Which of the following substances could act as a catalyst in a chemical reaction?

• A. Pt (platinum) in the hydrogenation of alkenes
• B. NaCl (sodium chloride) as a reactant in precipitation
• C. CO2 (carbon dioxide) as a product in combustion
• D. H2O (water) as a solvent in acid-base reactions

Answer: (A)

Verified step by step guidance

1. Understand the definition of a catalyst: a substance that increases the rate of a chemical reaction without being consumed or permanently changed in the process.
2. Analyze each option to see if it fits the role of a catalyst:
3. Pt (platinum) in the hydrogenation of alkenes is a classic example of a heterogeneous catalyst, where Pt provides a surface for the reaction to occur more efficiently without being consumed.
4. NaCl (sodium chloride) acts as a reactant in precipitation, meaning it is consumed and not regenerated, so it is not a catalyst.
5. CO2 (carbon dioxide) is a product in combustion reactions, so it cannot be a catalyst because it is formed rather than facilitating the reaction.
6. H2O (water) often acts as a solvent in acid-base reactions, providing a medium but not increasing the reaction rate by itself, so it is not considered a catalyst.