Which hazard class includes materials that are reactive with water?

Which hazard class includes materials that are reactive with water?

• A. Class 4.3 (Water-reactive Substances).
• B. Class 1 (Explosives).
• C. Class 3 (Flammable Liquids).
• D. Class 6 (Toxic Substances).

Answer: (A)

Reactivity with water is a distinct hazard defined for water-reactive substances. These materials release heat and often generate gases such as hydrogen (or other flammable/toxic gases) when they come into contact with water, and some can react violently. That specific behavior is what places them in Class 4.3. Because their defining risk is interaction with moisture, they’re kept separate from other hazards that involve burning, exploding, or toxicity in the absence of water contact. For example, calcium carbide and certain alkali metals like sodium react vigorously with water, illustrating why this class exists. In contrast, explosives (Class 1) are about rapid energy release, flammable liquids (Class 3) burn, and toxic substances (Class 6) harm people, but they aren’t defined by water reactivity.

Reactivity with water is a distinct hazard defined for water-reactive substances. These materials release heat and often generate gases such as hydrogen (or other flammable/toxic gases) when they come into contact with water, and some can react violently. That specific behavior is what places them in Class 4.3. Because their defining risk is interaction with moisture, they’re kept separate from other hazards that involve burning, exploding, or toxicity in the absence of water contact. For example, calcium carbide and certain alkali metals like sodium react vigorously with water, illustrating why this class exists. In contrast, explosives (Class 1) are about rapid energy release, flammable liquids (Class 3) burn, and toxic substances (Class 6) harm people, but they aren’t defined by water reactivity.