WJEC GCSE Chemistry
WJEC GCSE Chemistry 2.2 Acids, Bases and Salts Exam Questions
Free WJEC GCSE Chemistry revision for 2.2 Acids, Bases and Salts: what you need to know, common mistakes and exam-style questions with answers. Covers pH, strong and weak acids, neutralisation, preparing soluble and insoluble salts, the sulfate test, titration and concentration calculations.
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About WJEC GCSE Chemistry 2.2 Acids, Bases and Salts
Acids, bases and salts is a practical-heavy topic in Unit 2. You need to link pH to the concentration of hydrogen ions, distinguish strong from weak and concentrated from dilute acids, and write neutralisation equations, including the ionic equation. You must know how to prepare soluble salts from metals, oxides, hydroxides and carbonates, and insoluble salts by precipitation. The topic includes the test for sulfate ions, titration and calculations of concentration in mol/dm3. Specified practicals cover preparing crystals of a soluble salt and carrying out a titration. Examiners frequently set titration calculations and step-by-step method questions.
What you need to know
- pH and acids Acids produce H+ ions in water and alkalis produce OH- ions. pH below 7 is acidic, 7 neutral, above 7 alkaline. Strong acids are fully ionised, weak acids partly ionised; concentration describes how much acid is dissolved per dm3.
- Neutralisation H+(aq) + OH-(aq) → H2O(l). Acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide; acid + metal → salt + hydrogen.
- Soluble salts Add excess insoluble base or carbonate (e.g. copper(II) oxide) to warm acid, filter off the excess, evaporate some water and leave to crystallise. This is a specified practical.
- Insoluble salts and sulfate test Mix two solutions to form a precipitate, then filter, wash and dry it. Sulfate ions give a white precipitate of barium sulfate with barium chloride solution (acidified with hydrochloric acid).
- Titration Use a pipette to measure the alkali, add indicator, then add acid from a burette until the colour changes; repeat to get concordant results. This is a specified practical; the salt can then be made without indicator.
- Concentration Concentration (mol/dm3) = moles / volume (dm3). Convert cm3 to dm3 by dividing by 1000. Use the mole ratio from the balanced equation in titration calculations.
Common mistakes in 2.2 Acids, Bases and Salts
- Using 'strong' to mean 'concentrated'. Strong refers to how fully an acid ionises; concentrated refers to how much acid is in a given volume.
- Forgetting to convert cm3 to dm3 in concentration calculations. Divide the volume in cm3 by 1000 first.
- Heating a salt solution to dryness when crystals are wanted. Evaporate only some of the water, then leave the solution to cool and crystallise.
- Not adding the base in excess when making a soluble salt. Excess ensures all the acid is used up and the excess solid is then filtered off.
Try three 2.2 Acids, Bases and Salts exam-style questions
1. 25.0 cm3 of sodium hydroxide solution was neutralised by 20.0 cm3 of 0.100 mol/dm3 hydrochloric acid. Calculate the concentration of the sodium hydroxide. NaOH + HCl → NaCl + H2O (3 marks)
Show the mark scheme answer
Moles HCl = 0.100 x 20.0 / 1000 = 0.00200; 1:1 ratio so 0.00200 mol NaOH; concentration = 0.00200 / 0.0250 = 0.0800 mol/dm3.
2. Describe how to make pure dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid. (4 marks)
Show the mark scheme answer
Warm the acid and add copper(II) oxide until in excess / no more dissolves; filter to remove excess oxide; evaporate some of the water / heat to crystallisation point; leave to cool and crystallise, then dry crystals with filter paper.
3. Describe the test for sulfate ions. (2 marks)
Show the mark scheme answer
Add dilute hydrochloric acid then barium chloride solution; white precipitate forms.
2.2 Acids, Bases and Salts FAQs
What does WJEC GCSE Chemistry 2.2 Acids, Bases and Salts cover?
It covers pH, strong and weak acids, neutralisation, preparing soluble and insoluble salts, the test for sulfate ions, titration and calculations of concentration in mol/dm3.
Which unit is Acids, Bases and Salts examined in?
It is examined in Unit 2: Chemical Bonding, Application of Chemical Reactions and Organic Chemistry, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
How do I get titration calculations right?
Write moles = concentration x volume (in dm3) for the known solution, use the equation ratio, then divide by the volume of the unknown in dm3. Show every step and give units.
