Key Stage 4 science · Chemistry – Chemical changes

GCSE chemistry: acids, pH, electrolysis and redox

This topic brings together four big ideas in GCSE chemistry. Students study how acids react with metals and carbonates, what the pH scale actually measures, how electricity can split ionic compounds in electrolysis, and how oxidation and reduction describe reactions in which oxygen is gained or lost. Together they explain everyday things such as limescale removal, rusting, metal plating and how aluminium is made.

Acids, pH, electrolysis and redox, explained

Acids are substances that release hydrogen ions, H⁺, when dissolved in water. Students learn two reaction patterns that they are expected to predict. An acid reacting with a metal that is more reactive than hydrogen gives a salt and hydrogen gas: magnesium + hydrochloric acid → magnesium chloride + hydrogen. An acid reacting with a carbonate gives a salt, water and carbon dioxide: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. Copper does not react with dilute hydrochloric or sulfuric acid because it is below hydrogen in the reactivity series. The salt’s name comes from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.

The pH scale runs from 0 to 14 and measures the concentration of hydrogen ions. Below 7 is acidic, 7 is neutral, and above 7 is alkaline. The scale is not like a ruler: each step down the scale means ten times the concentration of hydrogen ions, so a solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4, and a hundred times that of one at pH 5. Universal indicator gives an approximate colour, and a pH meter gives a number. Many courses also separate strong acids, which ionise completely in water, from weak acids, which only partly ionise, and both are different from concentrated and dilute, which describe how much acid is dissolved.

Electrolysis uses direct current to break down an ionic compound that is either molten or dissolved in water. In a solid the ions are held in place in a lattice, so solid salt will not conduct, but when it is melted or dissolved the ions are free to move and carry the current. Positive ions travel to the negative electrode, the cathode, and negative ions to the positive electrode, the anode. In a molten compound the metal forms at the cathode and the non-metal at the anode, so molten lead bromide gives lead and bromine. In an aqueous solution water adds hydrogen and hydroxide ions to the mix, so students have to decide which ion is discharged at each electrode.

Oxidation and reduction always happen together, which is why chemists say redox. At this level they are defined by oxygen: oxidation is gaining oxygen and reduction is losing oxygen. When magnesium burns it is oxidised to magnesium oxide. When copper oxide is heated with hydrogen, the copper oxide loses oxygen and is reduced, while the hydrogen gains oxygen and forms water. Many schools also teach the electron version, oxidation is loss of electrons and reduction is gain of electrons, because it explains what happens at the electrodes in electrolysis. Ask which definition your child’s school uses.

Words your child will hear

Salt: A compound made when an acid’s hydrogen is replaced by a metal, such as magnesium chloride from magnesium and hydrochloric acid.
pH: A number from 0 to 14 showing hydrogen ion concentration: acidic below 7, neutral at 7, alkaline above 7.
Electrolyte: A molten or dissolved ionic compound that conducts electricity because its ions are free to move.
Cathode: The negative electrode, where positive ions are attracted and metals or hydrogen form.
Anode: The positive electrode, where negative ions are attracted and non-metals such as chlorine form.
Oxidation: Gaining oxygen. Many courses also say it is losing electrons.
Reduction: Losing oxygen. Many courses also say it is gaining electrons.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about the chemistry of acids; reactions with some metals and carbonates
Students should be taught about pH as a measure of hydrogen ion concentration and its numerical scale
Students should be taught about electrolysis of molten ionic liquids and aqueous ionic solutions
Students should be taught about reduction and oxidation in terms of loss or gain of oxygen

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Concentrated sodium chloride solution is electrolysed using inert electrodes. Predict the product at each electrode and say what is left in the solution.

List every ion present. The salt gives Na⁺ and Cl⁻. Water also contains a few H⁺ and OH⁻ ions. So there are four ions to consider.
At the cathode (negative), the positive ions Na⁺ and H⁺ are attracted. Sodium is more reactive than hydrogen, so sodium ions stay in solution and hydrogen ions are discharged, gaining electrons to form hydrogen gas. Half equations like this one use the electron idea, which many schools teach alongside the oxygen definition.
```
2H⁺ + 2e⁻ → H₂
```
At the anode (positive), Cl⁻ and OH⁻ are attracted. In a concentrated solution of a halide, the halide ion is discharged, so chloride ions lose electrons and form chlorine gas.
```
2Cl⁻ → Cl₂ + 2e⁻
```
Check the products with the gas tests: hydrogen makes a squeaky pop with a lit splint, and chlorine bleaches damp litmus paper.
Finally, work out what remains. Na⁺ ions were not discharged, and the hydroxide ions that came from the water were left behind as the hydrogen ions were removed. So the solution gradually becomes sodium hydroxide, which is alkaline.

Hydrogen forms at the cathode and chlorine at the anode, leaving sodium hydroxide solution, which turns universal indicator purple.

Common mistakes and how to help

Thinking pH 3 is just slightly more acidic than pH 4

Most scales children use are even, like a ruler or a thermometer, so a one-point difference feels small.

What helps: Say that every step on the pH scale is ten times. A drawing of a pH 4 drink being diluted ten times to reach pH 5 helps, and so does comparing it to an earthquake or sound scale where steps also multiply.

Believing strong acid means concentrated acid

In everyday speech “strong” covers both how powerful and how concentrated something is, such as strong tea.

What helps: Strong and weak describe how completely the acid ionises, while concentrated and dilute describe how much acid is in a given volume. A dilute solution of a strong acid and a concentrated solution of a weak acid can both exist.

Mixing up cathode and anode, or sending electrons through the solution

The names look similar, and the diagrams show a wire, a cell and a beaker all at once.

What helps: The memory aid PANIC (Positive Anode, Negative Is Cathode) works well. Remind your child that electrons travel in the wires, while ions carry the current through the liquid.

Thinking oxidation is the same as burning

Burning is the first example most children meet, and oxygen is in the definition.

What helps: Rusting is oxidation too: iron slowly gains oxygen, with no flame at all. Reduction happens at the same time in the same reaction, so ask “what gained oxygen, and what lost it?”

How to help at home

Make a natural indicator. Chop a few red cabbage leaves, crush them in a bowl with a little cold water and strain the purple liquid. With an adult watching, add a few drops to small cups of lemon juice, vinegar, clear lemonade and a bicarbonate of soda solution. The colour changes show a range from acid to alkali. Do not taste, and never mix cleaning products.
Spot oxidation happening. Put one steel nail on a damp paper towel in a saucer and another in a dry, lidded jar, and check them over a few days. Ask your child to explain why only one rusts, and what the iron is gaining. A cut apple browning, more slowly on the half with lemon juice squeezed over it, is another everyday reaction with oxygen in the air to talk about.
Clean a dull coin with acid. Soak a dull copper coin in a little vinegar with a pinch of salt for ten minutes. The acid reacts with the copper oxide on the surface, so the coin brightens. Rinse it and wash hands afterwards. This is an acid reacting with a metal oxide, a close relative of the reactions in class, and it makes a good talking point about why copper itself does not fizz in acid.
Link to the real world. Look at a tap, a bicycle part or a drinks can and ask whether it was probably made, coated or cleaned using electrolysis. Chrome plating, anodising and aluminium extraction all use it. Linking the lesson to a physical object helps it stick.

Builds on

Key Stage 3: Chemical reactions and equations
Key Stage 3: Acids, alkalis and neutralisation