How much heat is defined as a calorie?

A calorie is defined as the amount of heat energy required to raise the temperature of 1 gram of water by 1 degree Celsius (°C) at standard atmospheric pressure. This definition is fundamental in thermodynamics and is widely used as a unit of measurement for heat.

The correct choice captures the essence of the calorie perfectly: it specifies the amount of heat necessary to raise the temperature of 1 liter of water by 1 degree Celsius. It's important to note that since 1 liter of water is equivalent to 1000 grams, raising the temperature of this volume of water by 1 degree Celsius would require 1000 calories, which applies the definition appropriately in the context of volume (liter) rather than mass (gram).

The other options misrepresent the definition of a calorie or refer to different scenarios unrelated to the standard definition. For instance, raising the temperature by 2 degrees Celsius or 0.5 degrees Celsius does not align with how a calorie is quantified since they reference larger and smaller quantities of heat, respectively. The mention of heat needed for a chemical reaction in water also diverges from the definition, as it pertains to heat change associated with a chemical process rather than temperature change in water.