Chemical Reactions and Equations

How to write and balance chemical equations, and the main types of reaction: combination, decomposition, displacement, double displacement and redox.

Key ideas

• Signs of a chemical reaction: a change of state or colour, a gas given off, a change in temperature, or a precipitate.
• An equation is balanced when each element has the same number of atoms on both sides (mass is conserved).
• Combination: two or more substances form one product. Decomposition: one substance breaks into two or more.
• Decomposition needs energy: heat (thermal), electricity (electrolysis) or light.
• Displacement: a more reactive element pushes a less reactive one out of its compound.
• Double displacement: two compounds swap ions, often making a precipitate.
• Oxidation is gain of oxygen or loss of hydrogen; reduction is the opposite. When both happen together it is a redox reaction.
• Corrosion (rusting of iron) and rancidity (oils and fats going stale) are everyday oxidation.

Remember

Watch out

• Changing the small numbers in a formula to balance an equation: only change the numbers in front.
• Mixing up displacement (an element and a compound) with double displacement (two compounds).
• Saying the substance that gains oxygen is reduced (it is oxidised).

Worked example