Identification of common gases

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Introduction to Gas Tests

Each common gas has a specific test with a positive result that identifies it.
You need to know the tests for hydrogen, oxygen, carbon dioxide, and chlorine.
The tests rely on characteristic properties: burning, relighting, cloudiness, or bleaching.
Always use the correct splint or reagent as described for each gas.

Testing for hydrogen gas

Test for Hydrogen

Use a burning splint held at the open end of a test tube containing the gas.
If hydrogen is present, a squeaky pop sound is heard.
The pop is caused by the rapid combustion of hydrogen with oxygen to produce water.
Do not insert the splint into the tube; hold it at the mouth so air can mix with the gas.

Testing for hydrogen gas

Test for Oxygen

Use a glowing splint and place it inside a test tube of the gas.
If oxygen is present, the splint relights.
Sometimes the splint may not relight but glows very brightly; this is also a positive result.
In exams, state that the glowing splint relights.

Testing for oxygen gas

Test for Carbon Dioxide

Bubble the gas through limewater (an aqueous solution of calcium hydroxide).
If carbon dioxide is present, the limewater turns milky or cloudy.
Do not say the limewater turns white; use 'milky' or 'cloudy'.
Extinguishing a burning splint is not a definitive test for carbon dioxide because other gases can also do this.

Testing for carbon dioxide gas

Test for Chlorine

Use damp blue litmus paper.
If chlorine is present, the litmus paper turns red then is bleached white.
The red colour is due to acids produced when chlorine reacts with water.
Chlorine has a sharp, choking smell, but smell is not a valid test.
Always handle chlorine in a fume cupboard due to its toxicity.

Testing for chlorine gas

Distinguishing Between Tests

Remember: ligHted splint for Hydrogen, glOwing splint for Oxygen.
Limewater is only for carbon dioxide; it turns milky.
Chlorine bleaches damp litmus paper; it does not just turn it red.
The litmus paper must be damp, not dry, for the chlorine test.

Common Mistakes to Avoid

Confusing the tests for hydrogen and oxygen.
Saying limewater turns cloudy for any gas; it is specific to carbon dioxide.
Forgetting that chlorine bleaches the litmus paper white after turning it red.
Using dry litmus paper instead of damp for the chlorine test.
Describing limewater as turning white instead of milky or cloudy.

Exam Tips

Learn the exact observations: squeaky pop, relights, milky, bleached white.
When describing a test, include both the method and the positive result.
For chlorine, mention that damp litmus paper is bleached.
For carbon dioxide, state that limewater turns milky or cloudy.
For hydrogen, specify a burning splint and a squeaky pop.
For oxygen, specify a glowing splint and that it relights.

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Câu hỏi luyện tập

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1.Which gas relights a glowing splint?
Easy
- AOxygen
- BHydrogen
- CCarbon dioxide
- DChlorine
2.Which test is used to identify carbon dioxide gas?
Easy
- ABubble the gas through limewater and see if it turns milky.
- BHold a burning splint at the mouth of the test tube and listen for a pop.
- CInsert a glowing splint into the gas and see if it relights.
- DHold damp blue litmus paper in the gas and see if it is bleached.
3.A burning splint held at the open end of a test tube of hydrogen gas produces a loud 'squeaky pop'.
Easy
True or false?
4.Damp blue litmus paper turns red and then is bleached white in chlorine gas.
Easy
True or false?
5.Which observation is the positive result for the test for chlorine gas?
Medium
- ADamp blue litmus paper is bleached white.
- BA glowing splint relights.
- CA burning splint gives a squeaky pop.
- DLimewater turns cloudy.
6.Which statement is a property of carbon dioxide rather than a definitive test for it?
Medium
- AIt extinguishes a burning splint.
- BIt turns limewater milky.
- CIt bleaches damp litmus paper.
- DIt relights a glowing splint.
7.Which of the following are correct tests and positive results for gases? (select all that apply)
Medium
- AHydrogen: burning splint gives a squeaky pop.
- BOxygen: glowing splint relights.
- CCarbon dioxide: limewater turns milky.
- DChlorine: burning splint gives a squeaky pop.
- EOxygen: burning splint gives a squeaky pop.
8.Place the steps for testing an unknown gas with damp litmus paper in the correct order.
Medium
- Hold the damp litmus paper in the gas.
- Observe any colour change.
- State whether the litmus is bleached white.

