A calorie is __________.

Question

BlackTom AI · Accepted Answer

A calorie is a unit of energy; specifically the (small) calorie is defined as the amount of heat required to raise the temperature of 1 gram of water by 1°C. This is the primary concept: a calorie measures thermal (heat) energy and has a fixed relation to joules (1 cal = 4.184 J). The dietary “Calorie” printed on food labels is a kilocalorie (1 kcal = 1000 cal = 4184 J).

Step-by-step reasoning:
1. Relevant definitions:
   - small calorie (cal): energy to raise 1 gram of water by 1°C.
   - kilocalorie (kcal, usually written as Cal on food labels): 1000 small calories.
   - British thermal unit (BTU): energy to raise 1 pound of water by 1°F (a different unit).
2. Compare option b to the calorie definition: option b describes the BTU, not the calorie.
3. Quantitative check showing the difference:
   - Mass of 1 pound ≈ 453.592 g.
   - Temperature change 1°F = 0.555556°C.
   - Energy in joules to raise 1 lb water by 1°F: E = m·c·ΔT ≈ 453.592 g × 4.184 J/(g·°C) × 0.555556°C ≈ 1055.06 J.
   - Convert that to small calories: 1055.06 J ÷ 4.184 J/cal ≈ 252.0 cal. So the energy described in option b ≈ 1055 J ≈ 252 small calories, which is the BTU — not one calorie.
4. Therefore option a correctly states the calorie is a unit of measurement used to measure energy.

Why other options are incorrect:
- b: Describes the BTU, not the calorie.
- c (both a and b): Incorrect because b is false for “calorie.”
- d (neither): Incorrect because a is true.

Conclusion: The correct answer is a. a unit of measurement used to measure energy. This matches the provided answer.

A calorie is __________.单项选择题

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