AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical analysis › Identification of common gases
Practise Test for carbon dioxide. 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.
How to show that a gas is carbon dioxide: it turns limewater milky (cloudy). Usually a 2-mark test-and-result question, often linked to acids reacting with carbonates or to burning fuels.
Test: bubble the gas through (or shake it with) limewater
Result: limewater turns milky (cloudy)
acid + metal carbonate → salt + water + carbon dioxide
A splint going out is NOT a test for carbon dioxide
Notes
The test
Limewater is an aqueous solution of calcium hydroxide. It is colourless.
Bubble the gas through limewater (e.g. using a delivery tube), or shake the gas with limewater in a stoppered tube.
If the gas is carbon dioxide, the limewater turns milky (cloudy).
Where carbon dioxide comes from
diagram
Acids react with metal carbonates to form a salt, water and carbon dioxide, e.g. calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
Complete combustion of hydrocarbon fuels, such as methane, produces carbon dioxide and water.
To test the gas from a reaction, fit the reaction tube with a bung and delivery tube, and dip the end of the delivery tube into limewater in another tube.
Carbon dioxide turns limewater milky (cloudy).
Watch out
Carbon dioxide puts out a burning splint, but so do other gases such as nitrogen. This is not a test for carbon dioxide.
It is the limewater that turns milky, not the gas.
Fizzing only shows that a gas is given off. You need the limewater test to show that the gas is carbon dioxide.
How to answer each type of question
Describe the test for carbon dioxide and give the result
2 marksGrade 3
Test (1 mark): bubble the gas through limewater.
Result (1 mark): the limewater turns milky / cloudy.
Example. When dilute hydrochloric acid is added to marble chips, a gas is given off. Describe a test to show that the gas is carbon dioxide. Give the result of the test.
Show the model answerHide the model answer
Bubble the gas through limewater (1). The limewater turns milky / cloudy (1).
Describe how to collect and test the gas from a reaction
3 marksGrade 5
Put the reactants in a tube with a bung and a delivery tube.
Put the end of the delivery tube into limewater so the gas bubbles through it.
Give the result: the limewater turns milky.
Example. Copper carbonate reacts with dilute sulfuric acid. Describe how a student could show that carbon dioxide is produced in this reaction. Give the result.
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Put the copper carbonate and acid in a boiling tube fitted with a bung and delivery tube (1). Put the other end of the delivery tube into limewater in a second tube, so the gas bubbles through it (1). The limewater turns milky / cloudy (1).
Identify the gas and balance the equation
2 marksGrade 5
Acid + carbonate always gives carbon dioxide (plus a salt and water).
Name the salt from the metal and the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
Balance by counting each type of atom on both sides.
Example. Sodium carbonate reacts with dilute hydrochloric acid. The gas produced turns limewater milky. (a) Name the gas. (b) Balance the equation: Na2CO3 + ....HCl → ....NaCl + H2O + CO2
Writing 'the limewater turns white'. Use the words milky or cloudy.
Using a burning splint as the test. It goes out in carbon dioxide, but it goes out in other gases too.
Calling limewater 'calcium carbonate solution'. Limewater is calcium hydroxide solution; calcium carbonate is the precipitate that makes it cloudy.
Saying 'the gas turns milky'. It is the limewater that changes.
More tips
Memory tricks
Carbon dioxide → Cloudy: both start with a C sound.
Carbon dioxide makes limewater look like milk.
Acid + carbonate always fizzes: the fizz is carbon dioxide.
Exam technique
Two marks: 'bubble through limewater' (1) and 'turns milky / cloudy' (1).
If asked what limewater is, write 'calcium hydroxide solution'.
In practical questions, make sure the delivery tube goes from the reaction into the limewater.
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 carbon dioxideBubble the gas through limewater, or shake the gas with limewater.
Grade 3
State the positive result for carbon dioxideThe limewater turns milky (cloudy).
Grade 4
Name the chemical in limewaterLimewater is an aqueous solution of calcium hydroxide.
Grade 5
Describe how to test gas from a reactionUse a delivery tube to bubble the gas from the reaction through limewater.
Grade 5
Name reactions that produce carbon dioxideAcids with metal carbonates, and complete combustion of fuels that contain carbon.
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 solution used to test for carbon dioxide.
limewater
Complete the word equation. carbon + oxygen → ....................
carbon dioxide
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) Name the solution used to test for carbon dioxide.[1]
(b) Describe how the gas should be added to the solution.[1]
(c) Give the result if the gas is carbon dioxide.[1]
Show the answer and mark scheme
(a)Answer: limewater
limewater / calcium hydroxide solution
(b)
bubble the gas through the solution / shake the gas with the solution
(c)
(the solution) turns milky / cloudy
Question 2Medium3 marks
Limewater is calcium hydroxide solution. This equation shows the reaction of carbon dioxide with limewater. Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
(a) Use the equation to explain why limewater turns milky when carbon dioxide is bubbled through it.[2]
(b) A student bubbled exhaled breath through one tube of limewater and bubbled the same volume of air from the room through another tube of limewater. Suggest why the limewater turned milky more quickly with the exhaled breath.[1]
Show the answer and mark scheme
(a)
calcium carbonate is formed
calcium carbonate is insoluble / is a solid / forms a precipitate (shown by the state symbol (s))
(b)
exhaled breath contains a higher concentration / percentage of carbon dioxide (than air)