Every atom is built from three subatomic particles whose number and arrangement determine everything about an element.
🔍 Inside the atom
Protons carry a positive charge and sit in the nucleus. Neutrons carry no charge and also sit in the nucleus. Electrons carry a negative charge and orbit the nucleus in shells.
The atomic number of an element equals its number of protons — it's what makes each element unique.
Proton (+)Neutron (neutral)Electron (−)
🔢 Atomic mass (mass number)
The mass number (atomic mass) of an atom is approximately equal to the number of protons plus the number of neutrons.
In a neutral atom, the number of electrons always equals the number of protons.
📝 Worked examples
Example 1 · Reading an atom
Carbon-14 has atomic number 6 and mass number 14.
Protons = atomic number = 6
Electrons = protons (neutral) = 6
Neutrons = mass number − protons = 14 − 6 = 8
Example 2 · Sodium
Sodium: atomic number 11, mass number 23
Protons 11 · Electrons 11 · Neutrons 23 − 11 = 12
Example 3 · Isotopes
Isotopes are atoms of the same element (same protons) with different numbers of neutrons.
Carbon-12: 6 p, 6 n Carbon-13: 6 p, 7 n Carbon-14: 6 p, 8 n
📌 The three particles
Particle
Charge
Mass
Location
Proton
+1
1 amu
Nucleus
Neutron
0
1 amu
Nucleus
Electron
−1
almost 0
Shells around the nucleus
The number of protons defines the element. Change the protons and you change the element. Change the neutrons and you only get a different isotope.
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Atomic number
Atomic number equals the number of:
The atomic number identifies the element, and so does this particle.
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Where electrons live
Electrons are located:
They move around the nucleus in layers.
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Inside the nucleus
Protons and neutrons are both found:
Both heavy particles sit in the centre of the atom.
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An atom has 11 protons and 12 neutrons.
What is its atomic mass?
Enter an exact answer, without rounding.
Atomic mass = protons + neutrons: 11 + 12.
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An atom has 17 protons and 18 neutrons.
What is its atomic mass?
Enter an exact answer, without rounding.
Atomic mass = protons + neutrons: 17 + 18.
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A neutral atom has an atomic number of 6.
How many electrons does it have?
Enter an exact answer, without rounding.
In a neutral atom, electrons = protons.
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Which particle?
Which particle has no electric charge?
Its name sounds like "neutral".
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Isotopes differ in…
Isotopes of the same element have different numbers of:
What stays the same defines the element.
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Chlorine-35 has atomic number 17.
How many neutrons does it have?
Enter an exact answer, without rounding.
Subtract the atomic number from the mass number.
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A neutral atom has a mass number of 39 and 20 neutrons.
How many protons does it have?
Enter an exact answer, without rounding.
Mass number = protons + neutrons.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
What does the atomic number tell you?
Question 2 of 5
An atom has 8 protons and 9 neutrons. What is its mass number?
Question 3 of 5
How many electrons does a neutral atom with atomic number 15 have?
Question 4 of 5
Which particle has almost no mass?
Question 5 of 5
How many electrons does an Mg²⁺ ion have (Mg has 12 protons)?
Lesson 2 · Atoms
⚫ Bohr Diagrams: Modeling Electron Shells
The Bohr model gives us a simple, visual way to show exactly how an atom's electrons are arranged in shells around the nucleus.
⚫ Filling the shells
Electrons fill shells (energy levels) from the innermost outward. For the first 20 elements, shell 1 holds up to 2 electrons, shell 2 holds up to 8, and shell 3 holds up to 8.
Sodium (atomic number 11): 2 electrons in shell 1, 8 in shell 2, 1 in shell 3
Shell 1 (max 2)Shell 2 (max 8)Shell 3 (max 8)
📝 Worked examples
Example 1 · Filling shells
Shells fill from the inside out: 2, then 8, then 8 (for the first 20 elements).
Draw the nucleus and write the number of protons and neutrons inside it.
Draw circles for each shell needed.
Add electrons: 2 in the first shell, up to 8 in the second, and so on.
Count the outer-shell electrons: these are the valence electrons.
Full outer shell = stable. Atoms react to get a full outer shell, which is why Group 18 (the noble gases) hardly react at all.
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Shell 1 capacity
The first electron shell can hold a maximum of:
The innermost shell is the smallest.
