 175 g water → solution mass 200 g
concentration = mass of solute ÷ volume of solution
30 g in 0.5 L → 60 g/L
A saturated solution holds as much solute as it can at that temperature. The solubility of most solids increases with temperature. Particles in a suspension settle; a colloid (like milk) stays mixed but scatters light.
Solute is what dissolves; solvent is what does the dissolving. Water is the "universal solvent".
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Properties in a mixture
In a mixture, the different substances:
Nothing new is made in a mixture.
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20 g of salt is dissolved in 4 L of water.
What is the concentration, in g/L?
Enter an exact answer, without rounding.
g/L
Concentration = mass ÷ volume: 20 ÷ 4.
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What filtration separates
Filtration is used to separate:
The filter paper lets liquid through but traps solid bits.
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30 g of sugar is dissolved in 5 L of water.
What is the concentration, in g/L?
Enter an exact answer, without rounding.
g/L
Concentration = mass ÷ volume: 30 ÷ 5.
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Solute vs. solvent
In a solution, the substance being dissolved is called the:
Salt is dissolved in water. Which one is the salt?
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45 g of salt is dissolved in 9 L of water.
What is the concentration, in g/L?
Enter an exact answer, without rounding.
g/L
Concentration = mass ÷ volume: 45 ÷ 9.
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Solute
In sugar water, the sugar is the:
Which one dissolves?
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Saturated
A saturated solution is one that:
Think "full".
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30 g of salt is dissolved to make 0.5 L of solution.
What is the concentration in g/L?
Enter an exact answer, without rounding.
g/L
Divide mass by volume.
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25 g of sugar is dissolved in 175 g of water.
What is the mass of the solution?
Enter an exact answer, without rounding.
g
Mass is conserved.
📝 Lesson quiz · 5 questions
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Question 1 of 5
What is the solvent in salt water?
Question 2 of 5
Which mixture is a suspension?
Question 3 of 5
Milk is an example of a:
Question 4 of 5
For most solids, heating the solvent makes the solid:
Question 5 of 5
A concentrated solution has:
Lesson 9 · Mixtures
🔧 Separation Techniques
Once you know a mixture's components, chemists use several techniques — each based on a different physical property — to pull them apart again.
🔍 Pulling mixtures apart
A solution is saturated when no more solute can dissolve in it at a given temperature; solubility usually increases as temperature rises. Evaporation and crystallisation: heating a solution to evaporate the solvent leaves the dissolved solid behind as crystals — this is how you'd recover salt from salt water. Simple distillation: heating a solution so the liquid evaporates, then cooling the vapor back into a separate pure liquid — this separates a liquid from a solution based on boiling point. Paper chromatography: a mixture of dissolved pigments moves up a strip of paper at different speeds depending on solubility, separating into distinct coloured bands.
Simple distillation
Paper chromatography
PigmentPigmentPigmentPigment
📝 Worked examples
Sand + water → filtration
Salt dissolved in water → evaporation (or distillation to keep the water)
Ink dyes → chromatography
Ethanol + water → distillation (different boiling points)
Iron filings + sand → magnet
Rf = distance moved by spot ÷ distance moved by solvent
4 cm ÷ 8 cm = 0.5
Choose the method from the property that differs: size (filter, sieve), boiling point (distillation), magnetism (magnet), solubility (evaporation, chromatography).
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Saturated solutions
A solution that can't dissolve any more solute at a given temperature is called:
Saturated means full up.
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Evaporation and crystallisation
Evaporation and crystallisation are used to:
Heating drives off the liquid and leaves the solid behind.
✏️ Quick check · 3 of 10
Simple distillation
Simple distillation separates a liquid from a solution based on its:
The liquid boils off, is cooled and collected.
✍️ Type your answer · 4 of 10
Paper chromatography separates dissolved pigments into bands as they move up the paper at different speeds.
In a chromatography experiment with 4 different pigments, how many separated colour bands would you expect to see?
Enter an exact answer, without rounding.
Each pigment makes its own band.
✍️ Type your answer · 5 of 10
Filtration, evaporation/crystallisation, distillation, and chromatography are the separation techniques named across this lesson and the earlier Mixtures & Solutions lesson.
