Section 1 of 8The test
1
Moisten a piece of blue litmus paper with a little distilled water.
2
Hold the damp paper in the gas, or at the mouth of the vessel, using tweezers.
3
Observe the paper: chlorine first turns it red, because it forms an acidic solution, and then bleaches it white.
4
Dry litmus paper does not change, so the paper must be damp.
Section 2 of 8Why the paper is bleached
1Chlorine dissolves
- Chlorine dissolves The gas dissolves in the water on the paper.
2Acidic solution
- Acidic solution Chlorine reacts with water to give acids, so blue litmus turns red briefly.
3Bleaching action
- Bleaching action One of the products, chloric(I) acid, removes colour from the dye.
4White paper
- White paper The litmus is permanently decolourised.
Section 3 of 8Properties of chlorine
| Property | Detail |
|---|---|
| Appearance | Pale green gas |
| Smell | Sharp, choking and unpleasant |
| Solubility | Dissolves in water, giving an acidic and bleaching solution |
| Type of element | Halogen (group 7) non-metal |
| Molecule | Diatomic, Cl2, denser than air so it sinks |
Section 4 of 8Hazards and precautions
Handling chlorine
Toxic
- Toxic Breathing chlorine damages the lungs and eyes, and large doses can be fatal.
Fume cupboard
- Fume cupboard Prepare and test chlorine only in a fume cupboard.
Small quantities
- Small quantities Use as little as possible, and keep asthmatic students away from it.
Disposal
- Disposal Excess gas may be absorbed with sodium hydroxide solution.
Section 5 of 8Where chlorine appears
Electrolysis
| Type | Description |
|---|---|
| Electrolysis | Chlorine gas forms at the positive electrode during the electrolysis of sodium chloride solution. |
| Chloride ions lose electrons there | 2Cl− → Cl2 + 2e−. |
Uses
- Uses It is used to kill bacteria in drinking water and swimming pools and to make bleach and plastics such as PVC.
Section 6 of 8Practical: testing the gas from electrolysis
1
Set up the electrolysis of sodium chloride solution in a fume cupboard with inert electrodes.
2
Switch on the power supply and watch for bubbles at each electrode.
3
Collect the gas from the positive electrode in a test tube or syringe.
4
Hold damp blue litmus paper in the gas using tweezers.
5
Record that the paper turns red and is then bleached white, and conclude that the gas is chlorine.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Chlorine | A toxic, pale green halogen gas, Cl2. |
| Litmus paper | Paper that changes colour with acids and alkalis. |
| Bleaching | Removing the colour from a substance. |
| Halogen | An element in group 7 of the periodic table. |
| Fume cupboard | An enclosed workspace that removes harmful gases. |
Section 8 of 8Exam technique
1
Using dry litmus paper, which stays unchanged.
2
Saying chlorine only turns the paper red, when the key observation is bleaching.
3
Describing chlorine as a colourless gas.
4
Testing for chlorine on an open bench instead of in a fume cupboard.