Melting point is defined as the temperature at which a substance changes from solid to liquid.

• A. Temperature at which a liquid freezes
• B. Temperature at which a substance changes from solid to liquid
• C. Pressure at which a gas condenses
• D. The color change when heating

Answer: (B)

Melting point is the temperature at which a solid becomes a liquid because heat provides enough energy to overcome the forces holding the particles in a fixed structure. At that temperature, the solid and liquid phases are in equilibrium. For pure crystalline substances, this happens at a single temperature under standard pressure; impurities can make melting occur over a range. The other options describe a liquid freezing back to a solid, gas condensing into a liquid, or a color change, none of which define melting.