How many separation techniques are named in total?
Enter an exact answer, without rounding.
Count the techniques in the list.
✍️ Type your answer · 6 of 10
A solution holds 30 g of dissolved sugar at 20°C. Heating it to 40°C allows 10 more grams to dissolve, since solubility increases with temperature.
How many total grams of sugar can dissolve at 40°C?
Enter an exact answer, without rounding.
g
Add the extra 10 g to the 30 g.
✏️ Quick check · 7 of 10
Salt from water
Which method recovers salt from salt water?
The salt is dissolved, so a filter will not catch it.
✏️ Quick check · 8 of 10
Iron and sand
How could you separate iron filings from sand?
Which property is different?
✍️ Type your answer · 9 of 10
A 100 g mixture contains 30 g of iron filings.
What percent of the mixture is iron?
Enter an exact answer, without rounding.
%
Divide the part by the whole.
✍️ Type your answer · 10 of 10
In chromatography the solvent travels 8 cm and a spot travels 4 cm.
What is the Rf value?
Enter an exact answer, without rounding.
Divide spot distance by solvent distance.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Distillation separates liquids by their:
Question 2 of 5
Filtration separates:
Question 3 of 5
Chromatography is used to separate:
Question 4 of 5
Pouring off a liquid from a settled solid is called:
Question 5 of 5
Which method uses holes of a certain size?
Lesson 10 · Properties
🔍 Physical vs Chemical Properties
Before learning about changes, it helps to know the difference between a physical property (something you can observe without altering the substance) and a chemical property (something that describes how it reacts or transforms).
📋 Two kinds of properties
Physical properties can be observed or measured without changing what the substance is made of: colour, density, melting point, boiling point, hardness, solubility. Chemical properties describe how a substance can change into a different substance: flammability (ability to burn), reactivity with acid, ability to rust or corrode.
Water — 1 g/cm³Wood — 0.6 g/cm³Iron — 7.9 g/cm³
📝 Worked examples
Physical properties (can be observed without changing the substance):
colour, density, melting point, boiling point, hardness, conductivity
Chemical properties (describe how it reacts):
flammability, reactivity with acid, ability to rust
density = 27 g ÷ 10 cm³ = 2.7 g/cm³
Test: to find out a chemical property you have to try to change the substance into something new.
✏️ Quick check · 1 of 10
Physical properties
Physical properties can be observed:
You can measure colour or melting point without turning the substance into something else.
✏️ Quick check · 2 of 10
Flammability
Flammability (the ability to burn) is an example of a:
Burning makes new substances.
✏️ Quick check · 3 of 10
Melting point
Melting point is an example of a:
Melting changes the state, not the substance.
✍️ Type your answer · 4 of 10
Density = mass ÷ volume. An object has a mass of 20 g and a volume of 4 cm³.
What is its density, in g/cm³?
Enter an exact answer, without rounding.
g/cm³
Density = mass ÷ volume: 20 ÷ 4.
✍️ Type your answer · 5 of 10
Density = mass ÷ volume. An object has a mass of 45 g and a volume of 9 cm³.
What is its density, in g/cm³?
Enter an exact answer, without rounding.
g/cm³
Density = mass ÷ volume: 45 ÷ 9.
✍️ Type your answer · 6 of 10
Colour, density, melting point, boiling point, hardness, and solubility are all listed as physical property examples.
How many physical property examples are listed?
Enter an exact answer, without rounding.
Count the examples in the list.
✏️ Quick check · 7 of 10
Chemical property
Which is a chemical property?
It involves forming new substances.
✏️ Quick check · 8 of 10
Physical property
Which is a physical property?
You can measure it without a reaction.
✍️ Type your answer · 9 of 10
A metal block has a mass of 27 g and a volume of 10 cm³.
What is its density?
Enter an exact answer, without rounding.
g/cm³
Divide mass by volume.
✍️ Type your answer · 10 of 10
A substance melts at 0 °C and boils at 100 °C.
Over how many °C is it a liquid?
Enter an exact answer, without rounding.
°C
Subtract the melting point from the boiling point.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
Which is a physical property of copper?
Question 2 of 5
The ability of iron to rust is a:
Question 3 of 5
Boiling point is:
Question 4 of 5
Which property is chemical?
