Tests for Hydrogen

Identifying Hydrogen Gas

Hydrogen is a colourless and odourless gas, making it difficult to identify by sight or smell alone. It is the lightest element in the periodic table, which means it is much lighter than air. Hydrogen gas is commonly produced in the laboratory by reacting certain metals, such as zinc or magnesium, with acids like hydrochloric acid. This reaction releases hydrogen gas, which can then be collected for testing.

Because hydrogen is so light, it rises quickly in air. This property is useful when collecting the gas by upward delivery or displacement of water in a test tube. Upward delivery works because hydrogen is less dense than air, and displacement of water is effective since hydrogen is only slightly soluble in water.

Test for Hydrogen

The standard test to identify hydrogen gas involves using a lit splint. The procedure is:

• Collect the gas in a test tube by displacement of water or upward delivery.
• Bring a lit splint close to the mouth of the test tube.
• If hydrogen is present, it will react explosively with oxygen in the air, producing a characteristic 'pop' sound.

This 'pop' sound is a quick, sharp noise caused by the rapid combustion of hydrogen gas when it mixes with oxygen. The chemical reaction is: hydrogen reacts with oxygen to form water, releasing energy rapidly and causing the pop. It is a reliable and simple test used in GCSE practicals and exams to confirm the presence of hydrogen.

For instance, if you collect gas from the reaction of zinc with hydrochloric acid and bring a lit splint near the test tube, you should hear a pop if hydrogen is present.

Safety Precautions When Testing for Hydrogen

Because hydrogen is highly flammable and explosive when mixed with air, it is important to follow safety rules:

• Use a small amount of gas in the test tube to avoid large explosions.
• Keep the lit splint away from your face and body.
• Perform the test in a well-ventilated area or under a fume hood.

Additional Notes

Hydrogen’s properties such as being colourless and odourless mean that the 'pop' test is essential for identification. Other gases have different tests (e.g., oxygen relights a glowing splint, carbon dioxide turns limewater cloudy), so it is important to use the correct test for each gas.