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Shell 2 capacity
The second electron shell can hold a maximum of:
The second shell holds a full octet.
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Filling order
When drawing a Bohr diagram, electrons fill shells:
Electrons fill the shells closest to the nucleus first.
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Carbon has atomic number 6. Shell 1 holds 2 electrons, and the rest go into shell 2.
How many electrons are in shell 2?
Enter an exact answer, without rounding.
Take 2 from 6.
✍️ Type your answer · 5 of 10
Sodium has atomic number 11. Shell 1 holds 2, shell 2 holds 8, and the rest go into shell 3.
How many electrons are in shell 3?
Enter an exact answer, without rounding.
Take 2 + 8 from 11.
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Oxygen has atomic number 8. Shell 1 holds 2 electrons, and the rest go into shell 2.
How many electrons are in shell 2?
Enter an exact answer, without rounding.
Take 2 from 8.
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Shell capacity
What is the maximum number of electrons in the first shell?
The first shell is the smallest.
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Which element?
A neutral atom has shells 2, 8, 3. Which element is it?
Add the electrons to get the atomic number.
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Phosphorus has 15 electrons arranged as 2, 8, ...
How many valence electrons does it have?
Enter an exact answer, without rounding.
Subtract the electrons in the inner shells.
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Calcium has 20 electrons arranged 2, 8, 8, 2.
How many shells does it have?
Enter an exact answer, without rounding.
Count the numbers in the arrangement.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
Which element has the arrangement 2, 8, 1?
Question 2 of 5
How many valence electrons does chlorine (2, 8, 7) have?
Question 3 of 5
Which atom has a full outer shell?
Question 4 of 5
How many shells does magnesium (2, 8, 2) have?
Question 5 of 5
How many electrons are in the outer shell of nitrogen (2, 5)?
Lesson 3 · Elements
📊 The Periodic Table: Trends & Valence Electrons
An element's position on the periodic table reveals how many electrons it's willing to share or steal, and what kind of element it is.
⚡ Valence electrons
Valence electrons are the electrons in an atom's outermost shell, and they determine how an atom bonds with other atoms.
Elements in the same group (column) have the same number of valence electrons. Elements in the same period (row) have the same number of electron shells.
📊 The periodic table's great divide
Most elements are metals, found on the left and middle of the table — shiny and good conductors. Non-metals sit in the upper right — dull and poor conductors. A thin zigzag "staircase" line separates them — the elements touching that line are metalloids, with properties of both.
Metals: shiny, conduct heat/electricityNon-metals: dull, poor conductorsMetalloids: properties of both
📝 Worked examples
Example 1 · Locate and describe
Sodium: Group 1, Period 3 → 3 shells, 1 valence e⁻ (alkali metal)
Chlorine: Group 17, Period 3 → 3 shells, 7 valence e⁻ (halogen)
Neon: Group 18, Period 2 → 2 shells, 8 valence e⁻ (noble gas)
Example 2 · Predict reactivity
Alkali metals get more reactive going down the group: Li < Na < K. The outer electron is further from the nucleus, so it is lost more easily.
Example 3 · Same group, similar behaviour
Lithium, sodium and potassium all have 1 valence electron, so they all react vigorously with water.
🗺️ Map of the table
Group 1 · Alkali metals
Soft, very reactive, 1 valence electron.
Group 2 · Alkaline earth
Reactive metals, 2 valence electrons.
Group 17 · Halogens
Reactive non-metals, 7 valence electrons.
Group 18 · Noble gases
Almost unreactive, full outer shell.
Rows are periods (number of shells). Columns are groups (similar properties).
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Defining valence electrons
Valence electrons are:
Valence electrons are the ones on the outside.
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Groups on the periodic table
Elements in the same group (column) have the same number of:
Elements in a column behave alike because their outer shells match.
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Metalloids
Metalloids are found:
They sit between the metals and the non-metals.
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An element is in Group 1.
How many valence electrons does it have?
Enter an exact answer, without rounding.
For Group 1, the group number is the number of outer electrons.
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An element is in Group 17.
How many valence electrons does it have?
Enter an exact answer, without rounding.
For groups 13–18, take 10 away from the group number.
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An atom is in Period 3.
How many electron shells does it have?
Enter an exact answer, without rounding.
The period number is the number of shells.
