Latent heat

What is latent heat?

The heat absorbed by any matter without any rise in temperature is called as latent heat. Latent is another word for hidden.

When a matter changes from one state to another on application of heat, after a certain point, the temperature stops rising for some time. This shows that that heat is being absorbed but no temperature change is recorded. At this point, some of the particles have changed into another state, while some are still absorbing the heat to convert.

Experiment to observe latent heat:

Take some ice in the beaker and put a thermometer in it. Keep the heater on a burner and start heating. The thermometer indicates a hike in temperature slowly. The ice starts melting into water by the process of fusion.Once the ice has reached its melting point, that is 273.15K ( 0 degree Celsius), the temperature stops increasing for sometime. After a point, the temperature starts increasing again.

Where does the heat go?

To convert ice into water, the forces of attraction between the water molecules into the ice state needs to be overcome. The temperature did not increase in the above experiment after a point even after applying heat because the heat is used up in overcoming the forces of attraction between the molecules to change the ice into water. When enough energy is acquired by the particles, the molecules break free and start converting to water.

Types of latent heats

Whenever a state change takes place, there is a phase where some particles have acquired the required energy and  overcome the forces of attraction whereas some are still in their original form absorbing the heat. This is the point where the rise in temperature stops and the latent heat comes into picture. Once all the particles have broken free, the whole matter is in its new state and further application of heat increases the temperature too.

The amount of heat required to change one kilogram of a solid into liquid at atmospheric pressure at its melting point is called as latent heat of fusion.

Similarly, if we keep boiling the water in the above beaker to its boiling point (373.15K), the water will start changing it in its vapour form. At this stage, a constant temperature is observed for sometime. It is well known that steam at boiling point is more dangerous than water at the same temperature because the steam has gained more energy in the process of vaporisation.

The amount of heat required to change one kilogram of liquid into gas at atmospheric pressure at its boiling point is called latent heat of vaporisation.

NCERT Textbook Solutions – Page 12

Question 7: Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer: At 273K, ice starts changing from solid to water. At this temperature, the temperature becomes constant for sometime. Some of the particles of water are in solid and some of them are liquid. The particles that are liquid have already absorbed heat and overcome the forces of attraction. Hence, at the same temperature, water molecules have higher heat than the ice molecules. Hence, ic is more effective in cooling than water at the same temperature.

Question 8: What produces more severe burns, boiling water or steam?

Answer: Water starts boiling t 100 degree celcius. At this temperature, some of of the water molecules are in liquid and some are in gaseous form. To convert into steam, the water molecules absorb heat. Since steam posses more heat that boiling water, steam produces more severe burns.

Latent heat is the hidden heat that is used by particles of a matter to overcome the forces of attraction between them in order to change from one state to another. Since it gets hidden in the contents of the matter and no temperature change is recorded for sometime, it is called latent or hidden.