Question

what type of substance reacts with limestone (caco₃) and produces carbon dioxide gas?
○ a neutral substance
○ a base
○ an acid
○ both an acid and a base

Explanation:

Limestone ($\ce{CaCO_3}$) is a carbonate. Acids react with carbonates according to the general reaction: $\text{Acid} + \text{Carbonate}
ightarrow \text{Salt} + \ce{H_2O} + \ce{CO_2}\uparrow$. For example, $\ce{2HCl + CaCO_3 = CaCl_2 + H_2O + CO_2}\uparrow$. Neutral substances don't react with carbonates to produce $\ce{CO_2}$, bases also don't react with carbonates in this way, and a substance can't be both acid and base (except ampholytes, but they don't react with carbonates to produce $\ce{CO_2}$ like acids do). So the substance is an acid.

Answer:

an acid