Thêm câu hỏi

9.Match each gas to its correct test.
Medium
- Hydrogen
- Oxygen
- Carbon dioxide
- Chlorine
- Burning splint gives a squeaky pop
- Glowing splint relights
- Limewater turns milky
- Damp litmus paper is bleached white
10.Which of the following statements about gas tests and gas properties are correct? (select all that apply)
Medium
- AA glowing splint relights in oxygen.
- BA burning splint gives a squeaky pop in hydrogen.
- CCarbon dioxide turns limewater milky.
- DChlorine bleaches damp blue litmus paper white.
- EA burning splint being extinguished is a definitive test for carbon dioxide.
11.Match each gas to the correct positive test result.
Medium
- Hydrogen
- Oxygen
- Carbon dioxide
- Chlorine
- Relights a glowing splint
- Turns limewater milky
- Squeaky pop with a burning splint
- Bleaches damp blue litmus paper white
12.Place the steps for testing an unknown gas with damp blue litmus paper in the correct order.
Medium
- Hold the damp blue litmus paper in the gas.
- Observe the initial colour change of the litmus paper.
- Continue to observe whether the colour is bleached.
- Record the final colour of the litmus paper.
13.A student has four unlabelled gas jars. Which of the following procedures could correctly identify the gases? (select all that apply)
Hard
- AAdd limewater to each jar and look for a milky precipitate.
- BInsert a glowing splint into each jar and look for relighting.
- CHold a burning splint at the mouth of each jar and listen for a squeaky pop.
- DPlace damp blue litmus paper in each jar and look for bleaching.
- ESmell each gas and identify it by its characteristic odour.
14.Why must the litmus paper be damp when testing for chlorine gas?
Medium
- AChlorine only reacts with water to produce bleaching agents.
- BDry litmus paper does not change colour in any gas.
- CChlorine dissolves in water to form an acid, which turns litmus red before bleaching.
- DDamp litmus paper is more sensitive to all gases.
15.A glowing splint inserted into a test tube of oxygen gas will be extinguished.
Easy
True or false?
16.Which observation is the positive result for the test for carbon dioxide gas?
Medium
- ALimewater turns milky or cloudy.
- BA burning splint goes out.
- CDamp blue litmus paper turns red.
- DA glowing splint relights.
17.A student tests an unknown gas with damp blue litmus paper. The paper turns red and then is bleached white. What is the gas?
Medium
- AChlorine
- BHydrogen
- COxygen
- DCarbon dioxide
18.A student adds dilute hydrochloric acid to calcium carbonate in a test tube and collects the gas produced. Which test would identify the gas?
Medium
- AHold a burning splint at the mouth of the tube and listen for a squeaky pop.
- BInsert a glowing splint into the tube and see if it relights.
- CBubble the gas through limewater and see if it turns milky.
- DHold damp blue litmus paper in the gas and see if it is bleached white.
19.Hydrogen peroxide decomposes slowly to form water and oxygen. A glowing splint is placed in the gas given off. What is observed?
Easy
- AA squeaky pop is heard.
- BThe splint relights.
- CThe limewater turns cloudy.
- DThe splint is extinguished immediately.
20.During the electrolysis of sodium chloride solution, a gas is produced at the positive electrode. Which test would confirm the gas is chlorine?
Medium
- ACollect the gas and see if it extinguishes a glowing splint.
- BCollect the gas and check if it turns limewater milky.
- CCollect the gas and see if it relights a glowing splint.
- DCollect the gas and test with damp blue litmus paper.
21.A student bubbles an unknown gas through limewater and the limewater turns milky. Which gas is present?
Medium
- AHydrogen
- BOxygen
- CCarbon dioxide
- DChlorine
22.A student tests an unknown gas with damp blue litmus paper. The paper turns red and then is bleached white. Which gas is present?
Hard
- AHydrogen
- BOxygen
- CCarbon dioxide
- DChlorine
23.Which observation is the positive result when testing for hydrogen gas?
Medium
- AA glowing splint relights.
- BLimewater turns milky.
- CDamp litmus paper is bleached white.
- DA burning splint produces a squeaky pop.
24.A student wants to test a gas to see if it is oxygen. Which method should they use?
Medium
- AInsert a glowing splint into the gas and see if it relights.
- BHold a burning splint at the mouth of the tube and listen for a pop.
- CBubble the gas through limewater and look for cloudiness.
- DHold damp blue litmus paper in the gas and look for bleaching.
25.A student collects a colourless gas. When a burning splint is held at the mouth of the tube, a squeaky pop is heard. What is the gas?
Medium
- AHydrogen
- BOxygen
- CCarbon dioxide
- DChlorine
26.A student has a test tube of gas. They insert a glowing splint and it relights. Which gas is present?
Hard
- AHydrogen
- BOxygen
- CCarbon dioxide
- DChlorine
27.A student reacts an unknown metal with dilute hydrochloric acid and collects the gas produced. The gas gives a squeaky pop with a burning splint. Which metal could have been used?
Hard
- ACopper
- BMagnesium
- CGold
- DSilver
28.A student bubbles an unknown gas through limewater and it turns milky. Which gas is present?
Hard
- AHydrogen
- BOxygen
- CCarbon dioxide
- DChlorine
29.Which of the following is the correct test for chlorine gas?
Medium
- AHold a burning splint at the mouth of the tube.
- BInsert a glowing splint into the tube.
- CBubble the gas through limewater.
- DHold damp blue litmus paper in the gas.
30.A student places a glowing splint into a test tube of gas. The splint does not relight, but it glows much more brightly than usual. What is the correct conclusion?
Medium
- AThe gas is oxygen, because a brightly glowing splint is also a positive result.