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Which group?
The halogens are found in which group?
They are next to the noble gases.
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Reactivity trend
Which is more reactive: sodium or potassium?
Look at which is lower in the group.
✍️ Type your answer · 9 of 10
An element is in Group 2.
How many valence electrons does it have?
Enter an exact answer, without rounding.
The group number tells you.
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An atom has 4 electron shells.
Which period is it in?
Enter an exact answer, without rounding.
Shells = period.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Elements in the same group have similar properties because they have the same:
Question 2 of 5
Which group contains the noble gases?
Question 3 of 5
Where are the metals on the periodic table?
Question 4 of 5
Which is the most reactive?
Question 5 of 5
Which of these is a noble gas?
Lesson 4 · Bonding
🤝 Chemical Bonding: Ionic, Covalent & Lewis Dot Structures
Lewis dot structures give us a simple way to visualize valence electrons and predict exactly how two atoms will bond.
⚡ Lewis dot structures
A Lewis dot structure shows an element's symbol surrounded by dots representing its valence electrons.
Lewis dot structure: Na & Cl
Ionic vs Covalent
🔗 Ionic vs. covalent bonds
Ionic bonds form when a metal transfers one or more electrons to a non-metal, creating charged ions that attract each other — for example, sodium (Na) loses 1 electron and chlorine (Cl) gains 1, forming NaCl. Covalent bonds form when two non-metals share electrons instead of transferring them.
Naming simple binary compounds follows a pattern: metal name + non-metal root + "-ide". For example, NaCl = sodium chloride, and MgO = magnesium oxide.
Covalent bonding creates discrete molecules, like a single H₂O molecule — a fixed, countable group of atoms. Ionic bonding instead creates an ionic lattice, a repeating 3D grid of alternating positive and negative ions, like a NaCl crystal, with no single "molecule" unit.
📝 Worked examples
Example 1 · Ionic bond (transfer)
Sodium (1 valence e⁻) gives its electron to chlorine (7 valence e⁻).
Na → Na⁺ + e⁻
Cl + e⁻ → Cl⁻
Na⁺ and Cl⁻ attract → NaCl
Example 2 · Covalent bond (sharing)
In water, oxygen shares one pair with each hydrogen so that every atom gets a full outer shell.
H : O : H O has 2 bonds + 2 lone pairs
Example 3 · Double and triple bonds
Single bond = 2 shared electrons (H–H)
Double bond = 4 shared electrons (O=O)
Triple bond = 6 shared electrons (N≡N)
📌 Ionic vs. covalent
Ionic
Metal + non-metal. Electrons transferred. Usually high melting points and conduct when dissolved.
Covalent
Non-metal + non-metal. Electrons shared. Often low melting points.
The dots are drawn around the symbol for the outer electrons.
✏️ Quick check · 2 of 10
Ionic bonds
Ionic bonds form when:
One atom gives electrons away and the other takes them.
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Covalent bonds
Covalent bonds form when:
Co- means together: the atoms share.
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NaCl is named using the pattern metal + non-metal root + -ide.
How many words are in NaCl's full name (sodium chloride)?
Enter an exact answer, without rounding.
Sodium chloride.
✍️ Type your answer · 5 of 10
Sodium (Na) has 1 valence electron.
How many dots would appear in Na's Lewis dot structure?
Enter an exact answer, without rounding.
Draw one dot for each valence electron.
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Chlorine (Cl) has 7 valence electrons.
How many dots would appear in Cl's Lewis dot structure?
Enter an exact answer, without rounding.
Draw one dot for each valence electron.
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Bond type
What kind of bond forms between a metal and a non-metal?
Think of NaCl.
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Sharing
In a covalent bond, the atoms:
The word "co" means together.
✍️ Type your answer · 9 of 10
Oxygen gas has a double bond, O=O.
How many electrons are shared in a double bond?
Enter an exact answer, without rounding.
Each single bond shares 2 electrons.
✍️ Type your answer · 10 of 10
Carbon has 4 valence electrons and wants 8.
How many bonds does carbon usually form?
Enter an exact answer, without rounding.
8 − 4 = how many more are needed?
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which compound is ionic?
Question 2 of 5
How many electrons are shared in a triple bond?
Question 3 of 5
How many dots does a Lewis dot diagram for magnesium show?
