Latent Heat of Water

Have you ever thought about what happens when any matter changes its state? Changing of phase undergoes a heat transfer, but the temperature of the substances remains constant. So, the heat necessary for phase changes of water from solid to liquid or gas, or liquid to solid or gas, without any temperature alteration is known as latent heat of water.

Definition of Latent Heat

The energy radiated or absorbed by a thermodynamic system or a body during its change of state and without any change in its temperature is termed as latent heat (also called latent energy). Moreover, latent heat is generally represented in calories or joules per unit mass or mole of the body experiencing phase change.

Latent heat is a type of heat energy, described in energy per unit of mass,  that must be achieved for a phase change to occur in a substance. The most familiar changes are seen in water, such as freezing liquid water to create ice or boiling water to create gas. During this process, you are continuing to add heat energy to water, but the water temperature does not increase. Where is the energy going ? The answer lies in latent heat.

For instance, consider melting ice blocks. Ice melts as it absorbs heat and undergoes a rise in its temperature. Furthermore, during the melting process, ice absorbs latent energy which helps in the change of its state from solid to liquid. However, the temperature of ice does not change when it intakes latent heat.

Furthermore, three primary forms of latent heat are as follows:

• Latent Heat of Fusion

It refers to the energy related to freezing of liquid and melting of solid.

• Latent Heat of Vaporization

Whereas, latent heat of vaporization refers to the energy related with changing of solid or liquid to gas and condensation of vapour.

Take an example of a bucket of water which is boiling. Moreover, when water in the bucket is boiling, its temperature stays at 100 degree Celsius or 212 degrees Fahrenheit till it completely evaporates. This happens because water absorbs the heat applied as latent heat of vaporization. Further, it is taken by the evaporating molecules.

Additionally, in this same way, the temperature of ice when it melts is 0 degree Celsius or 32 degrees Fahrenheit. Also, the water which is formed with the effect of latent heat of fusion is 0 degree.

• Sensible Heat: Although sensible heat is often called latent heat, it isn't a constant-temperature situation, nor is a phase change involved. Sensible heat reflects heat transfer between matter and its surroundings. It is the heat that can be "sensed" as a change in an object's temperature.

Specific Latent Heat

Specific latent heat refers to the quantity of energy in heat form needed for the complete change of phase for one unit of mass of a particular matter. This is an intensive property. The specific latent heat expression is:

L = Q / m

Material or substance features are intensive properties which do not depend on the extent or shape of any matter. So, accordingly, to evaluate specific latent heat of water value for fusion and vaporization use the following expression:

Q = m x L

Where, Q is quantity of absorbing or radiating heat or during change of state of any matter (joules or calories)

m is mass of matter (kilograms)

L is specific latent heat for a particular matter (kJ kg-1); Lv for vaporization and Lf for fusion

Note: The latent heat of water at 0 degree Celsius for fusion is nearest to 334 joules per gram or 79.7 calories per gram. On the other hand, the latent heat of water at 1000C for vaporization is approximately 2230 joules per gram or 533 calories per gram.

Do It Yourself

(i) The Energy Absorbed or Released During a Change of State is Known as:

(a) thermal heat

(b) kinetic energy 

(c) fusion 

(d) latent heat

(ii) Water Molecules have the Greatest Kinetic Energy in

(a) Ice at 00C

(b) Water at 373 K 

(c) Water at 980C

(d) Steam at 1500C. 

Latent heat of water is a vital concept in Physics, and you must have a thorough knowledge of its related concepts for good grades in your examinations. Now you can even download our Vedantu app for convenient access to detailed study materials and interactive online sessions.

Sensible Heat and Meteorology 

While latent heat of fusion and vaporization are used in physics and chemistry, meteorologists also consider sensible heat. When latent heat is absorbed or released, it produces instability in the atmosphere, potentially producing severe weather. The change in latent heat alters the temperature of objects as they come into contact with warmer or cooler air. Both latent and sensible heat cause air to move, producing wind and vertical motion of air masses.

Examples of Latent and Sensible Heat

Daily life is filled with examples of latent and sensible heat:

1. Boiling water on a stove occurs when thermal energy from the heating element is transferred to the pot and in turn to the water. When enough energy is supplied, liquid water expands to form water vapor and the water boils. An enormous amount of energy is released when water boils. Because water has such a high heat of vaporization, it's easy to get burned by steam.

Similarly, considerable energy must be absorbed to convert liquid water to ice in a freezer. The freezer removes thermal energy, allowing the phase transition to occur. Water has a high latent heat of fusion, so turning water into ice requires the removal of more energy than freezing liquid oxygen into solid oxygen, per unit gram.

2. Latent heat causes hurricanes to intensify. Air heats as it crosses warm water and picks up water vapor. As the vapor condenses to form clouds, latent heat is released into the atmosphere. This added heat warms the air, producing instability and helping clouds to rise and the storm to intensify.

3. Sensible heat is released when soil absorbs energy from sunlight and gets warmer.

4. Cooling via perspiration is affected by latent and sensible heat. When there is a breeze, evaporative cooling is highly effective. Heat is dissipated away from the body due to the high latent heat of vaporization of water. However, it's much harder to cool down in a sunny location than in a shady one because sensible heat from absorbed sunlight competes with the effect from evaporation.

Tips to Memorize Latent Heat of Water:

1. Make notes so that you can refer to them easily before exams. Make sure you include only important points and write legibly.

2. You can make use of educational websites, such as Vedantu, where you can find useful solutions, such as notes, mock tests, and more. 

3. Go through the definitions and key points every day.