- BThe gas is hydrogen, because the splint did not go out.
- CThe gas is carbon dioxide, because the splint did not relight.
- DNo conclusion can be drawn because the splint did not relight.
31.A colourless gas is bubbled through limewater and the limewater turns milky. A burning splint is then held at the mouth of the gas tube and the flame is extinguished. Which statement best explains these observations?
Medium
- AThe gas is carbon dioxide; turning limewater milky is the definitive test, while extinguishing a burning splint is only a property.
- BThe gas is carbon dioxide; both observations are equally definitive tests for carbon dioxide.
- CThe gas is nitrogen; it turns limewater milky and extinguishes a burning splint.
- DThe gas is oxygen; it turns limewater milky and extinguishes a burning splint.
32.A gas is tested with damp blue litmus paper. The paper turns red and then is bleached white. Which gas is present?
Medium
- AChlorine
- BHydrogen
- COxygen
- DCarbon dioxide
33.A student has an unknown colourless gas. They bubble it through limewater and the limewater remains clear. They then hold a burning splint at the mouth of the tube and hear a squeaky pop. Which conclusion is correct?
Hard
- AThe gas is hydrogen; the limewater result rules out carbon dioxide.
- BThe gas is carbon dioxide; the limewater result was wrong.
- CThe gas is oxygen; the squeaky pop is the test for oxygen.
- DThe gas is chlorine; the squeaky pop is the test for chlorine.
34.A student has four unlabelled test tubes containing hydrogen, oxygen, carbon dioxide and chlorine. Which of the following procedures could correctly identify the gases? (select all that apply)
Hard
- AHold a burning splint at the mouth of each tube; the one that gives a squeaky pop is hydrogen.
- BInsert a glowing splint into each tube; the one that relights the splint is oxygen.
- CBubble each gas through limewater; the one that turns the limewater milky is carbon dioxide.
- DPut damp blue litmus paper into each gas; the one that turns the paper red then bleaches it white is chlorine.
- ESmell each gas; the one that smells like swimming pools is chlorine.
35.When testing for hydrogen, the burning splint should be inserted right into the test tube of gas.
Easy
True or false?
36.A glowing splint relighting is the definitive test for oxygen gas.
Medium
True or false?
37.Match each gas to the correct positive result for its test.
Medium
- Hydrogen
- Oxygen
- Carbon dioxide
- Chlorine
- Squeaky pop with a burning splint
- Glowing splint relights
- Limewater turns milky
- Damp blue litmus paper turns red then bleaches white
38.Place the steps for testing an unknown gas with a burning splint in the correct order.
Medium
- Collect the gas in a test tube.
- Light a splint.
- Hold the burning splint at the open end of the test tube.
- Listen for a squeaky pop.
39.Which gas is tested using a burning splint held at the open end of a test tube?
Easy
- AHydrogen
- BOxygen
- CCarbon dioxide
- DChlorine
40.What is the positive result for the test for oxygen gas?
Easy
- AA glowing splint relights
- BA burning splint gives a squeaky pop
- CLimewater turns milky
- DDamp litmus paper is bleached white
41.Complete the sentence about the test for carbon dioxide.
Easy
Carbon dioxide turns limewater ____.
42.Complete the sentence about the test for hydrogen.
Easy
Hydrogen gas burns with a loud ____ sound.
43.Name the solution used to test for carbon dioxide gas.
Easy
44.Complete the sentence about the test for hydrogen gas.
Medium
The test for hydrogen uses a ____ splint held at the open end of a test tube.
45.Select the correct words to complete the sentences about the test for hydrogen.
Easy
Hydrogen is tested with a ____ splint held at the ____ of the test tube.
46.Select the correct words to complete the sentences about the test for oxygen.
Easy
Oxygen is tested with a ____ splint inserted into the test tube. If oxygen is present, the splint will ____.
47.Select the correct words to complete the sentences about the test for carbon dioxide.
Easy
Carbon dioxide is bubbled through an aqueous solution of calcium hydroxide, which is also called ____. If carbon dioxide is present, the solution turns ____.
48.Select the correct words to complete the sentences about the test for chlorine.
Easy
Chlorine is tested with ____ litmus paper. If chlorine is present, the paper turns ____ and then is bleached white.
49.What is the positive result when damp blue litmus paper is exposed to chlorine gas?
Easy
50.Name the gas that relights a glowing splint.
Easy
51.Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3. Relative atomic masses: Ca = 40; C = 12; O = 16.
Medium
52.Calculate the percentage by mass of calcium in calcium carbonate, CaCO3. Use Ar(Ca) = 40 and Mr(CaCO3) = 100. Give your answer to the nearest whole number.
Medium
53.Describe a test to show that carbon dioxide is produced when calcium carbonate reacts with hydrochloric acid.
Hard
54.Describe a test that would identify chlorine gas. Give the result of the test.
Medium
55.Explain why the litmus paper must be damp when testing for chlorine gas.
Medium
56.A student has a mixture of two colourless gases, hydrogen and carbon dioxide. Describe a sequence of tests that would positively identify both gases in the mixture. Give the expected result for each test.
Hard
57.Explain why damp litmus paper is used rather than dry litmus paper when testing for chlorine gas.
Hard
58.Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3. Relative atomic masses: Ca = 40; C = 12; O = 16.
Medium
- A100
- B90
- C120
- D110
59.Calculate the percentage by mass of calcium in calcium carbonate, CaCO3. Use Ar(Ca) = 40 and Mr(CaCO3) = 100. Give your answer to the nearest whole number.
Medium
- A36 %
- B48 %
- C44 %
- D40 %

