Word and balanced symbol equations, conservation of mass, the reactions of acids with metals, bases and carbonates, neutralisation, the pH scale, and oxidation and reduction in terms of oxygen and electrons.

What this topic is asking

OCR wants you to write word and balanced symbol equations, apply conservation of mass, describe the reactions of acids with metals, bases and carbonates, explain neutralisation and the pH scale, and define oxidation and reduction.

Equations and conservation of mass

A word equation names the reactants and products, for example magnesium + oxygen produces magnesium oxide. A balanced symbol equation uses formulae and balancing numbers so that atoms are conserved, for example $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. You balance an equation by adjusting the numbers in front of the formulae (never changing a formula itself) until each element has equal atoms on both sides. Because mass is conserved, a sealed reaction does not change mass; an apparent change in an open container is usually because a gas has escaped or been taken in from the air.

Reactions of acids

Acids have three key reaction types that OCR expects you to know, each producing a salt:

• Acid + metal produces a salt + hydrogen. For example $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$. Test for hydrogen with a lighted splint: it gives a squeaky pop.
• Acid + base (metal oxide or hydroxide) produces a salt + water. This is neutralisation, for example hydrochloric acid + sodium hydroxide produces sodium chloride + water.
• Acid + metal carbonate produces a salt + water + carbon dioxide. Test for carbon dioxide by bubbling it through limewater, which turns cloudy.

Oxidation and reduction

Oxidation and reduction can be defined two ways. In terms of oxygen, oxidation is the gain of oxygen (for example magnesium burning to magnesium oxide) and reduction is the loss of oxygen (for example a metal oxide losing oxygen to become the metal). In terms of electrons, oxidation is the loss of electrons and reduction is the gain of electrons (a useful memory aid is "OIL RIG": Oxidation Is Loss, Reduction Is Gain, of electrons). When a metal reacts to form a positive ion, it loses electrons, so it is oxidised. These ideas link directly to the reactivity series and to electrolysis.