Item 21 - A student heated a blue solid which
gave off a gas. When tested, the gas
changed cobalt chloride paper to
pink. Which of the following is a
LIKELY component of the gas?
B.
Carbon dioxide
Hydrogen
Dust particles
Water vapour​

I believe the answer is hydrogen because

Explanation:

Procedure

Dissolve 5 g of hydrated cobalt(II) chloride in 100 cm3 of water (solution is TOXIC).

Soak filter paper in this solution, drain and dry in an oven (set at no more that 100 °C), so that it is a definite blue colour.

The filter paper can be cut into small strips and stored in a desiccator with dry silica gel.