AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemical analysis › Identification of common gases
Practise Test for oxygen. 11 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 oxygen: a glowing splint put into the gas relights. Usually a 2-mark 'describe a test and give the result' question, sometimes linked to reactions that make oxygen or to telling oxygen and hydrogen apart.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Describe the test for oxygenPut a glowing splint into a test tube of the gas.
3
State the positive result for oxygenThe glowing splint relights (bursts back into flame).
4
Tell the oxygen and hydrogen tests apartOxygen: glowing splint relights. Hydrogen: burning splint gives a pop.
5
Name reactions that produce oxygenDecomposition of hydrogen peroxide, electrolysis (at the positive electrode) and photosynthesis.
6
Explain why the splint relightsThe gas has a much higher concentration of oxygen than air (about 20%), so the splint burns fast enough to relight.
Notes
The test
Collect the gas in a test tube.
Light a splint, then blow it out so that it is just glowing.
Put the glowing splint into the test tube of gas.
If the gas is oxygen, the splint relights (bursts back into flame).
Why it works: burning needs oxygen. Air is only about 20% oxygen, so a glowing splint just smoulders in air. In a tube of oxygen the concentration is much higher, so it burns faster and bursts into flame. grade 6+
Where oxygen comes from
Hydrogen peroxide decomposes to water and oxygen. A catalyst such as manganese(IV) oxide speeds this up: 2H2O2 → 2H2O + O2.
In the electrolysis of solutions such as dilute sulfuric acid or sodium sulfate solution, oxygen forms at the positive electrode (anode).
Plants and algae give out oxygen in photosynthesis.
A burning splint just burns more brightly in oxygen, which is not the test. Always use a glowing splint for oxygen.
The result must be 'relights'. A splint that goes out shows the gas is not oxygen.
Cheatsheet
Test: glowing splint put into a test tube of the gas
Result: the splint relights
Air is only about 20% oxygen, so a glowing splint does not relight in air
2H2O2 → 2H2O + O2 (a catalyst speeds it up)
Electrolysis of sulfate solutions: oxygen at the positive electrode (anode)
Glowing splint = oxygen test; burning splint = hydrogen test
How to answer each type of question
Describe the test for oxygen and give the result
2 marks3
Test (1 mark): put a glowing splint into the gas.
Result (1 mark): the splint relights.
Example. When a catalyst is added to hydrogen peroxide solution, a colourless gas is given off. Describe a test to show that the gas is oxygen. Give the result of the test.
Show the model answer
Put a glowing splint into a test tube of the gas (1). The splint relights (1).
Tell two gases apart using gas tests
2 to 4 marks4
Choose the right test for each gas: glowing splint for oxygen, burning splint for hydrogen.
Give the test and the result for each gas.
Say which result identifies which gas.
Example. A student has two test tubes of colourless gas. One contains hydrogen and the other contains oxygen. Describe how the student could find out which gas is in each tube. Give the results.
Show the model answer
Put a glowing splint into each tube (1). The gas that relights the splint is oxygen (1). Hold a burning splint at the open end of the other tube (1). The gas that burns with a squeaky pop is hydrogen (1).
Explain why the splint relights
2 marks6
Say that burning needs oxygen, and air is only about 20% oxygen.
Say that the tube has a much higher concentration of oxygen, so the splint burns faster and bursts back into flame.
Example. A glowing splint relights in a test tube of oxygen but not in air. Explain why.
Show the model answer
Air contains only about 20% oxygen, but the tube contains (nearly) pure oxygen / a much higher concentration of oxygen (1). So the splint burns faster and bursts back into flame (1).
Shortcuts and memory tricks
Oxygen helps things burn, so it brings a dying (glowing) splint back to life.
Hydrogen burns, so it gets the burning splint; oxygen relights, so it gets the glowing splint.
'Glowing grows': in oxygen, a glowing splint grows back into a flame.
Where marks are lost
Using a burning splint for oxygen. It only burns more brightly, which is not the test.
Writing 'the splint glows brighter' or 'lights up'. The result is 'relights'.
Mixing up 'relights' (oxygen) and 'pop' (hydrogen).
Holding the splint at the mouth of the tube for oxygen. Put the glowing splint into the gas.
Exam technique
Use the spec words: 'glowing splint' and 'relights'.
When a question has two gases, give a test and a result for each one.
In electrolysis questions, link each gas to its electrode, e.g. for sodium sulfate solution: hydrogen at the negative electrode, oxygen at the positive electrode.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Pondweed was placed in a beaker of water in bright light. Bubbles of gas were given off. The gas was collected in an upside-down test tube filled with water. Name the process in the pondweed that produced the oxygen.
photosynthesis
Complete the sentences using words from the list. burning glowing goes out pops relights The test for oxygen uses a .................... splint. If the gas is oxygen, the splint ....................
glowing; relights
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
(a) Which test result shows that a gas is oxygen? Tick (✓) one box.[1]
A burning splint gives a squeaky pop
A glowing splint relights
Limewater turns milky
Damp litmus paper is bleached
(b) Air contains about 20% oxygen, but a test tube of air does not give a positive result in the test for oxygen. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: A glowing splint relights
(b)
the concentration / percentage of oxygen in air is too low (to relight a glowing splint)
Question 2Medium4 marks
A student added manganese(IV) oxide to hydrogen peroxide solution. The hydrogen peroxide decomposed. 2H2O2(aq) → 2H2O(l) + O2(g) The student collected the gas in test tubes over water.
(a) Describe the test for the gas produced and give the result.[2]
(b) The first test tube of gas collected gave a negative result in this test. The second test tube gave a positive result. Suggest why the first test tube of gas gave a negative result.[1]
(c) The second test tube of gas was left open for 10 minutes and then tested again. This time the result was negative. Suggest why.[1]
Show the answer and mark scheme
(a)
put a glowing splint into the test tube (of gas)
the splint relights
(b)
the first tube contained (mainly) air that was pushed out of the flask / delivery tube
(c)
the oxygen escaped / mixed with air (so its concentration was too low)
Question 3Hard5 marks
A student electrolyses dilute sulfuric acid using inert electrodes, and collects the gas produced at the positive electrode (anode) in a test tube.
(a) Predict the gas produced at the positive electrode. Explain your prediction.[2]
(b) Describe how the student could test this gas to confirm it is oxygen, and state the result.[2]
(c) The volume of gas collected at the positive electrode is about half the volume collected at the negative electrode in the same time. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: oxygen
oxygen
hydroxide ions (from the water) are oxidised / lose electrons at the positive electrode, forming oxygen gas
(b)
insert a glowing splint into the tube of gas
the splint relights
(c)
hydrogen and oxygen are formed in a 2 : 1 mole ratio (from water), and equal amounts in moles of any gas occupy equal volumes, so half as much oxygen as hydrogen is produced by volume