What is the definition of a small calorie?

The small calorie, often denoted as "cal," is defined specifically as the amount of heat energy needed to raise the temperature of 1 gram of water by 1 degree Celsius at standard atmospheric pressure. This definition is foundational in thermodynamics and is critical for understanding heat transfer, calorimetry, and the conversion of energy units.

The other options describe different concepts or units in thermodynamics or general physics. For instance, the definition regarding 1000 joules refers to a kilocalorie (sometimes referred to as a large calorie), which is significantly larger and not equivalent to a small calorie. The heat required to change the temperature of 1 liter of water by 1 degree Celsius relates to the concept of a kilocalorie as well since 1 liter of water is equivalent to 1000 grams. The total heat energy contained in 1 kilogram of a substance does not specifically relate to the caloric definition, as it would depend on the specific heat capacity of the substance involved.

Thus, the definition that accurately describes a small calorie is the first option.