Question 4 of 5
Which is a typical property of ionic compounds?
Question 5 of 5
Which molecule contains only single covalent bonds?
Lesson 5 · Reactions
⚗ Chemical Equations & Balancing
A chemical equation is a precise recipe, and balancing it makes sure every atom you start with is accounted for at the end.
📜 Reactants & products
The substances you start with are called reactants, written on the left of the arrow. The substances you end up with are called products, written on the right. An arrow separates them, meaning "reacts to form."
Reactants → Products
⚖️ Balancing equations
A balanced equation has the same number of each type of atom on both sides, adjusted using coefficients placed in front of each formula.
📝 Worked examples
Example 1 · Water
H₂ + O₂ → H₂O O: 2 on the left, 1 on the right
H₂ + O₂ → 2H₂O O balanced, but now H is 4 on the right
2H₂ + O₂ → 2H₂O H: 4 = 4, O: 2 = 2 ✓
Change only coefficients (the big numbers in front). Never change the small subscripts — that changes the substance.
Balance elements that appear in only one place first. Leave oxygen and hydrogen for last.
Finish by counting the atoms on both sides.
Why balance? Atoms cannot be created or destroyed, so every atom on the left must appear on the right.
✏️ Quick check · 1 of 10
Reactants
In a chemical equation, reactants are written:
Reactants are what you start with, so they come first.
✏️ Quick check · 2 of 10
Products
Products are written:
Products come after the arrow.
✏️ Quick check · 3 of 10
Balancing
Balancing an equation means:
Atoms cannot appear or vanish, so count each kind on both sides.
✍️ Type your answer · 4 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.
✍️ Type your answer · 5 of 10
Look at the balanced equation: 2H₂ + O₂ → 2H₂O.
How many oxygen atoms are on each side?
Enter an exact answer, without rounding.
Count the O atoms in O₂, then check 2H₂O.
✍️ Type your answer · 6 of 10
A reaction produces 3 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
Which is balanced?
Which equation is balanced?
Count Na and Cl on each side.
✏️ Quick check · 8 of 10
What can you change?
To balance an equation you may change:
Subscripts are part of the formula.
✍️ Type your answer · 9 of 10
Balance N₂ + _H₂ → _NH₃. Here N₂ makes 2 NH₃.
What coefficient goes in front of H₂?
Enter an exact answer, without rounding.
2NH₃ contains 6 hydrogen atoms.
✍️ Type your answer · 10 of 10
Balance C₃H₈ + _O₂ → 3CO₂ + 4H₂O.
What coefficient goes in front of O₂?
Enter an exact answer, without rounding.
Count all the oxygen atoms on the right, then divide by 2.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
In 2Mg + O₂ → 2MgO, what is the coefficient of MgO?
Question 2 of 5
H₂ + Cl₂ → _HCl. What number balances it?
Question 3 of 5
How many hydrogen atoms are in 2H₂O?
Question 4 of 5
In 2Al + 3Cl₂ → 2AlCl₃, how many chlorine atoms are on each side?
Question 5 of 5
Which change would be WRONG when balancing?
Lesson 6 · Reactions
💥 Types of Chemical Reactions: Synthesis, Decomposition, Replacement & Combustion
Chemical reactions fall into a handful of recognisable patterns, and learning to spot them makes predicting products far easier.
💥 Five reaction types
Synthesis: A + B → AB — substances combine into one.
Decomposition: AB → A + B — one substance breaks apart.
Single replacement: A + BC → AC + B — one element swaps into a compound.
Double replacement: AB + CD → AD + CB — two compounds trade partners.
Combustion: fuel + O₂ → CO₂ + H₂O + energy.
🔍 Evidence a reaction occurred
Four clues tell you a chemical reaction has taken place: a colour change, gas production (bubbles), a precipitate forming (a new solid appearing in a liquid), or a temperature change.
Shortcut: count the reactants and products. One product → likely synthesis. One reactant → likely decomposition.
✏️ Quick check · 1 of 8
Synthesis reactions
A + B → AB is an example of:
Two things join to make one.
✏️ Quick check · 2 of 8
Decomposition reactions
AB → A + B is an example of:
One thing breaks apart into two.
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Combustion reactions
Fuel + oxygen → carbon dioxide + water + energy is an example of:
Something burns in oxygen.
