In an experiment both hydrogen gas and carbon dioxide gas may form. What are the correct tests to identify them?

Correct answer: Pop sound for hydrogen and lime water for carbon dioxide

The governing concept is the use of characteristic gas tests. Hydrogen burns rapidly and produces a ‘pop’ when a burning splint is brought near the gas. Carbon dioxide does not give that test; it turns limewater milky because calcium carbonate forms. Thus option A correctly pairs each gas with its diagnostic test. Smell and colour are unreliable here, and reversing the tests gives option B, which is incorrect.