Question

which pair of substances would most likely result in the production of a gas when reacting with an acid?
○ litmus and methyl orange
○ base and salt
○ metal and carbonate
○ carbon dioxide and water

Explanation:

• Litmus and methyl orange are indicators, they don't react with acid to produce gas.
• Reaction between base and salt generally doesn't produce gas with acid.
• Metals react with acids to produce hydrogen gas (e.g., $\ce{Zn + 2HCl = ZnCl_{2}+H_{2}\uparrow}$), carbonates react with acids to produce carbon dioxide gas (e.g., $\ce{Na_{2}CO_{3}+2HCl = 2NaCl + H_{2}O + CO_{2}\uparrow}$).
• Carbon dioxide and water react with acid? No, they don't produce gas in reaction with acid. So metal and carbonate is the correct pair.

Answer:

C. metal and carbonate