In a typical heating curve for water, which arrows represent the phase transitions where heat energy is gained by the substance?

• A. The arrows corresponding to cooling within a single phase (temperature decrease)
• B. The arrows corresponding to melting (solid to liquid) and vaporization (liquid to gas)
• C. All arrows on the curve, regardless of direction
• D. The arrows corresponding to freezing (liquid to solid) and condensation (gas to liquid)

Answer: (B)

Verified step by step guidance

1. Understand that a heating curve shows how the temperature of a substance changes as heat is added, including both temperature increases within a phase and phase transitions where the temperature remains constant.
2. Identify that during phase transitions such as melting (solid to liquid) and vaporization (liquid to gas), the substance absorbs heat energy without a change in temperature, which is called latent heat.
3. Recognize that arrows representing cooling within a single phase indicate heat loss, not heat gain, so they do not correspond to heat energy being gained by the substance.
4. Note that freezing (liquid to solid) and condensation (gas to liquid) are phase transitions where the substance releases heat to the surroundings, so these arrows represent heat loss, not gain.
5. Conclude that the arrows corresponding to melting and vaporization represent the phase transitions where heat energy is gained by the substance.