A chemical equation shows what reacts and what forms. Atoms are never created or destroyed, so both sides of an equation must have the same atoms.
🧩 Types of reaction
Synthesis: A + B → AB. Decomposition: AB → A + B. Single displacement: a more reactive element replaces a less reactive one. Double displacement: two compounds swap partners, often forming a precipitate or water. Combustion: a fuel reacts with oxygen, giving carbon dioxide and water for hydrocarbons. Neutralisation: acid + base → salt + water.
A balanced equation has the same number of each atom on both sides. Change the numbers in front of formulae, never the small numbers inside them.
⚖️ Balancing and conservation of mass
Balance by changing coefficients (the big numbers in front), never the subscripts. Balance the elements that appear in one compound on each side first, then hydrogen and oxygen last. Because atoms are conserved, the total mass of the reactants equals the total mass of the products in a closed system.
N₂ + 3H₂ → 2NH₃ (1 + 3 + 2: coefficients)
4Fe + 3O₂ → 2Fe₂O₃
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
24 g Mg + 16 g O₂ → 40 g MgO
oxygen used = 40 − 24 = 16 g
Changing a subscript changes the substance. H₂O and H₂O₂ are different compounds.
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Single displacement
Zn + CuSO₄ → ZnSO₄ + Cu is a:
One element replaces another.
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Decomposition
2H₂O₂ → 2H₂O + O₂ is a:
One reactant, several products.
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Precipitate
NaCl + AgNO₃ → AgCl + NaNO₃ forms a white solid. It is a:
Partners swap.
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Balancing
When balancing an equation you may change:
The big numbers.
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Balance: __Fe + __O₂ → __Fe₂O₃. Use the smallest whole numbers.
What is the coefficient of O₂?
Enter an exact answer, without rounding.
Make Fe₂O₃ out of 4 Fe and 6 O.
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Balance: C₃H₈ + __O₂ → 3CO₂ + 4H₂O.
What is the coefficient of O₂?
Enter an exact answer, without rounding.
Count the O atoms on the right.
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24 g of magnesium burns with oxygen and produces 40 g of magnesium oxide.
What mass of oxygen reacted, in g?
Enter an exact answer, without rounding.
g
Total mass is conserved.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Combustion of a hydrocarbon in plenty of oxygen produces:
Question 2 of 5
In a neutralisation reaction the products are:
Question 3 of 5
A precipitate is:
Question 4 of 5
A closed flask holds reactants of total mass 50 g. After the reaction the contents weigh:
Question 5 of 5
The numbers written in front of formulas are called:
Lesson 2 · Gases
🎈 The Gas Laws
Gas pressure, volume and temperature are linked. The laws let you predict one when another changes.
🔧 Boyle, Charles and Gay-Lussac
Particles in a gas move randomly and hit the walls, causing pressure. Temperatures must be in kelvin (K = °C + 273). At constant temperature, Boyle’s law: P₁V₁ = P₂V₂. At constant pressure, Charles’s law: V₁/T₁ = V₂/T₂. At constant volume, Gay-Lussac’s law: P₁/T₁ = P₂/T₂.
Boyle: 2 L at 100 kPa squeezed to 200 kPa
V₂ = 100 × 2 / 200 = 1 L
Charles: 2 L at 300 K heated to 600 K
V₂ = 2 × 600 / 300 = 4 L
Gay-Lussac: 100 kPa at 300 K heated to 450 K
P₂ = 100 × 450 / 300 = 150 kPa
27 °C = 27 + 273 = 300 K
🖼️ See it
Halving the volume doubles the pressure at constant temperature.A straight line through the origin means volume is proportional to temperature in kelvin.
🧮 Molar volume
Equal volumes of gases at the same temperature and pressure contain equal numbers of particles (Avogadro’s law). At STP (0 °C and 1 atm) one mole of any gas occupies 22.4 L. This links moles, volume and mass.
volume = moles × 22.4 L/mol
0.5 mol → 11.2 L
44.8 L → 44.8 / 22.4 = 2 mol
2 mol of O₂ (32 g/mol) → 64 g, and occupies 44.8 L at STP
Always convert to kelvin. Using °C in gas law formulas gives wrong answers.
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Boyle
At constant temperature, if the volume of a gas is halved its pressure:
P × V is constant.
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Charles
At constant pressure, heating a gas makes its volume:
Hotter means more space.