Question 5 of 5
Malleability (can be hammered into shape) is a:
Lesson 11 · Changes
🔄 Physical & Chemical Changes
Matter can change in two very different ways: a physical change just changes appearance or state, while a chemical change creates a brand-new substance.
⚖️ Comparing the two
Physical change
No new substance forms. Examples: melting ice, cutting paper, dissolving sugar. Usually easy to reverse.
Chemical change
A new substance forms with different properties. Examples: rusting iron, burning wood, baking a cake. Usually hard to reverse.
🔍 Signs of a chemical change
A colour change that isn't just mixing (e.g. iron turning orange as it rusts)
Bubbles or gas being produced without boiling
A temperature change that isn't from heating or cooling
A new solid forming out of liquids (a precipitate)
A permanent change that can't easily be undone
Seeing one of these signs is a strong clue that a chemical reaction happened — but the only way to be 100% sure is to check whether a new substance with new properties has formed.
🖼️ See it
Ask one question: has a new substance been made? If yes, it is a chemical change.
📝 Worked examples
Physical: ice melting, tearing paper, dissolving sugar
Chemical: rusting, burning wood, baking a cake, milk going sour
Signs of a chemical change:
gas bubbles · colour change · precipitate · heat or light
Mass is conserved: 4 g H₂ + 32 g O₂ → 36 g H₂O
A new substance forms in a chemical change. A physical change only changes appearance or state.
✏️ Quick check · 1 of 10
Melting ice
Melting an ice cube is an example of a:
Melted ice is still water.
✏️ Quick check · 2 of 10
Rusting iron
Rusting of iron is an example of a:
Rust is a new substance, iron oxide.
✍️ Type your answer · 3 of 10
Think about what happens during a physical change.
How many new substances are formed in a physical change?
Enter an exact answer, without rounding.
A physical change does not make anything new.
✏️ Quick check · 4 of 10
Signs of a chemical change
Which of these is a sign that a chemical change may be happening?
Gas being made without heating means a new substance is forming.
✍️ Type your answer · 5 of 10
Think about what happens during a chemical change.
At least how many new substances are formed in a chemical change?
Enter an exact answer, without rounding.
A chemical change always makes something new.
✏️ Quick check · 6 of 10
Cutting paper
Cutting a piece of paper into smaller pieces is a:
The pieces are still paper.
✏️ Quick check · 7 of 10
Chemical
Which is a chemical change?
Look for new substances.
✏️ Quick check · 8 of 10
Physical
Which is a physical change?
The paper is still paper.
✍️ Type your answer · 9 of 10
A reaction releases bubbles, changes colour and gets warm.
How many signs of a chemical change are listed?
Enter an exact answer, without rounding.
Count each sign mentioned.
✍️ Type your answer · 10 of 10
4 g of hydrogen reacts with 32 g of oxygen to make water.
What is the mass of water formed?
Enter an exact answer, without rounding.
g
Mass is conserved.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Rusting of iron is a:
Question 2 of 5
Melting butter is a:
Question 3 of 5
A new substance forms in a:
Question 4 of 5
Cooking an egg is a:
Question 5 of 5
Chemical changes are usually:
Lesson 12 · Acids & Bases
🍋 Acids, Bases & pH
The pH scale measures how acidic or basic a substance is, from 0 to 14. It's one of the most useful tools in chemistry, biology, and everyday life.
📊 The pH scale
A pH below 7 is acidic, a pH of exactly 7 is neutral, and a pH above 7 is basic (also called alkaline).
Acidic (pH < 7)Neutral (pH = 7)Basic (pH > 7)
🧪 Properties of acids and bases
Acids
Taste sour, turn blue litmus paper red. Examples: lemon juice, vinegar, stomach acid.
Bases
Feel slippery, turn red litmus paper blue. Examples: soap, baking soda, bleach.
📝 Worked examples
pH scale: 0 ──── 7 ──── 14
acid neutral base
Lemon juice pH 2 (acid)
Pure water pH 7 (neutral)
Soap pH 10 (base)
Bleach pH 13 (strong base)
Universal indicator: red → acid, green → neutral, purple → base
Each step on the pH scale is a factor of 10 in acidity: pH 3 is 10 times more acidic than pH 4.
