Animated Solution for Physics - Properties of Solids and Liquids: Calorie is defined as the amount of heat required to raise temperature of 1 g of water by 1°C and it is defined under which of the following conditions ?

The concept of a calorie is one of the most fundamental ideas in thermodynamics and calorimetry. At first glance, the definition seems incredibly simple: it is the amount of heat required to raise the temperature of $1\text{ g}$ of water by $1^{\circ }\text{C}$.

However, nature is rarely that simple. If you actually perform this experiment, you will find that the amount of heat needed to heat water from $0^{\circ }\text{C}$ to $1^{\circ }\text{C}$ is slightly different from the heat needed to go from $99^{\circ }\text{C}$ to $100^{\circ }\text{C}$.

Why does this happen? The specific heat capacity of water is not perfectly constant; it varies slightly depending on the initial temperature of the water. If scientists around the world used different starting temperatures, their "calories" would all represent slightly different amounts of energy!

To avoid this chaos, the scientific community had to agree on a strict, standardized definition. They chose a specific $1^{\circ }\text{C}$ interval that was easy to replicate in a laboratory setting.

The internationally accepted standard is known as the $15^{\circ }\text{C}$ calorie. This specific calorie is defined as the exact amount of heat required to raise the temperature of $1\text{ g}$ of water from $14.5^{\circ }\text{C}$ to $15.5^{\circ }\text{C}$.

But temperature isn't the only variable. The specific heat of a substance also depends on the external pressure. If you boil water on top of Mount Everest, it behaves differently than at sea level. Therefore, the definition also strictly requires standard atmospheric pressure.

Standard atmospheric pressure is $1\text{ atm}$, which is equivalent to $760\text{ mm of Hg}$.

Combining these precise conditions, the rigorous definition of one calorie is the heat required to raise the temperature of $1\text{ g}$ of water from $14.5^{\circ }\text{C}$ to $15.5^{\circ }\text{C}$ at a pressure of $760\text{ mm of Hg}$.

This standardization ensures that whenever we talk about a calorie in physics or chemistry, we are all referring to the exact same amount of energy, which is approximately $4.184\text{ J}$.