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Kelvin
Why must gas law temperatures be in kelvin?
Zero must mean no motion.
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Molar volume
At STP one mole of any gas occupies:
A number worth remembering.
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A gas has a volume of 2 L at 100 kPa. It is compressed at constant temperature to 200 kPa.
What is the new volume, in L?
Enter an exact answer, without rounding.
L
P₁V₁ = P₂V₂.
✍️ Type your answer · 6 of 7
A gas at 300 K has a volume of 2 L. It is heated at constant pressure to 600 K.
What is the new volume, in L?
Enter an exact answer, without rounding.
L
V₁/T₁ = V₂/T₂.
✍️ Type your answer · 7 of 7
A sealed container holds gas at 100 kPa and 300 K. It is heated to 450 K.
What is the new pressure, in kPa?
Enter an exact answer, without rounding.
kPa
Pressure is proportional to the kelvin temperature.
📝 Lesson quiz · 5 questions
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Question 1 of 5
0 °C equals:
Question 2 of 5
A gas exerts pressure because its particles:
Question 3 of 5
How many moles are in 44.8 L of gas at STP?
Question 4 of 5
Heating a sealed rigid container of gas will:
Question 5 of 5
Avogadro’s law says equal volumes of gases at the same conditions contain:
Lesson 3 · Solutions
🥤 Solutions & Concentration
A solution is a uniform mixture of a solute in a solvent. Chemists describe how much solute there is as a concentration.
💧 Dissolving and solubility
A solute dissolves in a solvent (often water). A saturated solution holds the most solute possible at that temperature; an unsaturated one can dissolve more; a supersaturated one holds extra and is unstable. Most solids dissolve better when hot, while gases dissolve better when cold. Polar solvents dissolve polar and ionic solutes: like dissolves like.
percent by mass = mass of solute / mass of solution × 100
20 g salt in 80 g water → solution mass 100 g → 20%
solids: higher temperature → usually more dissolves
gases: higher temperature → less dissolves (fizzy drinks go flat when warm)
🖼️ See it
Concentration in mol/dm³ tells you how many moles are in each cubic decimetre (litre) of solution. Convert cm³ to dm³ by dividing by 1 000.
🧪 Molarity and dilution
Molarity (M) is moles of solute per litre of solution: M = n ÷ V. When you dilute, the moles of solute stay the same, so C₁V₁ = C₂V₂.
0.5 mol in 2 L → 0.5 / 2 = 0.25 M
mass for 2 L of 0.25 M NaOH (40 g/mol)
moles = 0.25 × 2 = 0.5 mol → mass = 0.5 × 40 = 20 g
dilute 50 mL of 2 M to 0.5 M
V₂ = 2 × 50 / 0.5 = 200 mL → add 150 mL water
Add the solid to water, then make up to the mark. The final volume of the solution is what counts.
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Saturated
A saturated solution is one that:
Full.
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Gases
Why does a warm fizzy drink go flat faster?
Gases like it cold.
✏️ Quick check · 3 of 7
Like dissolves like
Which solute is most likely to dissolve in water?
Ionic compounds.
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Dilution
When a solution is diluted, the moles of solute:
You only add water.
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0.5 mol of solute is dissolved to make 2 L of solution.
What is the molarity, in mol/L?
Enter an exact answer, without rounding.
M
M = n ÷ V.
✍️ Type your answer · 6 of 7
You need 2 L of 0.25 M NaOH. The molar mass of NaOH is 40 g/mol.
What mass of NaOH is needed, in g?
Enter an exact answer, without rounding.
g
Find the moles first.
✍️ Type your answer · 7 of 7
50 mL of 2 M solution is diluted to 0.5 M.
What is the final volume, in mL?
Enter an exact answer, without rounding.
mL
C₁V₁ = C₂V₂.
📝 Lesson quiz · 5 questions
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Question 1 of 5
In a salt solution, the solvent is:
Question 2 of 5
A supersaturated solution:
Question 3 of 5
20 g of salt is dissolved in 80 g of water. The percentage by mass of the solution is:
Question 4 of 5
Molarity measures:
Question 5 of 5
How can you speed up the dissolving of a solid?
Lesson 4 · Acids and bases
🍋 Acids, Bases & Titration
Acids and bases are described by the ions they release in water. The pH scale and titration turn that into numbers.