Acids taste sour and can corrode metals. Bases feel slippery. Never taste or touch lab chemicals.
✏️ Quick check · 1 of 10
Neutral pH
A substance with a pH of 7 is:
pH 7 is in the middle of the scale.
✍️ Type your answer · 2 of 10
Think about pure water.
What is the pH of pure water?
Enter an exact answer, without rounding.
Pure water is neither acidic nor basic.
✏️ Quick check · 3 of 10
Litmus paper and acids
Acids turn blue litmus paper:
Acids turn litmus the colour of a red light.
✍️ Type your answer · 4 of 10
Think about the full range of the pH scale.
What is the highest value on the standard pH scale?
Enter an exact answer, without rounding.
The scale goes from 0 up to this number.
✏️ Quick check · 5 of 10
Litmus paper and bases
Bases turn red litmus paper:
Bases turn litmus the opposite colour to acids.
✍️ Type your answer · 6 of 10
Think about the most acidic end of the pH scale.
What is the lowest value on the standard pH scale?
Enter an exact answer, without rounding.
The scale starts at this number.
✏️ Quick check · 7 of 10
Acid or base?
A solution with pH 3 is:
7 is neutral.
✏️ Quick check · 8 of 10
Indicator
Universal indicator turns what colour in a neutral solution?
Between red and purple.
✍️ Type your answer · 9 of 10
Lemon juice has pH 2 and vinegar has pH 5.
By how many pH units do they differ?
Enter an exact answer, without rounding.
Subtract.
✍️ Type your answer · 10 of 10
Each pH unit is a factor of 10.
How many times more acidic is pH 3 than pH 4?
Enter an exact answer, without rounding.
The difference is one unit.
📝 Lesson quiz · 5 questions
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Question 1 of 5
The pH of pure water is:
Question 2 of 5
Bases usually feel:
Question 3 of 5
Acids usually:
Question 4 of 5
Which has the strongest basic properties?
Question 5 of 5
Red litmus paper turns blue in:
Lesson 13 · Acids & Bases
⚖️ Neutralisation Reactions
When an acid and a base meet, they cancel each other out in a reaction called neutralisation — one of the most useful reactions in chemistry.
🔍 Acid + Base → Salt + Water
Neutralisation follows the word equation: Acid + Base → Salt + Water. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water. A universal indicator shows a full range of colours across the whole pH scale (unlike litmus paper, which only shows one colour change), making it useful for estimating the exact pH of a solution.
Acid + Base → Salt + Water
📝 Worked examples
acid + base → salt + water
HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Salt names:
hydrochloric acid → chloride
sulfuric acid → sulfate
nitric acid → nitrate
Antacid tablets neutralise excess stomach acid. Baking soda can ease a bee sting (acidic) and vinegar can ease a wasp sting (alkaline).
When a strong acid and strong base cancel exactly, the final solution has a pH of about 7.
✏️ Quick check · 1 of 10
Products of neutralisation
When an acid and a base react completely, they form:
Acid + base → ? + ?
✏️ Quick check · 2 of 10
The word equation
The word equation for a neutralisation reaction is:
The acid and the base go on the left of the arrow.
✏️ Quick check · 3 of 10
Universal indicator
A universal indicator is more useful than litmus paper because it:
Litmus only shows acid or base, not how strong.
✍️ Type your answer · 4 of 10
Hydrochloric acid reacts with sodium hydroxide (a base) in a neutralisation reaction.
How many total products form in a basic neutralisation reaction (salt and water)?
Enter an exact answer, without rounding.
A salt and water.
✍️ Type your answer · 5 of 10
A solution starts at pH 3 (acidic). Adding a base raises the pH by 4 units.
What is the new pH after adding the base?
Enter an exact answer, without rounding.
Add 4 to 3.
✍️ Type your answer · 6 of 10
A solution starts at pH 11 (basic). Adding an acid lowers the pH by 4 units.
What is the new pH after adding the acid?
Enter an exact answer, without rounding.
Take 4 from 11.
✏️ Quick check · 7 of 10
Products
The products of a neutralisation reaction are:
The H⁺ and OH⁻ form water.
