We have studied about the exchange of heat between system and surrounding. Now, we s hall focus on the measurement of heat. The heat transferred to the system appears as a rise of its temperature. The increase of temperature ( ΔT) is directly proportional to the quantity of heat (q) absorbed by the system. It can be put as

q = ΔT = or q = C.T

where C is called heat capacity of the system the value of which depends upon the size, composition and nature of the system.

Now, if the temperature difference (.1T) is unity, i. e., l^{0} celsius or 1° kelvin then, q = C. Thus, heat capacity of the system is defined as the quantity of heat required to raise its temperature through ] 0.

Equation (16.1) reveals that a given amount of heat will raise the temperature of the system to a smaller extent if its heat capacity is large and vice versa.

**MOLAR HEAT CAPACITY**

** **The molar heat capacity of a substance (C_{m}) is the heat required to raise the temperature of 1 mol of the system through 1°. If C is the heat capacity of n mol of system then its molar

heat capacity, C_{m} is given by

C m = Heat capacity / Number of moles = C / n

** **

**SPECIFIC HEAT CAPACITY (c)**

This term is used more frequently for solids and liquids. We know beat capacity of a system depends upon the quantity of matter in the system. For example, large block of aluminium has a higher heat capacity than does a small piece of the same metal. Specific heat capacity (c) is a heat capacity of 1 g of the sample. It is the quantity of heat required to raise the temperature of 1 gram of substance through 1 K (or JOC).

Specific heat capacity ( c) = Heat capacity / mass = C/m

Now , C = q / ΔT

• Specific heat capacity of water

= 4.184 kJ kg^{-1}K^{-1} or 4.184 Jg^{-1}K^{-1}

Specific heat capacity (c) = q / mΔT

or q = c x m x ΔT

Units of specific heat capacity are:

Jg^{-1} K^{-1} or Jg^{-1} (^{0}C^{-1})

The specific heat capacity (c) and molar heat capacity (C_{m}) of the substance are related as

C x molar mass = Cm