Chemical changes · GCSE Chemistry

Acids and alkalis

pH, strong and weak acids, neutralisation equations, salt names, carbonates, metals and the titration practical.

UNDERSTAND→RETRIEVE→REMEMBER
THE MEMORY HOOK
Acids donate H⁺. Alkalis contain OH⁻. Acid plus base gives a salt plus water. The salt name comes from the metal and from the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.

The important bits

What you need to know

  1. 1

    Acids produce hydrogen ions, H⁺(aq), in water. Alkalis produce hydroxide ions, OH⁻(aq). Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l).

  2. 2

    pH 0–6 is acidic, 7 is neutral, 8–14 is alkaline. Universal indicator or a pH probe measures pH. Each step on the pH scale is a ten-fold change in H⁺ concentration.

  3. 3

    Acid + metal → salt + hydrogen. Acid + metal oxide or hydroxide → salt + water. Acid + carbonate → salt + water + carbon dioxide.

  4. 4

    Hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates. The metal comes from the base, alkali or carbonate.

  5. 5

    Soluble bases are alkalis. Insoluble bases are still metal oxides or hydroxides; they still neutralise acids, but you add the solid until it is in excess and then filter.

  6. 6

    On Higher tier, strong acids ionise completely in water; weak acids ionise partially. pH of a weak acid is higher than a strong acid of the same concentration, but both can still be fully neutralised.

  7. 7

    Titration finds the exact volume of acid that neutralises an alkali (or the reverse). Use a pipette, burette, indicator, and concordant titres within 0.10 cm³, then take a mean of the concordant results.

Go deeper

Name the salt from the two halves of the reaction

The metal or ammonium comes from the base, alkali or carbonate. The other half comes from the acid. Sodium hydroxide plus hydrochloric acid is sodium chloride. Copper oxide plus sulfuric acid is copper sulfate. Calcium carbonate plus nitric acid is calcium nitrate. If hydrogen is produced, a metal was used, not a carbonate. If carbon dioxide is produced, a carbonate was used — limewater turning cloudy is the test. State symbols score: (aq) for solutions, (s) for insoluble solids, (g) for hydrogen and carbon dioxide, (l) for water. “Acid + alkali → salt + water” is the general sentence; the exam still wants the specific equation.

Go deeper

Strong versus weak is not the same as concentrated

Concentration is how much acid is dissolved per dm³. Strength is how completely it ionises. Hydrochloric acid is strong even when dilute: nearly all HCl molecules split into H⁺ and Cl⁻. Ethanoic acid is weak even when concentrated: only a fraction of molecules ionise. A 0.1 mol/dm³ HCl solution therefore has a lower pH than 0.1 mol/dm³ ethanoic acid. Both can still require the same volume of alkali for complete neutralisation if the concentrations of acid molecules are equal, because extra H⁺ is produced as the weak acid’s equilibrium shifts. That last point is Higher tier and is where many candidates stop thinking.

WORKED EXAMPLE

See the idea in action

25.0 cm³ of sodium hydroxide is titrated with 0.100 mol/dm³ hydrochloric acid. Concordant titres are 24.40 cm³ and 24.50 cm³. Mean titre = 24.45 cm³. The equation is HCl + NaOH → NaCl + H₂O, so the mole ratio is 1:1. Moles of HCl = 0.100 × (24.45/1000) = 0.002445 mol. That equals the moles of NaOH in 25.0 cm³, so concentration of NaOH = 0.002445 ÷ 0.0250 = 0.0978 mol/dm³. Report to three significant figures if the data support it: 0.0978 mol/dm³.

Exam technique

Turn knowledge into marks

Learn the three construction reactions until they are automatic: metal, base, carbonate. Then name the salt. In titration write concordant titres, mean, moles of the known solution, ratio, then concentration of the unknown. Include H⁺(aq) + OH⁻(aq) → H₂O(l) when asked for the ionic equation.

Common mistakes

Do not give these marks away

  1. 01

    Mixing up strength (ionisation) with concentration (moles per dm³).

  2. 02

    Naming the salt from the wrong acid, for example writing sodium chloride from sulfuric acid.

  3. 03

    Forgetting carbon dioxide when a carbonate reacts, or hydrogen when a metal reacts.

QUICK RETRIEVAL

What are the products when hydrochloric acid reacts with calcium carbonate?

ACalcium chloride and hydrogen

BCalcium chloride, water and carbon dioxide

CCalcium sulfate and water

DCalcium hydroxide and carbon dioxide

Show the answer

Calcium chloride, water and carbon dioxide. Acid plus carbonate always gives a salt, water and carbon dioxide. Hydrochloric acid produces a chloride, so the salt is calcium chloride.

Quick questions

If this is the bit you searched

How do you name the salt from a neutralisation?

Take the metal from the base, alkali or carbonate, and the ending from the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.

What is the difference between a strong acid and a concentrated acid?

Strong means fully ionised in water. Concentrated means a large amount of acid dissolved per dm³. They are not the same idea.