AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical changes › Reactions of acids
Practise Soluble salts. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How to make a pure, dry sample of a soluble salt from an acid and an insoluble solid (a metal, metal oxide, hydroxide or carbonate). This is required practical 1, so expect 4 to 6 mark method questions and questions on why each step is done.
The solid is added in excess (more than is needed) so that all the acid reacts. The leftover solid is easy to remove by filtering.
Method (required practical 1)
diagram
1. Measure dilute acid (e.g. 40 cm3 of sulfuric acid) into a beaker and warm it gently with a Bunsen burner. Do not boil it.
2. Add the insoluble solid a little at a time, stirring, until some solid is left over and will not dissolve. With a carbonate, the fizzing stops.
3. Filter the mixture to remove the excess solid. The filtrate is the salt solution.
4. Heat the filtrate in an evaporating basin, using a water bath or electric heater, until about half of the water has evaporated or crystals start to form.
5. Leave the solution to cool so that crystals form.
6. Filter off the crystals and dry them between sheets of filter paper or in a warm place.
React, filter, crystallise. The filtrate in step 2 is the salt solution; in step 3 the evaporating basin sits on a water bath.
Why each step matters
Warming the acid speeds up the reaction.
Excess solid makes sure no acid is left, so the salt is not contaminated with acid.
Filtering removes the unreacted solid, leaving only the salt solution.
Heating gently and not to dryness gives crystals; boiling it dry can make the solution spit and leaves a powder.
Wear eye protection: the acid and the hot solutions are hazards.
How to answer each type of question
Explain why a step is done
1 mark eachGrade 5
Link each step to its purpose: warming (faster), excess (all acid reacts), filtering (remove excess solid).
Give the specific reason, not just 'to make it work'.
Example. A student makes copper(II) chloride from copper(II) carbonate and dilute hydrochloric acid. (a) Why is excess copper(II) carbonate added? (b) How can the student tell that the copper(II) carbonate is in excess? (c) Why is the mixture then filtered?
Show the model answerHide the model answer
(a) To make sure all the acid has reacted / is used up (1) (b) Solid is left over / no more dissolves / the fizzing stops (1) (c) To remove the excess (unreacted) copper(II) carbonate (1)
Don’t lose marks
Adding excess acid instead of excess solid: the salt would then be contaminated with acid.
Writing 'evaporate all the water' or 'boil until dry'.
Forgetting to filter off the excess solid before crystallising.
Missing the final drying step, so the salt is not dry.
Choosing a very reactive metal such as sodium, which reacts dangerously with acid.
More tips
Memory tricks
Order of steps: We Eat Fresh Eggs, Cooked Daily (Warm, Excess, Filter, Evaporate, Crystallise, Dry).
Two filterings: the first removes excess solid (keep the filtrate); the second collects the crystals (keep what is in the filter paper).
Colour clue with copper(II) oxide: black solid left in a blue solution means the oxide is in excess.
Exam technique
In 6-mark method questions, write the steps in order and name the equipment: beaker, filter funnel and paper, evaporating basin, water bath.
Use the key words 'excess', 'filter', 'filtrate' and 'crystallise'.
A sensible volume of acid is fine to include, but the key steps and the reason for each matter most.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name reactants needed to make a saltFor example, copper(II) oxide and sulfuric acid make copper(II) sulfate.
Grade 4
Describe filtering and crystallisingFilter off the excess solid, heat to evaporate some water, then leave the solution to cool and crystallise.
Grade 5
Explain why excess solid is addedExcess solid makes sure all the acid reacts, so the salt solution contains no leftover acid.
Grade 5
Tell when the solid is in excessSolid stays undissolved at the bottom, and for a carbonate the fizzing stops.
Required practical:Making salts (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A student made copper sulfate crystals from copper oxide and dilute sulfuric acid. The steps of the method are shown in the wrong order:
A filter the mixture
B add copper oxide to warm acid until no more dissolves
C leave the solution to cool so that crystals form
D heat the filtrate to evaporate some of the water
Give the correct order of the steps.
B, A, D, C
A student made crystals of magnesium sulfate by reacting magnesium carbonate with dilute sulfuric acid. Write a word equation for the reaction.
A student makes copper(II) sulfate solution by adding copper(II) oxide to warm dilute sulfuric acid until some of the black solid is left undissolved. The student filters the mixture, then heats the filtrate to evaporate some of the water. The student then investigates how the place where the hot solution is left to cool affects the size of the crystals that form. The student pours 20 cm3 of the hot solution into each of three identical evaporating dishes. One dish is left in a refrigerator, one on the laboratory bench and one in a warm cupboard. After two days, the student measures the length of the largest crystal in each dish. What is the dependent variable in this investigation?
The length of the largest crystal.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
A student made copper sulfate crystals from copper oxide and dilute sulfuric acid. The steps of the method are shown in the wrong order:
A filter the mixture
B add copper oxide to warm acid until no more dissolves
C leave the solution to cool so that crystals form
D heat the filtrate to evaporate some of the water
(a) Give the correct order of the steps.[2]
(b) Explain why the student added copper oxide until no more dissolved.[1]
(c) Name the apparatus used to remove the excess copper oxide.[1]
Show the answer and mark scheme
(a)Answer: B, A, D, C
B first and C last
A before D: B, A, D, C
(b)
to make sure all of the acid had reacted / was used up
(c)
filter funnel and filter paper
Question 2Medium6 marks
A student made crystals of magnesium sulfate by reacting magnesium carbonate with dilute sulfuric acid.
(a) Write a word equation for the reaction.[1]
(b) The student warmed the acid before adding the magnesium carbonate. Give a reason why.[1]
(c) How would the student know when enough magnesium carbonate had been added?[2]
(d) The student evaporated the solution using a water bath instead of heating it strongly with a Bunsen burner. Suggest why.[1]
(e) Suggest how the student could dry the crystals.[1]