Chhetri AcademyGCSE & A level Paper Builder

4.8.3.3Carbonates

AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions

Practise Carbonates. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

Build a paper on this topic

Or take a quick quiz: 6 questions on this subtopic, marked as you tap. Open the Quiz tab and pick GCSE Chemistry Foundation (AQA).

▶ Find videos on Carbonates (YouTube search · Free Science Lessons) · Practise all of Identification of ions

Free downloads:Worksheet with answers (PDF)Revision notes (PDF)Open in the Notes section

Revision notes

Carbonate ions are identified by adding dilute acid: the mixture fizzes and the gas turns limewater milky. This ion test is in GCSE Chemistry only (not Combined Science). Questions ask for the method, the result and equations.

Key facts

  • Test: add dilute acid; bubble the gas through limewater
  • Result: fizzing, and limewater turns milky → carbonate ions (CO32−) present
  • acid + metal carbonate → salt + water + carbon dioxide
  • CaCO3 + 2HCl → CaCl2 + H2O + CO2
  • Acid is added before the halide and sulfate tests to remove carbonate ions

Notes

The test

diagram
  • Add dilute acid, e.g. dilute hydrochloric acid, to the solid or solution.
  • If carbonate ions (CO32−) are present, the mixture fizzes as carbon dioxide is given off.
  • Bubble the gas through limewater using a delivery tube. The limewater turns milky (cloudy), so the gas is carbon dioxide and a carbonate is present.
  • delivery tubebungsolid + diluteacid: fizzeslimewaterturns milkyThe gas bubbles through the limewater
    Fizzing plus milky limewater shows a carbonate.
  • Fizzing alone is not enough: a metal reacting with acid also fizzes (giving hydrogen). The limewater result proves the gas is carbon dioxide.

Equations

  • acid + metal carbonate → salt + water + carbon dioxide
  • Na2CO3 + 2HCl → 2NaCl + H2O + CO2
  • CuCO3 + H2SO4 → CuSO4 + H2O + CO2

Why this matters for the halide and sulfate tests

  • Carbonate ions also form precipitates with silver nitrate solution and with barium chloride solution, so they could give a false positive in the halide and sulfate tests.
  • That is why dilute acid is added first in those tests: it reacts with any carbonate ions and removes them as carbon dioxide.

How to answer each type of question

Describe the test for carbonate ions and give the result

2 to 3 marksGrade 3
  1. Add dilute acid to the sample.
  2. Bubble the gas through limewater.
  3. Result: fizzing, and the limewater turns milky.

Example. Describe a test to show that a white powder contains carbonate ions. Give the result.

Show the model answerHide the model answer
Add dilute hydrochloric acid to the powder (1). Bubble the gas given off through limewater (1). The mixture fizzes and the limewater turns milky / cloudy (1).

Don’t lose marks

  • Stopping at 'it fizzes'. You need the limewater test to prove the gas is carbon dioxide.
  • Writing 'the limewater turns white'. Write milky or cloudy.
  • Leaving water out of the products of acid + carbonate.
  • Writing the carbonate ion without its charge. It is CO32−.

More tips

Memory tricks

  • Carbonate test = 'fizz, then milk': bubbles, then milky limewater.
  • Salt names: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
  • Every acid + carbonate equation ends ‘+ H2O + CO2’.

Exam technique

  • For full marks, give the observation (fizzing) AND the limewater result.
  • Say 'dilute' acid, e.g. dilute hydrochloric acid.
  • To identify a whole compound, add a flame test or sodium hydroxide test for the metal ion.

What each grade needs

What you need to be able to do, from the first marks up to the top grade.

  1. Grade 3
    Describe the test for carbonate ionsAdd dilute acid and bubble any gas given off through limewater.
  2. Grade 3
    State the positive result for carbonatesFizzing, and the gas turns limewater milky (cloudy).
  3. Grade 5
    Name the products of acid and carbonateA salt, water and carbon dioxide.
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 gas produced when a carbonate reacts with an acid.
carbon dioxide
Balance the equation for the reaction.
Na2CO3(s) + ....HCl(aq) → ....NaCl(aq) + H2O(l) + CO2(g)
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
Calcium carbonate reacts with dilute hydrochloric acid. Name the salt formed in this reaction.
calcium chloride

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) Describe the test for carbonate ions.
Give the result of a positive test.[3]
(b) Name the gas produced when a carbonate reacts with an acid.[1]
Show the answer and mark scheme
(a)
  • add dilute acid (e.g. dilute hydrochloric acid)
  • bubbles / fizzing
  • the gas turns limewater milky / cloudy
(b) Answer: carbon dioxide
  • carbon dioxide / CO2
Question 2Medium4 marks
A student added dilute hydrochloric acid to a white solid, sodium carbonate.
(a) Balance the equation for the reaction.
Na2CO3(s) + ....HCl(aq) → ....NaCl(aq) + H2O(l) + CO2(g)[1]
(b) Describe how the student could show that the gas produced is carbon dioxide.
Give the result.[2]
(c) The student added dilute hydrochloric acid to a different white solid. There was no fizzing.
What does this show about the solid?[1]
Show the answer and mark scheme
(a) Answer: Na2CO3 + 2HCl → 2NaCl + H2O + CO2
  • 2HCl and 2NaCl
(b)
  • bubble the gas through limewater
  • limewater turns milky / cloudy
(c)
  • the solid is not a carbonate / does not contain carbonate ions

Related subtopics

Stuck? Get 1-to-1 help. Chhetri Academy tutors GCSE and A level Maths and Science online, with a free 30-minute trial lesson.

Book a free trial