📏 The pH scale
Acids release H⁺ ions in water; bases release OH⁻ ions (or accept H⁺). pH = −log[H⁺]. Each step of one pH unit is a tenfold change in H⁺ concentration. Strong acids ionise completely; weak acids only partly. At 25 °C, pH + pOH = 14.
[H⁺] = 0.001 M = 10⁻³ → pH = 3
pH 4 vs pH 6: 2 units apart → 10² = 100 times more acidic
pH 11 → pOH = 14 − 11 = 3
litmus: red in acid, blue in base
phenolphthalein: colourless in acid, pink in base
🖼️ See it
Each step on the scale is a tenfold change in hydrogen ion concentration.
🧫 Neutralisation and titration
In neutralisation, H⁺ + OH⁻ → H₂O. A titration finds an unknown concentration: add acid from a burette to a measured volume of base until the indicator changes colour (the end point). For a 1 : 1 reaction, C₁V₁ = C₂V₂.
HCl + NaOH → NaCl + H₂O (1 : 1)
25 mL of 0.2 M NaOH is neutralised by 20 mL of HCl
moles NaOH = 0.2 × 0.025 = 0.005 mol = moles HCl
C(HCl) = 0.005 / 0.020 = 0.25 M
Titrate slowly near the end point and read the burette at the bottom of the meniscus.
✏️ Quick check · 1 of 7
Acid ions
Acids in water release:
Hydrogen ions.
✏️ Quick check · 2 of 7
pH scale
Which solution is the most acidic?
Lowest number.
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Strong acid
A strong acid is one that:
Complete ionisation.
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End point
The end point of a titration is when:
A colour change.
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A solution has [H⁺] = 0.001 M (10⁻³ M).
What is its pH?
Enter an exact answer, without rounding.
Count the power of ten.
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Solution A has pH 4 and solution B has pH 6.
How many times more acidic is A than B?
Enter an exact answer, without rounding.
×
Each pH unit is a factor of 10.
✍️ Type your answer · 7 of 7
25 mL of 0.2 M NaOH is neutralised by 20 mL of HCl (1 : 1 reaction).
What is the concentration of the HCl, in mol/L?
Enter an exact answer, without rounding.
M
C₁V₁ = C₂V₂.
📝 Lesson quiz · 5 questions
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Question 1 of 5
A solution of pH 7 at 25 °C is:
Question 2 of 5
What colour does litmus turn in acid?
Question 3 of 5
For a solution of pH 11, the pOH is:
Question 4 of 5
Neutralisation of HCl with NaOH gives:
Question 5 of 5
Why use a pipette and burette in a titration?
Lesson 5 · Organic
🛢️ Organic Chemistry: Hydrocarbons
Organic chemistry is the chemistry of carbon compounds. Carbon bonds to itself in chains, so it forms millions of different molecules.
⛓️ Alkanes and alkenes
Hydrocarbons contain only carbon and hydrogen. Names use prefixes for the number of carbons: meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6, hept- 7, oct- 8. Alkanes (single bonds, saturated) have the formula CnH2n+2. Alkenes (one C=C double bond, unsaturated) are CnH2n. Alkenes decolourise orange bromine water; alkanes do not.
methane CH₄ ethane C₂H₆
propane C₃H₈ butane C₄H₁₀
ethene C₂H₄ propene C₃H₆
alkane with 6 C → 2×6 + 2 = 14 H (hexane C₆H₁₄)
alkene with 5 C → 2×5 = 10 H (pentene C₅H₁₀)
🖼️ See it
🧱 Functional groups, isomers and polymers
A functional group sets how a molecule reacts: –OH in alcohols, –COOH in carboxylic acids, –NH₂ in amines. Isomers share a molecular formula but have different structures: butane and methylpropane are both C₄H₁₀. Alkenes can join end to end by addition polymerisation to make polymers such as poly(ethene).
Count bonds, not just atoms. Each carbon forms four bonds.
✏️ Quick check · 1 of 7
Alkane formula
Which formula is an alkane?
2n + 2.
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Bromine water
Bromine water turns colourless when shaken with:
Unsaturated.
✏️ Quick check · 3 of 7
Isomers
Isomers have:
Same atoms, different arrangement.
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Functional group
The –OH group is the functional group of:
Ethanol.
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An alkane has 6 carbon atoms (CₙH₂ₙ₊₂).
How many hydrogen atoms does it have?
Enter an exact answer, without rounding.
