A student reacts a strip of magnesium with dilute hydrochloric acid in a test tube and collects the gas given off. When they hold a lit wooden splint near the mouth of the tube, they hear a sharp squeaky pop. What does this result confirm about the gas, and why does the pop happen?
- The gas is hydrogen: the flame supplies enough energy for the hydrogen to react rapidly with oxygen in the air, and the sudden release of hot gas and water vapour produces the popping sound
- The gas is oxygen: the splint's flame burns more brightly in the gas, and the pop is simply the sound of the wood catching fire faster than normal
- The gas is carbon dioxide: the pop happens because the gas extinguishes the flame instantly, and the sound is the splint's ember cracking as it cools
- The gas is chlorine: the pop is caused by the bleaching reaction between the gas and the burning wood fibres of the splint
Why A? And why not the others?
Correct answer: A. The gas is hydrogen: the flame supplies enough energy for the hydrogen to react rapidly with oxygen in the air, and the sudden release of hot gas and water vapour produces the popping sound
A lit splint held near hydrogen gas provides the activation energy needed for the hydrogen to react rapidly with oxygen in the surrounding air, and the sudden release of hot gas and water vapour from that fast reaction produces the sharp squeaky pop; this reaction is the standard confirmatory test for hydrogen gas. The option describing oxygen is wrong because the standard test for oxygen is a glowing splint relighting in the gas, not a popping sound, since oxygen supports combustion rather than being the fuel that combusts. The option describing carbon dioxide is wrong because carbon dioxide is tested by turning limewater cloudy, and a splint held in carbon dioxide is extinguished quietly rather than producing any pop. The option describing chlorine is wrong because chlorine is identified by bleaching damp litmus paper white, and it does not react explosively with a flame the way hydrogen does.
Source: RSC Education / BBC Bitesize GCSE Chemistry: Tests for gases