✍️ Type your answer · 4 of 8
In a double replacement reaction, AB + CD → AD + CB.
How many compounds are on the reactant side?
Enter an exact answer, without rounding.
Count the compounds before the arrow: AB and CD.
✏️ Quick check · 5 of 8
Classify it
2H₂O → 2H₂ + O₂ is which type of reaction?
One reactant, two products.
✏️ Quick check · 6 of 8
Single replacement
Which is a single replacement reaction?
Look for a free element swapping into a compound.
✍️ Type your answer · 7 of 8
A synthesis reaction combines simple substances.
How many products does a synthesis reaction have?
Enter an exact answer, without rounding.
The pattern is A + B → AB.
✍️ Type your answer · 8 of 8
Complete combustion of a hydrocarbon (like CH₄) forms CO₂ and H₂O.
How many different products are formed?
Enter an exact answer, without rounding.
Count the substances formed.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
2Mg + O₂ → 2MgO is:
Question 2 of 5
NaCl + AgNO₃ → AgCl + NaNO₃ is:
Question 3 of 5
Which is a combustion reaction?
Question 4 of 5
2KClO₃ → 2KCl + 3O₂ is:
Question 5 of 5
Which is a single replacement reaction?
Lesson 7 · Reactions
⚖️ Law of Conservation of Mass
No matter how dramatic a chemical reaction looks, the total amount of matter never actually changes — it's simply rearranged.
⚖️ Mass is conserved
The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of the reactants always equals the total mass of the products.
📦 Closed vs. open systems
In a closed system, no matter can enter or leave, so the total mass before and after a reaction stays exactly equal. In an open system, gas can escape into the air, making the measured mass appear to decrease even though no mass was truly lost.
Closed system
Open system
📝 Worked examples
Example 1 · Adding up
4 g H₂ + 32 g O₂ → ? g H₂O
mass of products = 4 + 32 = 36 g
Example 2 · The missing mass is a gas
50 g of reactants in an open beaker produce 30 g of solid. The other 20 g escaped as gas.
50 g = 30 g solid + 20 g gas
Example 3 · Count atoms
2H₂ + O₂ → 2H₂O
Left: 4 H, 2 O
Right: 4 H, 2 O → same atoms, same mass
📌 Why does the mass seem to change?
Antoine Lavoisier showed that in a closed system the total mass before equals the total mass after. If a candle seems to get lighter, the wax became gases (CO₂ and water vapour) that floated away. If iron rusts and gets heavier, it has joined with oxygen from the air.
Closed vs. open: in an open container, gases can enter or leave, so the measured mass may change even though no atoms were lost.
✏️ Quick check · 1 of 10
Conservation of mass
The law of conservation of mass states that mass is:
Atoms are only rearranged in a reaction.
✏️ Quick check · 2 of 10
Closed systems
A closed system:
Closed means nothing can get in or out.
✏️ Quick check · 3 of 10
Open systems
In an open system, if gas escapes during a reaction, the measured mass will appear to:
The escaping gas takes some mass with it.
✍️ Type your answer · 4 of 10
In a closed system, 50 g of reactants react completely.
What is the total mass of the products, in grams?
Enter an exact answer, without rounding.
g
In a closed system, mass before equals mass after.
✍️ Type your answer · 5 of 10
In a closed system, 20 g of one reactant combines with 30 g of another.
What is the total mass of the products, in grams?
Enter an exact answer, without rounding.
g
Add the two reactant masses.
✍️ Type your answer · 6 of 10
In an open system, a 40 g reaction releases 5 g of gas that escapes.
What mass would you measure remaining, in grams?
Enter an exact answer, without rounding.
g
Take the escaped 5 g from 40 g.
✏️ Quick check · 7 of 10
The law
The law of conservation of mass says that in a chemical reaction:
Think "conserved" means kept.
✏️ Quick check · 8 of 10
Burning log
Why is the ash from a burned log lighter than the log?
Where did the smoke and gases go?
✍️ Type your answer · 9 of 10
12 g of carbon reacts with 32 g of oxygen to make carbon dioxide.
What is the mass of CO₂ produced?
Enter an exact answer, without rounding.
g
Add the reactant masses.
✍️ Type your answer · 10 of 10
100 g of reactants make 60 g of solid product in an open dish; the rest is gas.
