AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions
Practise Sulfates. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Sulfate ions give a white precipitate of barium sulfate with barium chloride solution in the presence of dilute hydrochloric acid. This is in GCSE Chemistry only (not Combined Science). The sulfate test is often combined with other ion tests to identify a compound in the required practical.
Key facts
Test: dilute hydrochloric acid, then barium chloride solution
Result: white precipitate of barium sulfate → sulfate ions present
Hydrochloric acid removes carbonate ions, which would also give a white precipitate
Make a solution of the sample. Add a few drops of dilute hydrochloric acid, then a few drops of barium chloride solution.
If sulfate ions (SO42−) are present, a white precipitate of barium sulfate forms.
If no precipitate forms, there are no sulfate ions.
Barium chloride solution is toxic: wear eye protection and avoid skin contact.
Why dilute hydrochloric acid?
The acid removes carbonate ions, which would otherwise form a white precipitate (barium carbonate) with barium chloride.
Do not use sulfuric acid: it contains sulfate ions, so it would give a white precipitate every time.
Compare the halide test, which uses nitric acid because hydrochloric acid would add chloride ions. In the sulfate test, extra chloride ions do not matter.
Required practical: identifying an unknown compound
diagram
An ionic compound contains a positive ion and a negative ion, so you need a test for each.
Positive ion: flame test, or sodium hydroxide solution.
Negative ion: dilute acid and limewater (carbonate), nitric acid and silver nitrate (halide), or hydrochloric acid and barium chloride (sulfate).
One test for each negative ion; test the positive ion with a flame test or NaOH.
How to answer each type of question
Describe the test for sulfate ions and give the result
2 to 3 marksGrade 3
Add dilute hydrochloric acid to a solution of the sample.
Add barium chloride solution.
Result: a white precipitate.
Example. Describe a test to show that a solution contains sulfate ions. Give the result.
Show the model answerHide the model answer
Add dilute hydrochloric acid (1), then barium chloride solution (1). A white precipitate forms (1).
Describe tests to confirm both ions in a compound
4 marksGrade 5
Dissolve the solid in water.
Test one portion for the positive ion (flame test or sodium hydroxide solution) and give the result.
Test another portion for the negative ion and give the result.
Example. A solid is thought to be iron(III) sulfate. Describe tests to show that it contains iron(III) ions and sulfate ions. Give the results.
Show the model answerHide the model answer
Dissolve the solid in water and add sodium hydroxide solution to one portion (1). A brown precipitate shows iron(III) ions (1). To another portion, add dilute hydrochloric acid and then barium chloride solution (1). A white precipitate shows sulfate ions (1).
Don’t lose marks
Using sulfuric acid, which adds sulfate ions, instead of hydrochloric acid.
Mixing up the reagents: silver nitrate is for halides; barium chloride is for sulfates.
Writing 'goes cloudy' without saying that a white precipitate forms.
Writing the sulfate ion wrongly. It is SO42−.
More tips
Memory tricks
Barium for sulfate, silver for halides.
The acid matches the reagent's negative ion: barium chloride with hydrochloric acid; silver nitrate with nitric acid.
Both tests: acid first (to remove carbonate ions), then the reagent.
Exam technique
Name both reagents (dilute hydrochloric acid and barium chloride solution) and the result (white precipitate).
In 6-mark 'identify' questions, go test by test and give the result for every substance.
Say 'dissolve the solid in water' first: the halide and sulfate tests are done on solutions.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Describe the test for sulfate ionsAdd dilute hydrochloric acid, then barium chloride solution, to a solution of the sample.
Grade 3
State the positive result for sulfatesA white precipitate of barium sulfate forms.
Grade 5
Identify a compound using two ion testse.g. a blue precipitate with sodium hydroxide and a white precipitate with barium chloride → copper(II) sulfate.
Required practical:Identifying ions (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.
Name the acid that is added to the sample before this solution.
dilute hydrochloric acid
The solution contained sodium sulfate. Name the white precipitate formed.
barium sulfate
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
(a) Which solution is used in the test for sulfate ions? Tick (✓) one box.[1]
Barium chloride solution
Limewater
Silver nitrate solution
Sodium hydroxide solution
(b) Name the acid that is added to the sample before this solution.[1]
(c) Give the result of a positive test for sulfate ions.[1]
Show the answer and mark scheme
(a)Answer: Barium chloride solution
(b)Answer: dilute hydrochloric acid
(dilute) hydrochloric acid
(c)Answer: white precipitate
white precipitate
Question 2Medium6 marks
A student tests a solution for sulfate ions by adding dilute hydrochloric acid and then barium chloride solution.
(a) Why is dilute hydrochloric acid added first?[1]
(b) Explain why dilute sulfuric acid must not be used instead of hydrochloric acid.[2]
(c) The solution contained sodium sulfate. Name the white precipitate formed.[1]
(d) Complete the balanced equation for the reaction. BaCl2(aq) + Na2SO4(aq) → .................... + ....................[2]
Show the answer and mark scheme
(a)
to remove / react with carbonate ions (which would also form a white precipitate with barium ions)
(b)
sulfuric acid contains sulfate ions
so a white precipitate would form whatever the sample contained