Chemical Reactions and Equations

1. What is a chemical reaction?

A chemical reaction happens when the atoms in one or more substances rearrange to form new substances with different properties. Something genuinely new is made — you can't just filter or evaporate your way back to the original material, the way you could with a physical change like melting ice.

You can usually tell a chemical reaction has taken place by watching for one or more of these signs:

• Change of state — e.g. a solid forming where there wasn't one before.
• Change of colour — e.g. a shiny brown copper vessel turning green over time.
• Evolution of a gas — bubbles appearing, often with a smell.
• Change in temperature — the reaction flask feels warmer (exothermic) or colder (endothermic) to the touch.

2. Chemical equations and balancing

A chemical equation is shorthand for a reaction, written using formulae instead of full names. The substances you start with are reactants, written on the left; what you end up with are products, written on the right, with an arrow pointing from reactants to products.

A correctly written equation must be balanced — the number of atoms of each element must be the same on both sides, because matter is neither created nor destroyed in a chemical reaction (this is the law of conservation of mass). You balance equations by adjusting the numbers written in front of each formula (called coefficients) — never by changing the small subscript numbers inside a formula, since that would describe a different substance entirely.

Balancing by hit-and-trial — worked example

Balance: Fe + H₂O → Fe₃O₄ + H₂

You'll also see extra symbols in a fully-labelled equation: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water, Δ heat supplied, and ↑ / ↓ for a gas or precipitate escaping the reaction.

3. Five types of chemical reactions

a) Combination reaction

Two or more reactants combine to form a single product.

Combination reactions that release heat and light as they happen are also exothermic — burning coal (C + O₂ → CO₂) is both a combination and an exothermic reaction.

b) Decomposition reaction

A single compound breaks down into two or more simpler products — essentially the reverse of combination. There are three common sub-types depending on how the energy is supplied:

• Thermal decomposition (heat): CaCO₃ →(Δ) CaO + CO₂
• Electrolytic decomposition (electricity): 2H₂O →(electricity) 2H₂ + O₂
• Photolytic decomposition (sunlight): 2AgCl →(sunlight) 2Ag + Cl₂

c) Displacement reaction

A more reactive element pushes out (displaces) a less reactive element from its compound.

Here iron is more reactive than copper, so it displaces copper from copper sulphate — you'll actually see the blue solution fade and a reddish-brown coating of copper form on the iron nail.

d) Double displacement reaction

Two compounds exchange ions with each other. This is very common when one of the products is insoluble and separates out as a solid — called a precipitation reaction.

e) Oxidation and reduction (redox) reactions

Covered in detail in the next section — a reaction where one substance gains oxygen (or loses hydrogen) while another loses oxygen (or gains hydrogen), happening together.

4. Oxidation and reduction

Here CuO loses oxygen to become Cu (CuO is reduced), while H₂ gains oxygen to become H₂O (H₂ is oxidised). Both happen in the same equation — that's what makes it a redox reaction.

5. Corrosion and rancidity — oxidation in daily life

Corrosion

When a metal is slowly eaten away by reaction with substances around it — usually moisture, air, or acids — it's called corrosion. The most familiar example is rusting of iron, where iron reacts with oxygen and moisture in the air to form a flaky layer of hydrated iron(III) oxide. Rusting weakens bridges, railings and vehicle bodies over time and costs a lot to prevent — that's why iron is often painted, galvanised (coated with zinc), or alloyed into stainless steel.

Rancidity

When fats and oils in food are oxidised over time, they develop an unpleasant smell and taste — this is called rancidity, and it's why open packets of chips or pickles stored too long start tasting "off." Food companies slow this down by adding antioxidants, packaging snacks under nitrogen gas instead of air, and storing oily foods in airtight containers away from light.

6. Quick revision table

7. Practice questions

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