Chemical Reactions & Equations Notes

Last reviewed 30 Sept 2026 · 7 min read

Physical and chemical change

	Physical change	Chemical change
New substance formed?	No	Yes
Reversible?	Usually	Usually not
Examples	melting of ice, boiling of water, dissolving sugar, breaking glass	rusting of iron, burning of paper, curdling of milk, cooking food, digestion, photosynthesis

Signs of a chemical reaction: change of colour, evolution of a gas, formation of a precipitate, change of temperature (heat or light given out), change of state.

Chemical equations

A chemical equation represents a reaction with symbols and formulae. Reactants are on the left, products on the right, with an arrow between them.

Physical states: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. An arrow ↑ shows a gas escaping; ↓ a precipitate.
Conditions such as heat, catalyst, light or pressure are written on the arrow.

Balancing

By the law of conservation of mass, the number of atoms of each element must be the same on both sides. Balance by adding coefficients, never by changing the formulae.

Unbalanced: Fe + H₂O → Fe₃O₄ + H₂. Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Worked ExampleExample — balancing

Balance: $H_{2} + O_{2} \to H_{2} O$ .

Solution. The oxygen atoms: 2 on the left, 1 on the right, so put 2 in front of H₂O: $H_{2} + O_{2} \to 2 H_{2} O$ . Now hydrogen: 2 on the left, 4 on the right, so put 2 in front of H₂: $2 H_{2} + O_{2} \to 2 H_{2} O$ .

Types of chemical reactions

1. Combination (synthesis)

Two or more substances combine to form a single product: $A + B \to AB$ .

$CaO + H_{2} O \to Ca(OH)_{2}$ (quicklime + water → slaked lime, exothermic; used in whitewashing).
$2 Mg + O_{2} \to 2 MgO$ (burning magnesium with a dazzling white light).
$C + O_{2} \to CO_{2}$ ; $N_{2} + 3 H_{2} \to 2 NH_{3}$ (Haber process).

2. Decomposition

A single compound breaks into two or more simpler substances: $AB \to A + B$ .

Thermal decomposition: $CaCO_{3} heat CaO + CO_{2}$ (limestone → quicklime; used in cement making). Also $2 Pb(NO_{3})_{2} \to 2 PbO + 4 NO_{2} + O_{2}$ (brown fumes).
Electrolytic decomposition: $2 H_{2} O \to 2 H_{2} + O_{2}$ on passing electricity (hydrogen at the cathode, twice the volume of oxygen at the anode).
Photolytic decomposition: silver chloride and silver bromide turn grey in sunlight (basis of black-and-white photography): $2 AgCl light 2 Ag + Cl_{2}$ .

3. Displacement

A more reactive element displaces a less reactive one from its compound: $A + BC \to AC + B$ .

$Fe + CuSO_{4} \to FeSO_{4} + Cu$ (an iron nail in blue copper sulphate solution turns brownish and the solution turns pale green).
$Zn + CuSO_{4} \to ZnSO_{4} + Cu$ .

4. Double displacement

Two compounds exchange ions: $AB + CD \to AD + CB$ .

Precipitation: $Na_{2} SO_{4} + BaCl_{2} \to BaSO_{4} ↓ + 2 NaCl$ (white precipitate).
Neutralisation: acid + base → salt + water: $HCl + NaOH \to NaCl + H_{2} O$ .

5. Oxidation and reduction (redox)

Oxidation: gain of oxygen, or loss of hydrogen, or loss of electrons (OIL).
Reduction: loss of oxygen, or gain of hydrogen, or gain of electrons (RIG). Remember OIL RIG.
They occur together in a redox reaction. The substance that gets oxidised is the reducing agent; the one reduced is the oxidising agent.
$CuO + H_{2} \to Cu + H_{2} O$ : CuO is reduced (loses oxygen), H₂ is oxidised.
Examples of oxidising agents: oxygen, potassium permanganate, hydrogen peroxide, potassium dichromate, chlorine. Reducing agents: hydrogen, carbon, carbon monoxide, metals.

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