In a chemical reaction diagram, the substance labeled 'b' is shown reacting with hydrochloric acid to produce carbon dioxide gas. Which chemical is most likely represented by 'b'?

• A. KNO3
• B. NaCl
• C. MgSO4
• D. CaCO3

Answer: (D)

Verified step by step guidance

1. Identify the reaction described: a substance labeled 'b' reacts with hydrochloric acid (HCl) to produce carbon dioxide (CO\_2) gas.
2. Recall that carbon dioxide gas is typically produced when an acid reacts with a carbonate compound, as carbonates decompose to release CO\_2 upon acid treatment.
3. Examine the given options: KNO\_3 (potassium nitrate), NaCl (sodium chloride), MgSO\_4 (magnesium sulfate), and CaCO\_3 (calcium carbonate). Among these, only CaCO\_3 is a carbonate.
4. Understand the general reaction of a carbonate with hydrochloric acid: \[\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{CO}_2 + \text{H}_2\text{O}\]
5. Conclude that since the reaction produces CO\_2 gas, the substance 'b' must be calcium carbonate (CaCO\_3), as the other options do not produce CO\_2 when reacting with HCl.