Use 2n + 2.
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An alkene has 5 carbon atoms (CₙH₂ₙ).
How many hydrogen atoms does it have?
Enter an exact answer, without rounding.
Use 2n.
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Ethanol is C₂H₅OH (C = 12, H = 1, O = 16).
What is its molar mass, in g/mol?
Enter an exact answer, without rounding.
g/mol
Count 2 C, 6 H and 1 O.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Octane has how many carbon atoms?
Question 2 of 5
Alkanes are called saturated because they have:
Question 3 of 5
The addition polymerisation of ethene makes:
Question 4 of 5
Complete combustion of a hydrocarbon requires:
Question 5 of 5
The functional group –COOH is found in:
Lesson 6 · Metals
🔩 Metals & the Reactivity Series
Metals differ in how readily they lose electrons. Their order of reactivity explains how they are extracted and why some corrode.
📶 The reactivity series
From most to least reactive: K, Na, Ca, Mg, Al, (C), Zn, Fe, Pb, (H), Cu, Ag, Au. A more reactive metal displaces a less reactive one from its compound, and reacts more vigorously with water and acid. Metals above hydrogen react with dilute acids to give a salt and hydrogen gas.
Mg + CuSO₄ → MgSO₄ + Cu (Mg is more reactive than Cu)
Cu + MgSO₄ → no reaction
Zn + 2HCl → ZnCl₂ + H₂
Cu + HCl → no reaction (Cu is below hydrogen)
🖼️ See it
A more reactive metal displaces a less reactive one from its compounds.
⛏️ Extraction and corrosion
Metals less reactive than carbon (such as iron) are extracted by heating their oxide with carbon or carbon monoxide. Metals more reactive than carbon (such as aluminium) need electrolysis of the molten compound. Rusting needs both oxygen and water. It is prevented by painting, oiling or galvanising (coating with zinc, which corrodes in preference to the iron) and by making alloys such as stainless steel.
Fe₂O₃ + 3CO → 2Fe + 3CO₂
molar mass Fe₂O₃ = 2×56 + 3×16 = 160 g/mol
iron in 160 g = 112 g → 112 / 160 = 70% iron by mass
Al³⁺ + 3e⁻ → Al (3 electrons gained at the cathode)
Reduction is gain of electrons (OIL RIG: oxidation is loss, reduction is gain).
✏️ Quick check · 1 of 7
Displacement
Which reaction will occur?
The more reactive metal wins.
✏️ Quick check · 2 of 7
Acid reaction
Which metal does not react with dilute hydrochloric acid?
Below hydrogen.
✏️ Quick check · 3 of 7
Extraction
Aluminium is extracted by electrolysis because:
Above carbon.
✏️ Quick check · 4 of 7
Rusting
Rusting of iron requires:
Two things.
✍️ Type your answer · 5 of 7
Fe₂O₃ + __CO → 2Fe + 3CO₂
What is the coefficient of CO?
Enter an exact answer, without rounding.
Balance the carbon.
✍️ Type your answer · 6 of 7
Fe₂O₃ has a molar mass of 160 g/mol (Fe = 56, O = 16).
What mass of iron is in 160 g of Fe₂O₃, in g?
Enter an exact answer, without rounding.
g
There are 2 Fe atoms per formula.
✍️ Type your answer · 7 of 7
An aluminium ion is Al³⁺.
How many electrons does each ion gain to become an atom?
Enter an exact answer, without rounding.
Match the charge.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which metal is the most reactive?
Question 2 of 5
Galvanising protects iron by:
Question 3 of 5
An alloy is:
Question 4 of 5
What happens at the cathode in electrolysis?
Question 5 of 5
Which metal can displace iron from iron sulfate?
Final Challenge
🏆 12-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
0 / 12
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Reactions & Balancing Equations
Which equation is balanced?
Q2 · Reactions & Balancing Equations
The numbers written in front of formulas are called:
Q3 · The Gas Laws
Gas volume at STP for 0.5 mol is:
Q4 · The Gas Laws
Avogadro’s law says equal volumes of gases at the same conditions contain:
Q5 · Solutions & Concentration
Diluting a solution makes its concentration:
Q6 · Solutions & Concentration
How can you speed up the dissolving of a solid?
Q7 · Acids, Bases & Titration
A solution with pH 3 compared with pH 4 has an H⁺ concentration that is: