Chemical Tests for Gases
This topic covers the four essential chemical tests used to identify common laboratory gases. Mastering these simple, observation-based tests is crucial for questions involving reaction products.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Hydrogen (H2): A lit splint placed at the mouth of a test tube causes a 'squeaky pop' due to the explosive combustion of hydrogen with oxygen.
- Oxygen (O2): A glowing splint (one that has been lit and blown out) will relight when placed in a sample of the gas, as oxygen supports combustion.
- Carbon Dioxide (CO2): Bubbling the gas through limewater, an aqueous solution of calcium hydroxide (Ca(OH)2), will cause the solution to turn cloudy or milky due to the formation of a white precipitate, calcium carbonate (CaCO3).
- Chlorine (Cl2): Damp blue litmus paper placed in the gas will first turn red (as chlorine forms an acidic solution with water) and then be bleached to white.
Formulae
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) To understand the chemical basis for the carbon dioxide test. The solid calcium carbonate, CaCO3, is the white precipitate that makes the limewater cloudy.
2H2(g) + O2(g) → 2H2O(l) To understand the source of the 'squeaky pop' in the hydrogen test. This is the rapid, explosive reaction of hydrogen with oxygen from the air.
Definitions
- Limewater
- A common name for a saturated aqueous solution of calcium hydroxide, Ca(OH)2, used to test for carbon dioxide.
- Glowing splint
- A wooden splint that has been lit and then extinguished, leaving a red hot ember. It is used to test for oxygen.
- Bleaching
- The process of removing colour from a substance. In the chlorine test, the litmus paper is decolourised.
Worked example
An experiment produces a gas which is collected in a test tube. A student observes that the gas is pale green. They then introduce a piece of damp blue litmus paper into the test tube. What is the expected sequence of observations?
- 1
Step 1:
Identify the gas from its physical properties.
A pale green gas is characteristic of chlorine (Cl2).
- 2
Step 2:
Recall the chemical test for chlorine using damp litmus paper.
- 3
Step 3:
Remember that chlorine reacts with the water on the damp paper to form acidic substances.
This will turn the blue litmus paper red first.
- 4
Step 4:
Recall that chlorine is a powerful bleaching agent.
After turning red, the litmus paper will be decolourised and turn white.
- 5
Step 5:
Combine the observations into the correct sequence:
the paper turns red, and then it turns white.
Answer: The damp blue litmus paper will first turn red, and then it will be bleached white.
Common mistakes
- ×Confusing the tests for hydrogen and oxygen. Remember: a 'glowing' splint is for oxygen, while a 'burning' (lit) splint is for hydrogen.
- ×Forgetting the two-stage colour change for chlorine. It turns red first because it's acidic in water, then it bleaches white. A question might test this sequence.
- ×Overlooking the word 'damp' for the chlorine test. The test will not work with dry litmus paper as water is required for the reaction to occur.
- ×Mixing up the terms 'clear' and 'colourless'. Limewater is a clear, colourless solution; it becomes cloudy (translucent) and white, not clear, during a positive test for CO2.
No-calculator tips
- ✓This is a pure memory topic. Use mnemonics: 'A squeaky POP for Hydrogen', 'Oxygen makes things GLOW again'.
- ✓Visualise the experiments. Mentally picture a splint relighting or a clear solution turning milky. This can aid recall more effectively than just words.
- ✓Pay close attention to the descriptive words in a question, such as 'glowing', 'lit', 'damp', or 'cloudy', as they are direct clues to the identity of the gas.