✏️ Quick check · 8 of 10
Antacid
Antacid tablets treat indigestion because they:
The stomach has too much acid.
✍️ Type your answer · 9 of 10
A neutral solution has what pH?
What is the pH?
Enter an exact answer, without rounding.
It is the middle of the scale.
✍️ Type your answer · 10 of 10
HCl + NaOH → NaCl + H₂O. The reactants combine in a 1 : 1 ratio.
How many moles of HCl neutralise 1 mole of NaOH?
Enter an exact answer, without rounding.
Look at the coefficients.
📝 Lesson quiz · 5 questions
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Question 1 of 5
H₂SO₄ + 2NaOH → ? + 2H₂O
Question 2 of 5
HCl + KOH → ?
Question 3 of 5
Which substance can neutralise excess stomach acid?
Question 4 of 5
After mixing exactly equal amounts of strong acid and strong base, the pH is about:
Question 5 of 5
A bee sting is acidic. A good first-aid treatment is a:
Lesson 14 · Reactions
⚗ Chemical Reactions & Equations
A chemical equation is a shorthand way of describing a reaction: the starting materials, the ending materials, and how many of each atom are involved.
📜 Reactants & products
The substances you start with are called reactants. The substances you end up with are called products. An arrow separates them, meaning "reacts to form."
Reactants → Products
⚖️ Conservation of mass
The law of conservation of mass says matter can't be created or destroyed in a chemical reaction — every atom you start with must end up somewhere in the products. That's why equations must be balanced: the same number of each atom on both sides.
📝 Worked examples
Word equation:
methane + oxygen → carbon dioxide + water
Symbol equation (balanced):
CH₄ + 2O₂ → CO₂ + 2H₂O
Left: C 1, H 4, O 4 Right: C 1, H 4, O 4 ✓
N₂ + 3H₂ → 2NH₃ (6 H on each side)
Reactants are on the left and products on the right. Balance by changing coefficients only, never subscripts.
Why balance? The law of conservation of mass says atoms are never created or destroyed.
✏️ Quick check · 1 of 10
Law of conservation of mass
The law of conservation of mass states that mass in a chemical reaction is:
Atoms are only rearranged, never lost.
✍️ Type your answer · 2 of 10
Look at the balanced equation: 2H₂ + O₂ → 2H₂O.
How many hydrogen atoms are on the reactant side?
Enter an exact answer, without rounding.
2H₂ means 2 molecules with 2 H atoms each.
✏️ Quick check · 3 of 10
Reactants
The substances you start with in a chemical reaction are called:
They react together at the start.
✍️ Type your answer · 4 of 10
Look at the balanced equation: 2H₂ + O₂ → 2H₂O.
How many oxygen atoms are on each side of the equation?
Enter an exact answer, without rounding.
Count the O atoms in O₂, then check the 2H₂O side.
✏️ Quick check · 5 of 10
Products
The substances formed by the end of a chemical reaction are called:
They are produced by the reaction.
✍️ Type your answer · 6 of 10
A reaction produces 2 molecules of CO₂.
How many carbon atoms are in the products?
Enter an exact answer, without rounding.
Each CO₂ molecule has one carbon atom.
✏️ Quick check · 7 of 10
Reactants
In a chemical equation, reactants are written:
The arrow points to the products.
✏️ Quick check · 8 of 10
Balanced?
Which equation is balanced?
Count the atoms on each side.
✍️ Type your answer · 9 of 10
Balance Mg + O₂ → MgO.
What coefficient goes in front of MgO?
Enter an exact answer, without rounding.
There are 2 O atoms on the left.
✍️ Type your answer · 10 of 10
Balance N₂ + _H₂ → 2NH₃.
What coefficient goes in front of H₂?
Enter an exact answer, without rounding.
Count the hydrogen atoms in 2NH₃.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Products are the substances that:
Question 2 of 5
How many hydrogen atoms are in 2H₂O?
Question 3 of 5
Equations must be balanced because of:
Question 4 of 5
In 2Na + Cl₂ → 2NaCl, how many Na atoms react?
Question 5 of 5
How many chlorine atoms are in 3Cl₂?
Lesson 15 · Reactions
🔥 Testing for Gases & Energy Changes
Chemists can identify invisible gases with simple tests, and every reaction either releases or absorbs energy as it happens.
🧪 Gas tests and energy changes
Oxygen relights a glowing splint. Carbon dioxide turns limewater cloudy (milky). Hydrogen makes a "squeaky pop" sound when a lit splint is held near it. Reactions are also classified by energy change: exothermic reactions release energy and get warmer (like burning or neutralisation); endothermic reactions absorb energy and get colder (like some dissolving processes).
Exothermic
Releases energy to the surroundings and gets warmer. Examples: burning, neutralisation.
Endothermic
Absorbs energy from the surroundings and gets colder. Examples: some dissolving processes.
📝 Worked examples
Hydrogen: lit splint → squeaky pop
Oxygen: glowing splint relights
Carbon dioxide: limewater turns milky
Chlorine: bleaches damp litmus paper
Ammonia: damp red litmus turns blue
Temperature change: 22 °C → 31 °C = +9 °C (exothermic)
Exothermic reactions release energy and the surroundings get warmer. Endothermic reactions absorb energy and feel cold.
Zinc + acid → hydrogen: collect the gas and test with a lit splint. A squeaky pop confirms hydrogen.
✏️ Quick check · 1 of 10
Testing for oxygen
Which test confirms the presence of oxygen gas?
Oxygen helps things burn.
✏️ Quick check · 2 of 10
Testing for carbon dioxide
Which test confirms the presence of carbon dioxide gas?
Bubble the gas through limewater.
✏️ Quick check · 3 of 10
Testing for hydrogen
Which test confirms the presence of hydrogen gas?
Hydrogen burns with a small bang.
✏️ Quick check · 4 of 10
Exothermic reactions
A reaction that releases energy and gets warmer is called:
Exo- means out: heat goes out.
✍️ Type your answer · 5 of 10
A reaction's temperature rises from 20°C to 35°C as it proceeds.
By how many degrees did the temperature rise?
Enter an exact answer, without rounding.
°C
Take 20 from 35.
✍️ Type your answer · 6 of 10
A reaction's temperature drops from 25°C to 18°C as it proceeds.
By how many degrees did the temperature drop?
Enter an exact answer, without rounding.
°C
Take 18 from 25.
✏️ Quick check · 7 of 10
Oxygen test
Which test identifies oxygen?
It relights.
✏️ Quick check · 8 of 10
CO₂ test
Carbon dioxide turns limewater:
Limewater is clear to start with.
✍️ Type your answer · 9 of 10
A reaction mixture rises from 22 °C to 31 °C.
By how many degrees did the temperature rise?
Enter an exact answer, without rounding.
°C
Subtract the starting temperature.
✍️ Type your answer · 10 of 10
An endothermic reaction cools a flask from 25 °C to 19 °C.
By how many degrees did the temperature fall?
Enter an exact answer, without rounding.
°C
Subtract the final temperature.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which gas gives a squeaky pop with a lit splint?
Question 2 of 5
Which gas relights a glowing splint?
Question 3 of 5
Which gas turns limewater milky?
Question 4 of 5
An endothermic reaction feels:
Question 5 of 5
Ammonia turns damp red litmus paper:
Lesson 16 · Elements
🔩 Metals & Non-Metals
Every element on the periodic table can be sorted into one of three families: metals, non-metals, and metalloids — each with its own set of typical properties.
📊 The periodic table's great divide
Most elements are metals, found on the left and middle of the table. Non-metals sit in the upper right. A thin zigzag "staircase" line separates them — the elements touching that line are metalloids, with properties of both.
Metals: shiny, conduct heat/electricity, malleableNon-metals: dull, brittle, poor conductorsMetalloids: properties of both
🔧 Typical properties
Metals
Shiny, good conductors of heat and electricity, malleable (can be hammered into sheets), ductile (can be pulled into wires).
Non-metals
Dull, poor conductors, brittle when solid — they shatter instead of bending.
📝 Worked examples
Reactivity series (most → least):
K, Na, Ca, Mg, Al, Zn, Fe, Cu, Ag, Au
A more reactive metal displaces a less reactive one:
Fe + CuSO₄ → FeSO₄ + Cu
(iron is above copper, so iron wins)
Rusting needs BOTH oxygen and water.
Metals are shiny, conduct heat and electricity and can be hammered. Non-metals are usually dull, brittle and poor conductors. An alloy is a mixture of metals (steel is iron with a little carbon).
Prevent rust by painting, oiling, galvanizing (zinc coating) or using stainless steel.
✏️ Quick check · 1 of 10
Conductivity of metals
Metals are typically good conductors of:
Saucepans and electric wires are made of metal.
✏️ Quick check · 2 of 10
Malleable
A "malleable" material is one that can be:
Think of beating gold into gold leaf.
✍️ Type your answer · 3 of 10
Think about how the periodic table's elements are grouped.
How many major families are elements divided into (metals, non-metals, and what else)?
Enter an exact answer, without rounding.
Metals, non-metals and the ones in between.
✏️ Quick check · 4 of 10
Non-metal properties
Which property is typical of non-metals?
Solid non-metals like sulfur snap or crumble.
✏️ Quick check · 5 of 10
Metalloids
Elements found along the periodic table's "staircase" line are called:
They have some properties of metals and some of non-metals.
✍️ Type your answer · 6 of 10
Carbon is a non-metal with an atomic number of 6.
How many protons does a carbon atom have?
Enter an exact answer, without rounding.
The atomic number is the number of protons.
✏️ Quick check · 7 of 10
Displacement
Which metal will displace copper from copper sulfate solution?
Which one is above copper in the series?
✏️ Quick check · 8 of 10
Properties
Which is a typical property of metals?
Think of electrical wires.
✍️ Type your answer · 9 of 10
Rusting needs oxygen and water.
How many conditions must be present for iron to rust?
Enter an exact answer, without rounding.
Count them.
✍️ Type your answer · 10 of 10
Mg, Cu, Fe, Zn are listed. In the reactivity series Mg and Zn are above Fe, while Cu is below.
How many of the four are more reactive than iron?
Enter an exact answer, without rounding.
Count the ones above Fe.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
Non-metals are usually:
Question 2 of 5
Gold is useful in jewellery because it is:
Question 3 of 5
Which metal is the most reactive?
Question 4 of 5
Rusting requires:
Question 5 of 5
Steel is an alloy of iron and:
Final Challenge
🏆 26-Question Challenge Quiz
One question from every topic you just learned. Pick an answer for each question, then hit Check My Answers.
0 / 26
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Matter & States
What is the change of state from a liquid to a solid called?
Q2 · Atoms & Elements
The atomic number of an atom tells you the number of:
Q3 · Periodic Table
The vertical columns of the periodic table are called:
Q4 · Molecules & Compounds
How many total atoms are in one molecule of H₂O?
Q5 · Mixtures & Solutions
Filtration is used to separate:
Q6 · Physical & Chemical Changes
Rusting of iron is an example of a:
Q7 · Acids, Bases & pH
What pH value is considered neutral?
Q8 · Chemical Reactions
The law of conservation of mass states that mass is:
Q9 · Metals & Non-Metals
Metals are typically good conductors of:
Q10 · Matter
Which formula correctly calculates density?
Q11 · Matter & States of Matter
Evaporation differs from boiling because it:
Q12 · Elements, Compounds & Mixtures
How many different elements are in H₂SO₄?
Q13 · Atoms & Elements
Which particle has a negative charge?
Q14 · The Periodic Table
The modern table is arranged by increasing:
Q15 · Families of the Periodic Table
Halogens form ions with a charge of:
Q16 · Molecules & Compounds
The formula CO₂ tells us a molecule has:
Q17 · Chemical Bonding
Oxygen gas (O₂) has what kind of bond between its atoms?
Q18 · Mixtures & Solutions
A concentrated solution has:
Q19 · Separation Techniques
Which method uses holes of a certain size?
Q20 · Physical vs Chemical Properties
Malleability (can be hammered into shape) is a:
Q21 · Physical & Chemical Changes
Chemical changes are usually:
Q22 · Acids, Bases & pH
Red litmus paper turns blue in:
Q23 · Neutralisation Reactions
A bee sting is acidic. A good first-aid treatment is a: