What is a chemical reaction?
Think about an iron nail left outside in the rain. Over time, a reddish-brown layer of rust appears. The iron has reacted with oxygen and moisture to form a new substance. This is a chemical reaction.
A chemical reaction is a process in which one or more substances, called reactants, are transformed into new substances, called products. The atoms are rearranged, but they are not created or destroyed.
A change in colour, the release of a gas, a change in temperature, or the formation of an insoluble solid can be evidence of a chemical reaction. These observations are useful clues; you still need to consider whether a new substance has formed.
Reactants → Products. A chemical reaction rearranges atoms to make new substances.
Writing a chemical equation
A chemical equation is a compact way of describing a reaction using symbols and formulae. When magnesium burns in oxygen, it forms magnesium oxide.
The word equation is: magnesium + oxygen → magnesium oxide.
The skeletal equation is: Mg + O₂ → MgO. The arrow means ‘produces’. Reactants appear on the left; products appear on the right. Notice that the oxygen atoms do not yet match on both sides. This equation needs balancing.
Why do equations need balancing?
The law of conservation of mass tells us that mass is neither created nor destroyed in a chemical reaction. In an ordinary chemical reaction, each type of atom must therefore have the same count before and after the reaction.
For magnesium: 2Mg + O₂ → 2MgO. There are now two magnesium atoms and two oxygen atoms on each side.
Change the coefficients in front of formulae to balance an equation. Never change the subscripts inside a formula: MgO and MgO₂ would describe different substances.
Balance coefficients, never chemical formulae.
Four useful reaction patterns
Combination: two or more substances form one product. For example, CaO + H₂O → Ca(OH)₂.
Decomposition: one compound breaks into simpler substances, often with heat, light, or electricity. For example, heating calcium carbonate gives CaCO₃ → CaO + CO₂.
Displacement: a more reactive element displaces a less reactive element from its compound. For example, Zn + CuSO₄ → ZnSO₄ + Cu.
Double displacement: two compounds exchange ions. For example, Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. Barium sulphate forms a white precipitate.
Check your understanding
1. Why is rusting a chemical change? 2. Balance: H₂ + O₂ → H₂O. 3. What is the difference between a coefficient and a subscript?
Answers: Rusting produces a new substance. The balanced equation is 2H₂ + O₂ → 2H₂O. A coefficient counts formula units; a subscript tells you the number of atoms of an element within a formula unit.