Specific Heat of Water

Specific Heat of Water

Heat is a form of energy, often called thermal energy. Energy can be transformed from one form to another (a blender transforms electrical energy into mechanical energy), but it cannot be created or destroyed; rather, energy is conserved. In basic thermodynamics, the higher the temperature of a material, the more thermal energy it possesses. In addition, at a given temperature, the more of a given substance there is, the more will be the total thermal energy the material possesses.

On an atomic level, absorbed heat causes the atoms of a solid to vibrate, much as if they were bonded to one another through springs. As the temperature is raised, the energy of the vibrations increases. In a metal, this is the only motion possible. In a liquid or gas, absorbed heat causes the atoms in the molecule to vibrate, and the molecule to both rotate and move from place to place. Because there are more “storage” possibilities for energy in liquids and gases, their heat capacities are larger than in metals.

What is Heat Capacity?

Heat capacity, Cp, is the amount of heat required to change the heat content of 1 mole of a material by exactly 1°C.

What is Specific Heat?

Specific heat, Csp, is the amount of heat required to change the heat content of exactly 1 gram of a material by exactly 1°C.

Specific heat values can be determined in the following way: When two materials, each initially at a different temperature, are placed in contact with one another, heat always flows from the warmer material into the colder material until the both the materialas attains the same temperatrure. From the law of conservation of energy, the heat gained by the initially colder material must equal the heat lost by the initially warmer material.

We know that when heat energy is absorbed by a substance, its temperature increases. If the same quantity of heat is given to equal masses of different substances, it is observed that the rise in temperature for each substance is different. This is due to the fact that different substances have different heat capacities. So heat capacity of a substance is the quantity of the heat required to raise the temperature of the whole substance by one degree. If the mass of the substance is unity then the heat capacity is called Specific heat capacity or the specific heat.

Specific Heat Capacity Formula

Q = C m ∆t
Where Q = quantity of heat absorbed by a body

m = mass of the body

∆t = Rise in temperature

C = Specific heat capacity of a substance it depends on the nature of the material of the substance.

S.I unit of specific heat is J kg-1 K-1.

Heat capacity Formula

Heat capacity = Specific heat x mass

Its S.I unit is J K-1.

Specific Heat of Water

For liquid at room temperature and pressure, the value of specific heat capacity (Cp) is approximately 4.2 J/g°C. This implies that it takes 4.2 joules of energy to raise 1 gram of water by 1 degree Celsius. This value for Cp is actually quite large. This (1 cal/g.deg) is the specific heat of water as a liquid or specific heat capacity of liquid water.

One calorie= 4.184 joules; 1 joule= 1 kg(m)2(s)-2 = 0.239005736 calorie.

The specific heat capacity of water vapor at room temperature is also higher than most other materials. For water vapor at room temperature and pressure, the value of specific heat capacity (Cp) is approximately 1.9 J/g°C.

As with most liquids, the temperature of water increases as it absorbs heat and decreases as it releases heat. However, the temperature of liquid water falls & rises more slowly than most other liquids. We can say that water absorbs heat without an immediate rise in temperature. It also retains its temperature much longer than other substances.

We use this property of water in our body to maintain a constant body temperature. If water had a lower Csp value, then there would a lot of cases of overheating and underheating.

Explanation:

We can explain the reason for the high specific heat of water due to the hydrogen bonds. In order to increase the temperature of the water with the multitude of joined hydrogen bonds, the molecules have to vibrate. Due to the presence of so many hydrogen bonds, a larger amount of energy is required to make the water molecules break by vibrating them.

Similarly, for hot water to cool down, it takes a bit of time. As heat is dissipated, temperature decreases and the vibrational movement of water molecules slow down. The heat that is given off counteracts the cooling effect of the loss of heat from the liquid water.

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Practise This Question

Methylene blue, from its aqueous solution, is absorbed on activated charcoal 25 C. For this process, the correct statement is