 # Specific Heat Capacity, Specific Heat At Constant Pressure And Constant Volume

What is specific heat capacity?

Let’s understand the concept of specific heat capacity with the help of examples. Let’s take an equal quantity, say 1 liter, of three liquids. Water, Mustard Oil, and Mercury. The concept of specific heat is related to a body’s thermal properties. Let’s heat up these three liquids using the same stove, maintaining the same conditions in all three experiments. The liquids are at a room temperature of 20oC and need to be brought to 60oC. Will all three samples take the same amount of time?

The chances are close to none. While doing the experiment, you will notice that to undergo the same temperature rise, under the same conditions, all three liquids take different times. The mercury is heated up the fastest, followed by Mustard Oil followed then by Water. This property is measured by the heat capacity. The Heat capacity represents the change in temperature in the sample for a given amount of heat. The specific heat in SI Units is joule per kelvin (J kg-1). Specific Heat Capacity which is different from Heat Capacity represents the amount of heat needed to raise the temperature of a unit mass of a substance by 1oC. The specific heat formula is;

$S (Heat Capacity)$ =$\frac{Q}{\Delta T}$

$s = \frac{S}{m} = \frac{1}{m}\frac{Q}{\Delta T}$

Specific heat capacity is different from heat capacity only in the fact that specific heat capacity accounts for the mass of the body and hence it is more specific and accurate than heat capacity. The SI unit of specific heat is Joule per kelvin per kg (J kg-1 K-1). It is important to mention that the specific heat capacity of water is 4.186 joule/gram oC which is higher than any other common substance. As a result, water plays a huge role in temperature regulation and consequently weather. If specific heat capacity is considered from the point of view of moles and not mass, that then becomes molar specific heat capacity of the substance. Molar specific heat formula is;

$C$ =$\frac{S}{\mu }$ =$\frac{1}{\mu}\frac{Q}{\Delta T}$

Here, the m  represents the number of moles of a substance and the SI unit of molar specific heat capacity is J mol-1 K-1.

One thing we must know here is that none of the relationships discussed earlier apply in case of a phase change because the heat added or removed during a phase change does not change the temperature. The need for being specific is great. The molar specific heat capacity is not accurate enough. One of the factors we did not consider while heating the sample was whether to leave the top open to evaporation or keep it at a constant pressure. Doing one of either will definitely change the results of the experiment. To address this, scientists sub-divided specific heat capacity into two groups.

• Molar Specific Heat Capacity at Constant Pressure: If the heat transfer to the sample is done when it is held at constant pressure, then the specific heat obtain using such a method is called Molar Specific Heat Capacity at Constant Pressure.
• Molar Specific Heat Capacity at Constant Volume: If the heat transfer to the sample is done when the volume of the sample is held constant, then the specific heat obtain using such a method is called Molar Specific Heat Capacity at Constant Volume.