OxfordAQA International GCSE Chemistry
OxfordAQA International GCSE Chemistry 3.5 Acids, Bases and Salts Exam Questions
Free OxfordAQA International GCSE Chemistry revision for 3.5 Acids, Bases and Salts: what you need to know, common mistakes and exam-style questions with answers. Covers acids, bases and alkalis, pH and indicators, neutralisation, naming salts and preparing soluble and insoluble salts.
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About OxfordAQA International GCSE Chemistry 3.5 Acids, Bases and Salts
Acids, Bases and Salts in OxfordAQA International GCSE Chemistry covers what makes a solution acidic or alkaline and how salts are made. You learn that metal oxides and hydroxides are bases, that soluble hydroxides are alkalis, how the pH scale and universal indicator work, and the ionic equation for neutralisation. You must name the salt formed from hydrochloric, nitric and sulfuric acids, and choose a method to make a named salt: from a metal, an insoluble base, an alkali using an indicator, or by precipitation for insoluble salts. Examiners often ask for a full method to prepare pure dry crystals, so learn the steps in order.
What you need to know
- Acids and alkalis H+(aq) ions make a solution acidic and OH-(aq) ions make it alkaline. The pH scale runs from 0 to 14 with 7 neutral; universal indicator gives an approximate pH from its colour.
- Bases Metal oxides and hydroxides are bases; soluble hydroxides are alkalis. Ammonia dissolves in water to give an alkaline solution and is used to make ammonium salts.
- Neutralisation Acid + base → salt + water. The ionic equation is H+(aq) + OH-(aq) → H2O(l). Limewater reacts with carbon dioxide to form calcium carbonate.
- Naming salts Hydrochloric acid gives chlorides, nitric acid gives nitrates and sulfuric acid gives sulfates. The first part of the name comes from the metal or ammonium in the base.
- Salts from metals and insoluble bases Add the metal or insoluble base (for example copper oxide) to warm acid until no more reacts, filter off the excess solid, then crystallise the filtrate. Some metals are too reactive and some not reactive enough to use.
- Salts from alkalis Use an indicator or titration to find the exact volumes for neutralisation, then repeat without indicator and crystallise. This is used for sodium, potassium and ammonium salts.
- Insoluble salts Mix two solutions that contain the ions needed, for example barium chloride and sodium sulfate to make barium sulfate; filter, wash and dry the precipitate. Precipitation also removes unwanted ions in water and effluent treatment.
Common mistakes in 3.5 Acids, Bases and Salts
- Saying all bases are alkalis. Only soluble bases (hydroxides in solution) are alkalis.
- Forgetting to add excess insoluble base. Adding excess ensures all the acid reacts, then the excess is filtered off.
- Evaporating all the water to dryness when making crystals. Heat to concentrate, then leave to cool so crystals form.
- Using an indicator when making salts from insoluble bases. The excess solid shows the reaction is complete; an indicator is used with alkalis.
- Naming the salt from sulfuric acid as sulfide. Sulfuric acid gives sulfates.
Try three 3.5 Acids, Bases and Salts exam-style questions
1. Describe how to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid. (5 marks)
Show the mark scheme answer
Warm the dilute sulfuric acid; add copper oxide a little at a time, stirring, until no more dissolves (excess); filter to remove the excess copper oxide; heat the filtrate to evaporate some water then leave to cool and crystallise; filter off crystals and dry them between filter paper.
2. Write the ionic equation for neutralisation, including state symbols. (2 marks)
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H+(aq) + OH-(aq); → H2O(l).
3. 4.0 g of copper(II) oxide reacts completely with sulfuric acid: CuO + H2SO4 → CuSO4 + H2O. Calculate the mass of copper(II) sulfate formed. (Ar: Cu = 63.5, O = 16, S = 32, H = 1) (3 marks)
Show the mark scheme answer
Mr CuO = 79.5, moles = 4.0 / 79.5 = 0.0503 mol; 1:1 ratio so 0.0503 mol CuSO4; Mr CuSO4 = 159.5, mass = 0.0503 x 159.5 = 8.0 g.
3.5 Acids, Bases and Salts FAQs
What does OxfordAQA International GCSE Chemistry 3.5 Acids, Bases and Salts cover?
It covers acids, bases and alkalis, pH and indicators, neutralisation and its ionic equation, naming salts, and methods for preparing soluble and insoluble salts.
Which paper is 3.5 Acids, Bases and Salts examined in?
OxfordAQA International GCSE Chemistry has two equally weighted papers, Paper 1 and Paper 2, each 1 hour 30 minutes and 90 marks. Content from any part of the specification can be assessed in either paper, so revise this topic for both.
How do I choose a method to make a named salt?
First decide if the salt is soluble. If it is insoluble, use precipitation from two solutions. If it is soluble, use an excess of a metal or insoluble base and filter, unless the salt is of sodium, potassium or ammonium, in which case use an alkali and an indicator or titration.
