Why is tetraoxosulphate(VI) acid NOT used for making carbon dioxide from calcium carbonate? It is

Analyze the question: We want to make CO₂ in the lab from calcium carbonate (marble chips or limestone). The usual acid is dilute HCl. The question asks why dilute H₂SO₄ is not used.

Concept: When an acid reacts with a carbonate, it gives a salt, water and CO₂. The reaction only continues if the salt formed can move away from the surface of the carbonate.

Solution:

With sulphuric acid:

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Calcium tetraoxosulphate(VI), CaSO₄, is only sparingly soluble in water. It forms an insoluble coating on the surface of the marble chips. This coating blocks the acid from reaching the remaining CaCO₃, so the reaction quickly slows down and stops. Only a small amount of CO₂ is produced.

With hydrochloric acid:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

CaCl₂ is very soluble, so no coating forms and the reaction continues.

Checking the other options:

• A. Dehydrating agent applies to concentrated H₂SO₄, but it is not the reason here.
• B. Being a strong acid would make it work well, not badly.
• C. Being an oxidising agent is irrelevant, because CO₂ is not affected.
• D. Being dibasic does not stop the reaction.
• E. It forms an insoluble CaSO₄ layer. This is the correct reason.

Tip: Remember the rule "insoluble coating stops the reaction." The same reason explains why dilute H₂SO₄ is a poor choice for reacting with lead(II) compounds (PbSO₄ is insoluble) or with marble chips. For CO₂ preparation, always think "marble chips + dilute HCl."