What do acids react with, and how is a soluble salt prepared?
The reactions of acids with metals, bases and carbonates, the salts produced, and how to prepare a soluble salt.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on the reactions of acids, covering reactions with metals, bases and carbonates, the salts produced, and the method for preparing a soluble salt.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe the reactions of acids with metals, bases and carbonates, the salts produced, and how to prepare a soluble salt.
The reactions of acids
For example: hydrochloric acid + magnesium to magnesium chloride + hydrogen; sulfuric acid + copper oxide to copper sulfate + water; nitric acid + calcium carbonate to calcium nitrate + water + carbon dioxide.
Naming the salt
So a reaction of sulfuric acid with zinc makes zinc sulfate; nitric acid with sodium hydroxide makes sodium nitrate.
Preparing a soluble salt
To make a pure, dry sample of a soluble salt from an insoluble base (or metal or carbonate):
- Warm the acid and add the base (e.g. copper oxide) until no more dissolves - the base is in excess, so all the acid is used up.
- Filter to remove the leftover excess base.
- Evaporate the filtered solution until crystals just start to form (the point of crystallisation).
- Leave to cool so crystals form, then pat dry with filter paper.
Using an excess of the base and then filtering makes sure there is no leftover acid in the final salt.
Testing the gases produced
The gases from acid reactions can be identified with simple tests, which exam questions often combine with the reactions. The hydrogen from acid + metal gives a squeaky pop with a lit splint. The carbon dioxide from acid + carbonate turns limewater cloudy (milky). These tests confirm which reaction has happened and which gas is given off. Being able to state the reaction, name the gas, and give the test result is a common multi-mark question, so it is worth learning the tests alongside the reactions.
Why the salt name has two parts
The name of every salt has two parts: the metal comes first, and the acid decides the second part. This is because the salt is formed when the hydrogen in the acid is replaced by a metal. So sulfuric acid (which contains the sulfate part) makes sulfates, hydrochloric acid makes chlorides, and nitric acid makes nitrates. Once you know which acid is used, you immediately know the ending of the salt's name, and the metal from the base or carbonate gives the first part. Practising naming salts from the acid and metal used is a reliable way to pick up marks, because these names appear in almost every acids question on the paper.
Try this
Q1. What gas is given off when an acid reacts with a metal? [1 mark]
- Cue. Hydrogen.
Q2. What salt does nitric acid form? [1 mark]
- Cue. A nitrate.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksGive the general word equations for the reactions of an acid with (a) a metal, (b) a carbonate, naming the products.Show worked answer →
A Unit 5 recall question worth 4 marks. Reward: acid + metal to salt + hydrogen (2); acid + carbonate to salt + water + carbon dioxide (2). Markers credit each general equation with the correct products. A common error is to forget the carbon dioxide from a carbonate, or to add water to the metal reaction (only the carbonate and base reactions make water).
WJEC style5 marksDescribe how you would prepare pure, dry copper sulfate crystals from copper oxide and sulfuric acid.Show worked answer →
A Unit 5 practical question worth 5 marks. Reward: warm the acid and add copper oxide (the base) until no more dissolves (it is in excess), so all the acid reacts (1); filter to remove the excess copper oxide (1); evaporate the solution to the point of crystallisation (1); leave to cool so crystals form (1); then pat dry with filter paper (1). Markers credit excess base, filtering, evaporating, crystallising and drying. A common error is to leave out the excess solid or the filtering step.
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