Gas Tests Virtual Lab: Identifying Six Common Gases

This gas tests virtual lab covers the chemical tests for gases in GCSE and IGCSE Chemistry. Students run six tests on six gases: a lighted splint, a glowing splint, limewater, damp red and blue litmus paper and acidified potassium manganate(VII). They record each result in a table, find the one test that identifies each gas, then switch to unknown tubes and name them. There is no chlorine, sulfur dioxide or ammonia in the room, and no hydrogen near a flame. One lesson gives a complete results table and practice at writing the observations that exam questions ask for.

Curriculum links

• AQA GCSE Chemistry 4.8.2 (identification of common gases): hydrogen, oxygen, carbon dioxide and chlorine. In Combined Science: Trilogy the same content is 5.8.2.
• Cambridge IGCSE Chemistry 0620, topic 12.5 (identification of ions and gases): adds ammonia with damp red litmus and sulfur dioxide with acidified potassium manganate(VII).
• Other courses: the same tests appear in middle-school and upper-secondary chemistry in many countries.

Simulic is not affiliated with or endorsed by AQA, Cambridge or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"You bubble carbon dioxide through limewater, then sulfur dioxide through fresh limewater. What happens each time?"

Most students learned "limewater test = carbon dioxide" and expect only CO₂ to turn it milky. Don't correct them yet.

Method in the simulation

1. Set Bench to "Gas tests" and Mode to "Known gases (practice)". Keep Damp litmus paper ticked.
2. Choose a Sample, for example H₂ – hydrogen, and a Test, for example Lighted splint. Press the step buttons in order and watch the tube.
3. Choose what you saw in the buttons below the steps; it goes into the results table. Press Repeat test to start a fresh tube.
4. Run all six tests on all six gases.
5. Untick Damp litmus paper and test ammonia with damp red litmus again. Compare.
6. Switch Mode to "Unknown gases". Test each unknown, choose the gas for each one and press Check my answers. New unknowns gives a fresh set.

Gas	Lighted splint	Glowing splint	Limewater	Damp red litmus	Damp blue litmus	Acidified KMnO₄
H₂
O₂
CO₂
Cl₂
NH₃
SO₂

Expected results

These observations come from the simulation in Known gases mode. A dash means no change.

Gas	Lighted splint	Glowing splint	Limewater	Damp red litmus	Damp blue litmus	Acidified KMnO₄
H₂	squeaky pop	goes out	–	–	–	–
O₂	burns more brightly	relights	–	–	–	–
CO₂	goes out	goes out	milky	–	red	–
Cl₂	dull, smoky flame	goes out	–	bleached white	red, then bleached	–
NH₃	goes out	goes out	–	turns blue	–	–
SO₂	goes out	goes out	milky	–	red	purple to colorless

• Each gas has one result that no other gas gives (in bold). Limewater alone cannot tell CO₂ from SO₂.
• With Damp litmus paper unticked, ammonia and chlorine leave the paper unchanged, and the sim warns that the gas must first dissolve in the water on damp paper.

Questions for students

1. (Prediction, asked again after the lab) What happens when CO₂ and SO₂ are bubbled through limewater?
2. What does the test with dry red litmus paper show?
3. What do you see when a lighted splint is held at the mouth of a tube of oxygen?
4. A gas puts out a lighted splint, does not relight a glowing splint, leaves limewater clear and turns damp red litmus blue. Which gas is it?
5. How can you tell carbon dioxide from sulfur dioxide, and why does the test work?

Answers for teachers: (1) Both turn the limewater milky. (2) The litmus does not change, so the gas must dissolve in water on the paper to act. Dry paper is a comparison: dampness is the only difference. (3) No pop; the splint burns more brightly. (4) Ammonia. (5) Bubble each gas through acidified potassium manganate(VII). SO₂ turns it from purple to colorless because it is a reducing agent; CO₂ leaves it purple.

Common misconceptions

• "Oxygen gives a squeaky pop." The pop is hydrogen burning. Oxygen makes a lighted splint burn more brightly and relights a glowing one.
• "Milky limewater proves carbon dioxide." Sulfur dioxide does it too. In a mixture of possible gases, test with acidified KMnO₄ as well.
• "Chlorine turns litmus red, so it is just an acid gas." Chlorine bleaches the paper white. Damp blue litmus turns red first, then loses its color.

Extension

• Set Bench to "Ion tests + NH₄⁺, OH⁻, Group 2" and run the ammonium test: warm the salt with NaOH and hold damp red litmus at the mouth of the tube.
• Ask students to write a word or symbol equation for each positive test; Known gases mode shows the equations to check against.

FAQ

Can the class link open straight on the gas bench?

Yes. On the Share page, pin Bench to "Gas tests" and Mode to "Known salts (practice)" in the link's starting values; on the gas bench that mode shows as Known gases. Otherwise the first step of the method tells students to switch.

Is this the same simulation as the ion tests lab?

Yes. The identifying ions virtual lab uses the Ion tests bench for flame and precipitate tests. This lab uses the Gas tests bench, so the two can run in back-to-back lessons.

Why does the question set use Known gases mode?

In Unknown gases mode each student gets random tubes, so fixed answers are impossible. Use Unknown gases as the closing challenge, where Check my answers gives instant feedback.

Related simulations and guides

2H₂ + O₂ → 2H₂O – sparking a hydrogen explosion Ammonia fountain experiment – compared with HCl and N₂

For more chemistry activities, see interactive chemistry lesson ideas.