Thẻ ghi nhớ

What is the test for hydrogen gas?

Hold a burning (lit) splint at the open end of a test tube of the gas. Keep it at the mouth of the tube, not inside, because the gas needs air to burn.

What is the positive result for the test for hydrogen?

A loud 'squeaky pop' is heard. This comes from the rapid combustion of hydrogen with oxygen to produce water.

Why must the burning splint be held at the mouth of the test tube, not inserted into it, when testing for hydrogen?

The gas needs air to burn, so the splint is held just at the open end of the tube.

What is the test for oxygen gas?

Place a glowing splint inside a test tube of the gas. If oxygen is present, the splint relights.

What is the positive result for the test for oxygen?

The glowing splint relights. It may also glow very brightly, but 'relights' is the answer to give in an exam.

What is the test for carbon dioxide gas?

Bubble the gas through limewater, which is an aqueous solution of calcium hydroxide.

What is the positive result for the test for carbon dioxide?

The limewater turns milky or cloudy.

What is limewater?

An aqueous solution of calcium hydroxide, used to test for carbon dioxide.

What is the test for chlorine gas?

Put damp blue litmus paper into the gas. If chlorine is present, the paper turns red and is then bleached white.

What is the positive result for the test for chlorine?

Damp blue litmus paper turns red and then is bleached white.

Why does damp litmus paper turn red first in the test for chlorine?

Acids are produced when chlorine comes into contact with water, so the litmus turns red before being bleached.

Why must the litmus paper be damp for the chlorine test?

The chlorine must come into contact with water to produce the acids that turn the litmus red before it is bleached.

Why is 'extinguishing a burning splint' not an acceptable test for carbon dioxide?

Other gases, such as nitrogen, also extinguish a burning splint, so it is not a definitive test and should not be quoted in an exam answer.

Why is the smell of chlorine not an acceptable test for chlorine gas?

A sharp, choking smell is a characteristic property of chlorine, not a test. Chlorine should be handled in a fume cupboard because it is toxic.

How can you remember which splint is used for hydrogen and which for oxygen?

A ligHted splint has an H for Hydrogen; a glOwing splint has an O for Oxygen.

A gas relights a glowing splint. What is the gas?

Oxygen.

A gas gives a squeaky pop with a burning splint. What is the gas?

Hydrogen.

A gas turns limewater milky. What is the gas?

Carbon dioxide.

A gas bleaches damp blue litmus paper white. What is the gas?

Chlorine.

What is the difference between a property of a gas and a test for a gas?

A property is something you observe about the gas, such as a smell or extinguishing a flame. A test is a specific procedure with a definitive positive result that identifies the gas.

What safety precaution should be taken when testing for chlorine gas?

Carry out the test in a fume cupboard, because chlorine is toxic.

Which gas turns damp litmus paper red and then bleaches it white?

Chlorine.

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