Which option lists water-reactive substances and their hazards correctly?

• A. Sodium chloride — becomes explosive with water; Calcium carbonate — releases chlorine gas.
• B. Sodium metal — reacts with water to release hydrogen gas and heat; Calcium carbide — produces acetylene gas; both can be flammable.
• C. Water-reactive substances do not exist; all reactants are stable with water.
• D. Sodium metal — forms salt; Calcium carbide — dissolves in water without hazard.

Answer: (B)

Water-reactive substances are materials that react with water, often releasing heat and gases that can ignite or create pressure hazards. Sodium metal reacts with water to form sodium hydroxide and hydrogen gas; the reaction is highly exothermic, and the liberated hydrogen can ignite, posing a flame or explosion risk. Calcium carbide reacts with water to produce calcium hydroxide and acetylene gas; acetylene is a flammable gas that can form explosive mixtures with air, so this reaction also presents significant fire and explosion hazards.

The other options misstate the chemistry or the hazards. Sodium chloride is essentially inert in water, so it does not explode or react to release gases. Calcium carbonate does not release chlorine gas when simply mixed with water. Water-reactive substances do exist, so that claim is false. Sodium metal does not form a simple salt with water; it forms sodium hydroxide and hydrogen. Calcium carbide does not dissolve harmlessly in water without hazard; it reacts to generate acetylene gas.