You are told that three different gas jars contain carbon dioxide, hydrogen and oxygen
respectively.
Which single method of testing for gases will enable you to identify all three gases?

Answer:

2) B

A – Wrong
Bromine water is the test for the presence of alkenes.

B – Correct
A burning splint makes a popping sound in hydrogen, goes out in carbon dioxide and then the glowing splint relights in oxygen. 

C – Wrong
Limewater turns milky in the presence of carbon dioxide. However, no observations are made in the presence of hydrogen and oxygen. 

D – Wrong
This is the test for acids. Blue litmus turns red in acid.

E – Wrong
This is the test for alkali. Red litmus turns blue in alkali.