Phase Transition: Definition, Examples, Types, Important Questions

When we leave a glass of cold water for some time, the surface of the glass gets fully covered with tiny droplets of water. Similarly, if we put water into the freezer, it turns into ice. These changes are the result of the phase change, also known as the phase transition.

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Phase Transition

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Phase transition is the transformation of a substance from one state to another state. Every element and every substance can change from one phase to another under certain conditions of temperature and pressure.

The temperature and pressure at which a substance changes its state, are highly dependent on the intermolecular forces acting on the molecules and atoms of the substance. 

Also Read: Phase Transition in Detail

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Two-phase state-

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It is possible that both phases coexist in one container at the same time. This usually happens when the substance transforms from one phase to another. This state of matter is known as a two-phase state. 

For Example- When the ice melts, there is both solid and liquid water in the cup.

There are six ways to change the substance between the three stages, i.e. Solid, Liquid and Gas are- melting, freezing, evaporation, condensation, sublimation and precipitation. These processes are reversible and occurred between phases in different ways:

• Melting: The transition from solid to liquid
• Freezing: The transition from liquid to solid phase.
• Evaporation: Change from liquid phase to gas phase.
• Condensation: The transition from the gas phase to the liquid phase.
• Sublimation: The transition from solid phase to gas phase.
• Deposition: The transition from gas to solid phase.

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Conditions Responsible for Phase Transition

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When considering phase changes, two variables should be considered: pressure (P) and temperature (T). For the gaseous state, the relationship between temperature and pressure is determined by the Ideal gas law:

PV=nRT

According to Van der Waals Equation of State:

(P+a∗n₂/ V₂) (V−nb) = nRT

Here, V is the volume, R is the gas constant, and n is the number of moles of gas. 

The ideal gas law assumes that there is no intermolecular force acting on the gas, while the Van der Waals equation contains two constants a and b, which take into account the intermolecular force acting on the gas molecules.

Check Important Notes for Avogadro’s Law (Volume-Amount Relationship)

Temperature

Temperature can change the phase of a substance. To explain this let's take the example of Ice. If the temperature of the ice is changed, it will start melting resulting in the change of state from solid to liquid. If the temperature is increased then it will start evaporating resulting in the change of state from liquid to gas. 

In the image above, we have an entity in the container. When we put it on a heat source like a burner. The heat is transferred to the material, which increases the kinetic energy of the material molecules. The temperature of the material is gradually increased until the substance reaches its melting point. As more and more heat is transferred above its melting point, the substance begins to melt and become liquid. 

This type of phase change is called an isobaric process because the pressure in the system remains constant.

Pressure

A solid can be converted into liquid and then to gas by decreasing the pressure. Every state of matter of any substance is stable at a corresponding pressure and temperature. When the pressure exerted on a substance increases, it can cause the substance to condense. Decreasing pressure can cause it to vaporize. 

In the picture above, we have a container with a piston to seal the gas. As the piston compresses the gas, the pressure increases. Once it reaches the boiling point. The gas condenses into liquid, as long as the piston continues to compress the liquid, the pressure will rise until it reaches the melting point. The liquid converts into a solid state. This type of phase change is an isothermal process in which the temperature is constant and only the pressure changes.

Read Also: Rotational Kinetic Energy Derivations

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Types of Phase Transition

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Melting 

Melting is a process which converts a substance from solid to liquid. This phase transition can only take place by increasing the temperature of the substances. Every substance has a melting point. The melting point is the temperature at which a solid becomes a liquid. Different pressures require different temperatures to melt the substance. 

Every pure element in the periodic table has a normal melting point, which is the temperature at which the element converts into liquid under 1 atmosphere of pressure.

Boiling 

Every substance also has a boiling point. The boiling point is the temperature at which a liquid turns into a gas. The boiling point changes with temperature and pressure. Like melting point, every pure element has a normal boiling point, which is the temperature at which liquid converts into vapour under 1 atmosphere of pressure.

Condensation

Condensation is the process by which the physical state of a substance changes from a gaseous phase to a liquid phase. It can also be defined as the transformation of water vapor into water droplets when it comes in contact with a hard surface. 

This method can be used to separate solutes and solvents from their solutions. The solution is heated, the solvent is evaporated, and then collected separately after condensation.

Heating a Metal

The structural, magnetic and electrical properties of metals change when it is heated. Metal expands when heated and its length, volume and surface area increase. This process is called thermal expansion. 

Different metals have different degrees of thermal expansion. This process is useful in designing metal structures such as pipes and fittings.

Freezing

Freezing is a process in which the physical state of a substance changes from liquid to solid. This phase change process occurs when the temperature drops and the liquid reaches its freezing point. This procedure is used to store fruits and vegetables (frozen food) or Animal samples, because it can slow down the decomposition and growth of bacteria.

Sublimation

Sublimation is the transition of a substance from a solid state to a gas state directly without being converted into the liquid state. Sublimation is an endothermic process that occurs at a temperature and pressure below the triple point in the phase diagram of a substance, which corresponds to the lowest pressure at which a substance can exist as a liquid. 

Deposition

The reverse process of sublimation is deposition. In this process the matter is present in the gaseous state, and as the reaction occurs, it gets converted into solid state without being converted into a liquid. A sub-zero temperature is needed to achieve this state. In higher altitudes and south and north poles, the vapours of water get transformed into solid ice without getting into their liquid state. 

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