What is phase change?

A physical process in which a conversion among the basic states or phases of matter, i.e., solid, liquid, and gas takes place under the effect of a certain temperature and pressure is referred to as a phase change. Generally, the phase change of a substance occurs when heat transfer takes place between the substance and its surroundings. Based on the direction in which heat transfer takes place, different types of phase changes can occur.

How does phase change occur in a substance?

The two types of phase changes that usually take place in a substance are from solid to liquid and from liquid to gas. In order to understand the different types of phase changes that occur in a substance, let us take the simplest example of water.

Water can exist in three different phases, i.e., ice in the solid phase, water in the liquid phase, and steam in the gaseous phase. Let us start with the solid form of water.

Consider few ice cubes (solid form of water) are placed in a container. Heat is given to this container by placing it over a constant source of heat and the temperature of the contents of this container is noted continuously in small intervals. The variation of the temperature of the water with heat supplied is shown in the graph below:

The various phase changes that take place in water are as follows:

Melting

The heat supplied to the container is used in increasing the temperature of ice. The temperature of ice increases until the temperature of the whole of the ice becomes $32°\mathrm{F}$. This process is represented by line OP in the graph.

After point P, the heat energy supplied to the ice does not increase its temperature further but  helps in overcoming the force of attraction between the molecules of the ice. Therefore, the force of attraction between the molecules becomes weak, thereby increasing the intermolecular separation between them. Now, the molecules are free to move forming a liquid. Thus, at this point, the ice gets converted into its liquid form, i.e., water.

This process of phase change from solid to liquid is referred to as melting. It is represented by line PQ in the graph. It can be clearly seen from the graph that the temperatures of the ice and water mixture are the same, i.e., $32°\mathrm{F}$ during the phase change PQ. Thus, we can say that ice (solid state) and water (liquid state) are in thermal equilibrium with each other during the phase change.

The temperature at which the solid and liquid phases of a substance exist in thermal equilibrium is referred to as the melting point of that substance. Thus, the value of the melting point of ice is $32°\mathrm{F}$.

• The melting points of different substances are different.
• The melting point of a substance varies with pressure.

The reverse of this phase change, i.e., the conversion of liquid into a solid is known as fusion. In the process of fusion, heat is released into the surroundings in the form of thermal energy.

Vaporization

At point Q, the whole of the ice is in liquid form, i.e., water. Now, if heat is again supplied to the container, then the temperature of the water increases. The temperature of water keeps increasing until its final temperature becomes $212°\mathrm{F}$. This process is represented by line QR in the graph.

From R to S, the heat energy supplied to the water does not increase its temperature further but is used in overcoming the force of attraction between the molecules of the water. Therefore, the force of attraction between the molecules becomes extremely weak. Thus, molecules are no longer bound together and are free to move to form a gas. Thus, at this point, water gets converted into steam.

This process of phase change from liquid to gas (vapor) is known as vaporization. It is represented by line RS in the graph. It can be clearly seen from the graph that the temperatures of the water and steam are the same, i.e., $212°\mathrm{F}$ during the phase change RS. Thus, we can say that the water (liquid state) and steam(gaseous state) remain in thermal equilibrium with each other during the phase change.

The temperature at which the liquid and gaseous phases of a substance exist in thermal equilibrium is known as the boiling point of that substance. Thus, the value of the boiling point of water is $212°\mathrm{F}$.

• The value of the boiling point of different substances is different.
• The value of the boiling point of a substance is directly proportional to the pressure, i.e., it decreases with the decrease in pressure and increases with the increase in pressure.

The reverse of this phase change, i.e., the conversion of steam into liquid is known as condensation. In the process of condensation, heat is released into the surroundings in the form of thermal energy.

Sublimation

The process of phase change in which the solid phase converts directly into the gaseous phase without involving the liquid phase is called sublimation. The solid and gaseous phases remain in thermal equilibrium with each other during this phase change. The heat energy supplied during this process is high enough and so the substance does not pass through the liquid phase. It therefore directly gets converted into the gaseous state.

Some substances that undergo sublimation at normal temperature and pressure are dry ice (solid carbon dioxide), naphthalene balls, and iodine.

Latent heat

During the phase change of a given mass of a substance either from solid to liquid phase or from liquid to the gaseous phase, the temperature of the substance remains constant. Therefore, the entire heat supplied to the substance during the phase change is used in changing the phase of the substance. Also, reverse processes, such as the phase change of a given mass of a substance either from liquid to solid or from gas to liquid, involve the release of heat to the surroundings at a constant temperature. This heat which is supplied or released during the phase change is generally referred to as latent heat.

Thus, latent heat is defined as the amount of heat energy supplied or released to change the phase of a unit mass of a substance without changing its temperature. It is represented by “L.”

The expression for the amount of heat supplied to change the phase of a substance of mass “m” and latent heat “L” is given as:

$∆Q=mL$

The units of measurement of latent heat are $\mathrm{J} {\mathrm{kg}}^{-1}$ or $\mathrm{cal} {\mathrm{g}}^{-1}$.

