Which of the following observations indicates that a liquid has transferred thermal energy to the air?

• A. The liquid freezes.
• B. The temperature of the air decreases.
• C. The liquid becomes more viscous.
• D. The temperature of the liquid decreases.

Answer: (D)

Verified step by step guidance

1. Understand the concept of thermal energy transfer: Thermal energy always flows from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached.
2. Identify the direction of heat flow: If a liquid transfers thermal energy to the air, the liquid must be losing heat, so its temperature decreases, and the air gains heat, so its temperature increases.
3. Analyze each observation: - 'The liquid freezes' indicates the liquid is losing heat, but freezing is a phase change, not a direct measure of heat transfer to air. - 'The temperature of the air decreases' means the air is losing heat, which contradicts the idea that the liquid is transferring heat to the air. - 'The liquid becomes more viscous' relates to physical properties changing with temperature but does not directly indicate heat transfer.
4. Recognize that the key indicator of heat transfer from the liquid to the air is the decrease in the liquid's temperature, showing it is losing thermal energy.
5. Therefore, the correct observation indicating that the liquid has transferred thermal energy to the air is that the temperature of the liquid decreases.