How many grams of gas escaped?
Enter an exact answer, without rounding.
g
Subtract the solid from the total.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
10 g of A reacts with 6 g of B and makes only one product. Its mass is:
Question 2 of 5
Who is credited with proving that mass is conserved?
Question 3 of 5
If there are 6 hydrogen atoms in the reactants, how many are in the products?
Question 4 of 5
Steel wool is burned in air and gets heavier. Why?
Question 5 of 5
Which setup lets you prove mass is conserved?
Lesson 8 · Reactions
🔥 Reaction Rates & Energy: Exothermic vs Endothermic
Some reactions release energy in a rush, others quietly absorb it, and several everyday factors can speed a reaction up or slow it down.
🔥 Exothermic vs. endothermic
Exothermic
Releases energy into the surroundings and gets warmer — for example, burning fuel.
Endothermic
Absorbs energy from the surroundings and gets colder — for example, some dissolving processes.
⚡ What speeds up a reaction?
Four factors can increase how fast a reaction happens:
1. Higher temperature — particles move faster and collide more often. 2. Higher concentration — more particles means more collisions. 3. Greater surface area — crushed solids react faster than whole ones. 4. Catalysts — speed up a reaction without being used up themselves.
🖼️ See it
Exothermic: the products have less energy than the reactants, so heat is given out and the surroundings get warmer.Endothermic: the products have more energy than the reactants, so heat is absorbed and the surroundings get cooler.
📝 Worked examples
Example 1 · Temperature
A reaction takes 20 s at 20 °C. As a rule of thumb the rate doubles for each 10 °C rise.
30 °C → 10 s
40 °C → 5 s
Example 2 · Surface area
A sugar cube dissolves slowly. The same mass of powdered sugar dissolves much faster because more particles touch the water.
Example 3 · Energy changes
Exothermic → releases heat (a hand warmer, burning)
Endothermic → absorbs heat (an instant cold pack)
📌 Collision theory
For particles to react they must collide, and with enough energy. Anything that increases the number or energy of collisions speeds the reaction up.
Heat the mixture → faster, harder collisions
Increase concentration → more collisions
Crush into powder → more surface area
Add a catalyst → easier route, not used up
Remember: a catalyst changes the speed, not the amount of product.
✏️ Quick check · 1 of 9
Exothermic reactions
A reaction that releases energy and gets warmer is called:
Exo- means out: heat goes out.
✏️ Quick check · 2 of 9
Surface area
Crushing a solid reactant into powder before a reaction:
More surface means more particles can collide.
✏️ Quick check · 3 of 9
Catalysts
A catalyst speeds up a reaction:
A catalyst is still there at the end of the reaction.
✍️ Type your answer · 4 of 9
A reaction's temperature rises from 20°C to 33°C.
By how many degrees did it rise?
Enter an exact answer, without rounding.
°C
Take 20 from 33.
✍️ Type your answer · 5 of 9
A reaction's temperature drops from 30°C to 22°C.
By how many degrees did it drop?
Enter an exact answer, without rounding.
°C
Take 22 from 30.
✏️ Quick check · 6 of 9
Speed it up
Which change would speed up a reaction?
Think about collisions.
✏️ Quick check · 7 of 9
Cold pack
An instant cold pack gets cold. This is an:
Endo means "in".
✍️ Type your answer · 8 of 9
A reaction takes 20 s at 20 °C. Each +10 °C halves the time.
How many seconds does it take at 40 °C?
Enter an exact answer, without rounding.
s
Halve the time twice.
✍️ Type your answer · 9 of 9
The rate doubles for every 10 °C rise.
By what factor does the rate increase for a 30 °C rise?
Enter an exact answer, without rounding.
Three steps of 10 °C each.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
A catalyst:
Question 2 of 5
Why do crushed tablets dissolve faster?
Question 3 of 5
A reaction that makes a test tube feel warm is:
Question 4 of 5
Collision theory says a reaction needs collisions that:
Question 5 of 5
Diluting a solution will usually make a reaction:
Lesson 9 · Acids & Bases
🍋 Acids & Bases: pH, Ions & Neutralisation
Acids and bases are defined by which ion they release in water, and understanding that single difference explains the entire pH scale.
📊 The pH scale
Acids release H+ (hydrogen) ions in water. Bases release OH− (hydroxide) ions in water.