The latent heat is of two types:

• Latent heat of fusion
• Latent heat of vaporization

Latent heat of fusion

The amount of heat supplied to change the phase of a unit mass of a substance from solid to liquid (melting) or vice-versa (fusion) at its melting point is known as the latent heat of fusion of that substance. It is denoted by Lf. The latent heat of fusion of ice is $80 \mathrm{cal} {\mathrm{g}}^{-1}$.

Latent heat of vaporization

The amount of heat supplied to change the phase of a unit mass of a substance from liquid to gas (vaporization) or vice-versa (condensation) at its boiling point is referred to as the latent heat of vaporization of that substance. It is denoted by $L_v$. The latent heat of vaporization of water is $540 \mathrm{cal} {\mathrm{g}}^{-1}$. 

Specific heat

The amount of heat supplied to increase the temperature of a unit mass of a substance through a unit degree is called the specific heat of that substance. It is denoted by “s.”

Thus, the amount of heat supplied to increase the temperature of a substance of mass “m” and specific heat “s” by $∆T$ is:

$∆Q=ms∆T$

The units of measurement of specific heat are $\mathrm{J} {\mathrm{kg}}^{-1}$ or $\mathrm{cal} {\mathrm{g}}^{-1}$.

Common Mistakes

Students often get confused between specific heat and latent heat. Although both are types of heat but are entirely different.

• Specific heat is the amount of heat supplied to increase the temperature of a unit mass of a substance, whereas latent heat is the amount of heat supplied to change the phase of that substance.
• When latent heat is supplied to a substance, its temperature remains constant. While, the specific heat supplied to a substance changes its temperature.
• Latent heat is measured in the units of J/kg or cal/g, whereas specific heat is generally measured in the units of J/kg/K or cal/g/C.

Formulas

The amount of heat supplied to change the phase of a substance of mass “m” is given as:

$∆Q=mL$

where, L is the latent heat of the substance

The amount of heat supplied to increase the temperature of a substance of mass “m” and specific heat “s” by $∆T$ is given as:

$∆Q=ms∆T$

where, s is the specific heat of the substance

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses such as:

• Bachelor of Science in Applied Physics
• Bachelor of Technology in Material Science and Engineering
• Bachelor of Science in Environmental Science
• Master of Science in Applied Physics
• Master of Technology in Material Science and Engineering
• Master of Science in Environmental Science

Related Concepts

• Triple Point
• Molar Specific Heat
• Heat Transfer
• Thermal Energy

Practice Problems

Q1. Which of the following statements is incorrect?

1. The value of melting point of a substance varies with pressure.
2. The value of melting point of a substance is different for different substances.
3. The value of melting point of every substance is same at a particular temperature.
4. None of these

Correct option: (c)

Explanation: All the substances that exist in nature do not have the same value of melting point temperature. The melting point generally depends on the ability, and the way of atoms and molecules are closely held together.

Q2. The process of phase change from liquid to solid is:

1. melting
2. boiling
3. fusion
4. condensation

Correct option: (c)

Explanation: Any substance transformed from the solid to liquid state can be termed as fusion. It is also known as the meting phenomenon. It is opposite to the phenomenon of the freezing process.

Q3. Which of the following substances undergo sublimation at normal temperature and pressure?

1. Common salt
2. Sulphur
3. Sand
4. Dry ice

Correct option: (d)

Explanation: Dry ice will undergo sublimation at standard (normal) temperature and pressure. The substances that undergo sublimation are iodine, air fresheners, camphor, freeze-drying, water, menthol, naphthalene, mothballs, anthracene, and many more.

Q4. What happens to the temperature of a substance during the phase change?

1. increases
2. decreases
3. does not vary
4. becomes zero

Correct option: (c)

Explanation: The temperature of any substance remains constant or unchanged during the phase change process. Energy is being utilized during this process, and there is no heat left to rise the temperature.

Q5. How much heat is required to convert 500 g of ice into water?

1. $4\times {10}^4 \mathrm{cal}$
2. $4.5\times {10}^4 \mathrm{cal}$
3. $5\times {10}^4 \mathrm{cal}$
4. $6\times {10}^4 \mathrm{cal}$

Correct answer: (a)

Explanation: The latent heat of ice is $L=3.34\times {10}^5 \mathrm{J}/\mathrm{kg}$.

The mass of the ice is $m=500 \mathrm{g}=0.5 \mathrm{kg}$

The expression for the heat required to convert the ice into water is: 

$Q=mL$

Substitute the values in above expression.

$$\begin{gathered} Q=0.5 \text{kg}\times 3.34\times {10}^5 \mathrm{J}/\mathrm{kg} \\ Q=1.67\times {10}^5 \mathrm{J} \\ \mathrm{Q}=3.99\times {10}^4 \mathrm{cal} \\ \mathrm{Q}\approx 4\times {10}^4 \mathrm{cal} \end{gathered}$$