It's all about ions in water

When dissolved in water:

• An acid is a source of hydrogen ions, H⁺ — it releases H⁺ into the solution.
• An alkali is a soluble base that releases hydroxide ions, OH⁻ into the solution.
• A base is any substance that neutralises an acid (a metal oxide, hydroxide or carbonate). An alkali is simply a base that dissolves in water.

Measuring acidity

The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. pH 7 is neutral; below 7 is acidic; above 7 is alkaline. The lower the pH, the more acidic.

Watch out: pH 7 is neutral, not "no acidity". A lower pH means more acidic (more H⁺), so pH 1 is far more acidic than pH 6.

Reading the pH scale

Acid meets alkali

When an acid reacts with an alkali, the H⁺ from the acid and the OH⁻ from the alkali join to make water. This is neutralisation:

The other ions stay in solution as the salt. So overall: acid + alkali → salt + water. As neutralisation proceeds, the pH moves towards 7.

Core practical: you can follow a neutralisation by adding alkali to acid from a burette and recording the pH with universal indicator or a pH probe, or run an acid–alkali titration with a single indicator to find the exact volume needed.

What is neutralisation?

Three reactions of acids

Acids react with metals and with bases. Learn these three general patterns:

• Acid + metal → salt + hydrogen
  e.g. Mg + 2HCl → MgCl₂ + H₂ (squeaky pop test for the gas)
• Acid + metal oxide / metal hydroxide → salt + water (oxides & hydroxides are bases)
  e.g. CuO + H₂SO₄ → CuSO₄ + H₂O
• Acid + metal carbonate → salt + water + carbon dioxide
  e.g. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ (turns limewater milky)

Name comes from the acid

The salt's name has two parts. The first part comes from the metal (or ammonium); the second part comes from the acid used:

• Hydrochloric acid (HCl) → chloride salts
• Sulfuric acid (H₂SO₄) → sulfate salts
• Nitric acid (HNO₃) → nitrate salts

So copper + sulfuric acid gives copper sulfate; sodium + hydrochloric acid gives sodium chloride; potassium + nitric acid gives potassium nitrate.

Watch out: the salt name depends on which acid you use, not the metal. Sulfuric acid always makes a sulfate, whatever metal or base it reacts with.

Name the salt

Acid + base → which salt?

Tap a reaction on the left, then its salt on the right.

A soluble salt from an insoluble base

The Edexcel core practical: prepare pure, dry crystals of a soluble salt from an acid and an insoluble base (e.g. copper sulfate from copper oxide + sulfuric acid). The method:

• Warm the acid, then add the insoluble base (e.g. copper oxide) until it is in excess — when no more dissolves, all the acid has reacted.
• Filter to remove the leftover excess base.
• Evaporate the filtrate to the point of crystallisation, then leave to crystallise.
• Pat the crystals dry between filter paper.

Using excess insoluble base guarantees no acid is left over and the excess solid is easy to filter out — that's the trick that gives a pure salt.

Why use excess base?

Strong vs weak acids

Strong and weak describe how much an acid ionises (dissociates) in water:

• A strong acid (HCl, H₂SO₄, HNO₃) is fully ionised — almost every molecule releases its H⁺.
• A weak acid (e.g. ethanoic, citric, carbonic) is only partially ionised — ionisation is reversible: CH₃COOH ⇌ CH₃COO⁻ + H⁺.

For the same concentration, a strong acid has a higher H⁺ concentration, so a lower pH. As pH decreases by 1, the H⁺ concentration goes up by a factor of 10.

Watch out — strong ≠ concentrated. Strong/weak is about the degree of ionisation; concentrated/dilute is about how much acid is dissolved in a given volume. A dilute strong acid and a concentrated weak acid are different things.

Strong or concentrated?

Splitting compounds with electricity

Electrolysis uses electricity to break down an ionic compound (the electrolyte) into its elements. The ions must be free to move, so the compound is molten or dissolved.

Watch out: the cathode is negative and attracts the positive ions (cations). It is wrong to say the cathode is positive.

The rules for aqueous solutions

In a solution, water also provides H⁺ and OH⁻ ions, so there is a competition. Use these rules:

• At the cathode (−): hydrogen is produced unless the metal is less reactive than hydrogen (e.g. copper, silver), in which case the metal is deposited.
• At the anode (+): oxygen is produced (from OH⁻) unless a halide (Cl⁻, Br⁻, I⁻) is present, in which case the halogen is produced.

So electrolysing dilute sodium chloride solution gives hydrogen at the cathode and (with concentrated NaCl) chlorine at the anode.

Predict the products

Cathode or anode?

Tap a product, then tap the electrode where it forms.

Electrolysing copper sulfate

The Edexcel core practical: electrolyse copper sulfate solution and investigate the products at inert (carbon/platinum) electrodes.

• Cathode (−): copper is less reactive than hydrogen, so a red-brown layer of copper is deposited.
• Anode (+): no halide is present, so oxygen bubbles off (the SO₄²⁻ stays in solution).

With copper electrodes instead, the anode copper dissolves and copper transfers to the cathode — this is how copper is purified.

Half equations at the electrodes

A half equation shows the electrons gained or lost at one electrode. Electrons are written as e⁻:

• Cathode (reduction — gain of electrons):
  Cu²⁺ + 2e⁻ → Cu   or   2H⁺ + 2e⁻ → H₂
• Anode (oxidation — loss of electrons):
  2Cl⁻ → Cl₂ + 2e⁻   or   4OH⁻ → O₂ + 2H₂O + 4e⁻

Remember: at the cathode the positive ions gain electrons (reduction); at the anode the negative ions lose electrons (oxidation). Both charge and atoms must balance.

Balance the half equation

The key ideas to know

You've covered the whole of Edexcel Topic 3 — acids, neutralisation, reactions of acids, making salts and electrolysis. Press Finish to see your score.