Heat is a form of energy, often termed thermal energy. As we are familiar with the Law of conservation of energy which states that energy can neither be created nor destroyed, it can only be transformed from one form to another. For example – A toaster draws electric current (electrical energy) from a wall outlet and converts these moving electric charges into heat (thermal energy) in the filaments that turn red hot to cook your toast.
In basic thermodynamics, the higher the temperature of a material, the more thermal energy it possesses. In addition, at a given temperature, the more mass of a given substance there is, the more will be the total thermal energy the material possesses.
If we look at the atomic level, the 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 in addition 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 is the amount of heat required to change the heat content of 1 mole of a material by exactly 1°C. It is denoted by Cp. The S.I. unit of heat capacity is joule per kelvin.
Heat capacity is an extensive property of matter, meaning that it is proportional to the size of the system. When expressing the same phenomenon as an intensive property, the heat capacity is divided by the amount of substance, mass, or volume, thus the quantity is independent of the size or extent of the sample.
What is Specific Heat?
Specific heat is the amount of heat required to change the heat content of exactly 1 gram of a material by exactly 1°C. It is denoted by Csp. In other words. specific heat is the heat capacity per unit mass of a material.
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 both the materials attain the same temperature. 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 very well 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 ∆tWhere 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
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. What this implies is 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. For comparison sake, it only takes 0.385 Joules of heat to raise the temperature of 1 gram of copper by 1°C.
One calorie= 4.184 joules; 1 joule= 1 kg(m)2(s)-2 = 0.239005736 calorie.
Even 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. Again which means that it takes 1.9 joules of energy to raise the temperature of 1 gram of water vapor by 1 degree Celsius.
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.
If you leave a bucket of water outside in the sun in summer it will certainly get warm, but not hot enough to boil an egg. But, if you walk barefoot on the sand in summer, you’ll burn your feet. Dropping an egg on the metal of a car hood on a summer day will produce a fried egg. Metals have a much lower specific heat capacity than water. If you’ve ever held onto a needle and put the other end in a flame you know how fast the needle gets hot, and how fast the heat is moved through the length of the needle to your finger. Not so with water.
Lucky for me, you, and our fish in the pond, water does indeed have a very high specific heat capacity. 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.
The oceans and lakes help regulate the temperature ranges that billions of people experience in their towns and cities. The water surrounding or near cities take longer to heat up and longer to cool down than do land masses, so cities near the oceans will tend to have less change and less extreme temperatures than inland cities.
Why water has a high specific heat?
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.
As a summary, the reasons are:
- The heated water will contribute much of the heat to loosening, bending or breaking the hydrogen bonds.
- Water had three rotational degrees of freedom. In addition to vibration, rotations happen a lot to water molecules. This will result in a higher heat capacity.
- Specific heat capacity is defined as the amount of heat required per unit mass to increase the temperature by a degrees Celsius. The relatively low molar mass of water allows more moles of it to be there in a mass unit (either kg or g).
To know more, you can refer to Calorimetry : System, Boundary, Surroundings, Specific Heat.