Which of the following is a base?

• A. Hydrochloric acid
• B. Sodium chloride
• C. Ammonia
• D. Carbon dioxide

Answer: (C)

Bases are substances that accept protons or increase the concentration of hydroxide ions in solution. Ammonia does this in water: NH3 + H2O ⇌ NH4+ + OH-. The formation of OH− raises the solution’s pH, so ammonia acts as a base. The other substances don’t fit this role: hydrochloric acid donates protons and lowers pH, sodium chloride is a neutral salt that doesn’t significantly alter pH, and carbon dioxide dissolves to form carbonic acid, which behaves as an acid rather than a base. In hazmat contexts, recognizing bases helps predict how a substance will react with acids and participate in neutralization processes; ammonia is a weak base that forms ammonium salts when it reacts with acids.