What is formed when sulfur trioxide gas is absorbed in concentrated sulfuric acid?

Equal lengths of magnesium ribbon are taken in two test tubes $A$ and $B$. $H_{2}SO_{4}$ is added to test tube $A$ and $H_{2}CO_{3}$ in test tube $B$ in equal amounts.
$(a)$ Identify the test tube showing vigorous reaction.
$(b)$ Give reason to support your answer.
$(c)$ Name the gas liberated in both the test tubes. How will you prove its liberation?

$A$ dry pellet of a common base $B$,when kept in open,absorbs moisture and turns sticky. The compound is also a by-product of the chlor-alkali process. Identify $B$. What type of reaction occurs when $B$ is treated with an acidic oxide? Write a balanced chemical equation for one such reaction.

Which of the following gases acts as a mild bleaching agent?

$(a)$ What will you observe when dilute hydrochloric acid is added to a small amount of copper oxide in a beaker?
$(b)$ Aqueous solution of $HCl$ shows acidic character. But the aqueous solution of glucose fails to do so. Why?

$A$ student prepared solutions of $(i)$ an acid and $(ii)$ a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless,how will she distinguish between the two?