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)
⚖️ Neutralisation
Neutralisation follows the pattern: Acid + Base → Salt + Water.
Acid + Base → Salt + Water
📝 Worked examples
Example 1 · The pH scale
Stomach acid pH 2 (strong acid)
Lemon juice pH 2–3
Pure water pH 7 (neutral)
Baking soda pH 9 (weak base)
Bleach pH 13 (strong base)
Example 2 · Neutralisation
HCl + NaOH → NaCl + H₂O
acid + base → salt + water
Example 3 · Indicators
Blue litmus turns red in acid. Red litmus turns blue in base.
📌 Acid or base?
Acids (pH < 7)
Sour taste, can corrode metals, turn blue litmus red.
Bases (pH > 7)
Bitter, slippery to touch, turn red litmus blue.
Safety: never taste or touch chemicals in a lab. Strong acids and strong bases are both dangerous.
✏️ Quick check · 1 of 10
Acids and ions
Acids release which ion in water?
Acids give hydrogen ions.
✏️ Quick check · 2 of 10
Bases and ions
Bases release which ion in water?
Bases give hydroxide ions.
✏️ Quick check · 3 of 10
Neutral pH
A pH of exactly 7 is considered:
7 is the middle of the scale.
✍️ Type your answer · 4 of 10
A solution starts at pH 3. Adding a base raises the pH by 4 units.
What is the new pH?
Enter an exact answer, without rounding.
Add 4 to 3.
✍️ Type your answer · 5 of 10
A solution starts at pH 12. Adding an acid lowers the pH by 5 units.
What is the new pH?
Enter an exact answer, without rounding.
Take 5 from 12.
✍️ Type your answer · 6 of 10
Neutralisation follows Acid + Base → Salt + Water.
How many products form in a neutralisation reaction?
Enter an exact answer, without rounding.
Salt and water.
✏️ Quick check · 7 of 10
Acid, base or neutral?
A solution has a pH of 3. It is:
7 is the dividing line.
✏️ Quick check · 8 of 10
Litmus test
Blue litmus paper turns red in:
Acids turn blue litmus red.
✍️ 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 the pH values.
✍️ Type your answer · 10 of 10
Each pH unit is a factor of 10 in acidity.
How many times more acidic is pH 4 than pH 6?
Enter an exact answer, without rounding.
Two steps, each ×10.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
What is the pH of pure water?
Question 2 of 5
An acid reacting with a base in neutralisation produces:
Question 3 of 5
Which of these is a base?
Question 4 of 5
Which has the lowest pH?
Question 5 of 5
Red litmus turns blue in:
Final Challenge
🏆 19-Question Challenge Quiz
One question from every topic you just learned. Pick an answer for each question, then hit Check My Answers.
0 / 19
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Atoms
Atomic number equals the number of:
Q2 · Periodic Table
Elements in the same group have the same number of:
Q3 · Bonding
Ionic bonds form by:
Q4 · Equations
In 2H₂ + O₂ → 2H₂O, how many oxygen atoms are on each side?
Q5 · Conservation of Mass
The law of conservation of mass states mass is:
Q6 · Reaction Rates
A reaction that absorbs energy and gets colder is called:
Q7 · Acids & Bases
What pH value is considered neutral?
Q8 · Bonding
Naming binary compounds follows the pattern:
Q9 · Reaction Rates
Which of these increases a reaction's rate?
Q10 · Atoms
Electrons carry which charge?
Q11 · Subatomic Particles & Atomic Structure
How many electrons does an Mg²⁺ ion have (Mg has 12 protons)?
Q12 · Bohr Diagrams: Modeling Electron Shells
How many electrons are in the outer shell of nitrogen (2, 5)?
Q13 · The Periodic Table: Trends & Valence Electrons
Which of these is a noble gas?
Q14 · Chemical Bonding: Ionic, Covalent & Lewis Dot Structures
Which molecule contains only single covalent bonds?
Q15 · Chemical Equations & Balancing
Which change would be WRONG when balancing?
Q16 · Types of Chemical Reactions: Synthesis, Decomposition, Replacement & Combustion
Which is a single replacement reaction?
Q17 · Law of Conservation of Mass
Which setup lets you prove mass is conserved?
Q18 · Reaction Rates & Energy: Exothermic vs Endothermic