Solution

$$\bf{Hint-}$$
Study about the reactions of charcoal.

$$\bf{Concept-}$$
Charcoal contains carbon which on burning forms carbon dioxide gas.

$$\bf{Explanation-}$$
The nature of the gas can be found by adding water to the gas in the test tube. And then put some blue litmus solution in the jar. The blue litmus solution turns red, this means that the gas is acidic in nature.
$$\bf{Equation-1}$$ Burning of charcoal:
$$C+ O_2 \rightarrow CO_2$$
$$\bf{Equation-2}$$ Adding water to the gas to make the solution:
$$CO_2 + H_2O \rightarrow H_2CO_3$$