Which elements are classic examples of water-reactive chemicals?

• A. Sodium and Potassium
• B. Chlorine and Fluorine
• C. Iron and Nickel
• D. Neon and Argon

Answer: (A)

The concept here is that some elements, particularly alkali metals, react very readily with water. Sodium and potassium are classic examples because when they meet water they react vigorously, producing hydrogen gas and a metal hydroxide (NaOH or KOH). The reaction is highly exothermic and often visible as fizzing, surface melting of the metal, and sometimes flames from the liberated hydrogen. This intense, immediate water reactivity is a hallmark of these elements.

The other options don’t fit as well: halogens like chlorine and fluorine do react with water but in a different, less dramatic way and aren’t the typical “water-reactive metal” demonstration; iron and nickel react only very slowly with water (rusting is a slow process, not a rapid reaction with liquid water); noble gases like neon and argon are inert and do not react with water at all.