Change of State – Class 11 | Chapter – 11 | Physics Short Notes Series PDF for NEET & JEE

Change of State: A change of state refers to the process by which matter undergoes a physical transformation from one state to another, such as from a solid to a liquid or from a liquid to a gas. These changes can occur due to variations in temperature, pressure, or other external factors that influence the properties of matter.

The three common states of matter are solid, liquid, and gas. Matter can also exist in a fourth state called plasma, which is a highly ionized gas that conducts electricity.

Change of State – Phase Changes

Phase changes occur because of changes in temperature, pressure, or both. The behavior of matter is determined by the kinetic energy of its particles. The particles in a solid have the lowest kinetic energy, while the particles in a gas have the highest kinetic energy.

When energy is added to or removed from a system, it affects the kinetic energy of the particles and can cause a phase change. For example, when a solid is heated, its particles absorb energy and start to vibrate more rapidly. As the particles vibrate faster, the attractive forces between them weaken, causing the solid to melt and become a liquid.

Similarly, when a liquid is cooled, its particles lose kinetic energy and move more slowly. As the particles slow down, the attractive forces between them become stronger, causing the liquid to solidify into a solid.

Changes in pressure can also cause phase changes. For example, increasing the pressure on a gas can cause it to condense into a liquid, while decreasing the pressure on a liquid can cause it to evaporate into a gas.

Overall, phase changes occur because matter is constantly seeking a state of equilibrium, where the energy and forces within the system are balanced. When changes in temperature or pressure disturb this equilibrium, matter will undergo a phase change in order to reach a new equilibrium state.

Changes Between Liquids and Solids

The transition between a liquid and a solid is a common phase change that can occur due to changes in temperature or pressure. This transition is called freezing or solidification when a liquid changes to a solid and melting when a solid changes to a liquid.

When a liquid is cooled, the average kinetic energy of its molecules decreases, and they start to move more slowly. As the temperature decreases further, the molecules lose enough energy to the point where they can no longer overcome the attractive forces between them, and they form a regular lattice structure. This process is called freezing or solidification.

On the other hand, when a solid is heated, the average kinetic energy of its molecules increases, and they start to move more rapidly. As the temperature increases further, the molecules gain enough energy to break free from the attractive forces holding them in a regular lattice structure, and they start to move more freely. This process is called melting.

The temperature at which a liquid freezes or a solid melts is called the melting point or freezing point. The melting point and freezing point of a substance are the same temperature, and they depend on the properties of the substance, such as its molecular structure and intermolecular forces.

There are some substances that do not have a clear-cut melting point or freezing point, and they undergo a gradual change from a solid to a liquid or vice versa. This process is called melting or freezing point depression or elevation, respectively, and it occurs when a solute is added to a solvent. In this case, the solute molecules disrupt the regular lattice structure of the solvent molecules, causing the melting or freezing point to be lowered or raised, respectively.

Changes Between Liquids and Gases

The transition between a liquid and a gas is another common phase change that can occur due to changes in temperature or pressure. This transition is called vaporization when a liquid changes to a gas and condensation when a gas changes to a liquid.

When a liquid is heated, the average kinetic energy of its molecules increases, and they start to move more rapidly. As the temperature increases further, the molecules gain enough energy to overcome the attractive forces holding them together in the liquid state, and they break free to become a gas. This process is called vaporization, and it can occur in two ways:

• Boiling: Vaporization occurs throughout the liquid, including below the surface. This occurs when the temperature of the liquid reaches its boiling point, which is the temperature at which the vapor pressure of the liquid equals the pressure of the surrounding atmosphere.
• Evaporation: Vaporization occurs only at the surface of the liquid. This occurs when the temperature of the liquid is below its boiling point, and molecules at the surface gain enough energy to break free from the liquid’s surface.

On the other hand, when a gas is cooled, the average kinetic energy of its molecules decreases, and they start to move more slowly. As the temperature decreases further, the molecules lose enough energy to the point where they can no longer overcome the attractive forces between them, and they condense into a liquid. This process is called condensation.

The temperature at which a liquid boils or a gas condenses is called the boiling point or condensation point. The boiling point and condensation point of a substance are the same temperature, and they depend on the properties of the substance, such as its molecular structure and intermolecular forces.

There is also a transition called sublimation, which occurs when a solid directly changes into a gas without first becoming a liquid. This occurs when the temperature and pressure conditions are such that the solid’s vapor pressure exceeds the surrounding atmospheric pressure. The reverse of sublimation, where a gas directly changes into a solid, is called deposition.

Changes Between Solids and Gases

The transition between a solid and a gas is a phase change that can occur due to changes in temperature or pressure. This transition is called sublimation when a solid changes directly into a gas and deposition when a gas changes directly into a solid.

Sublimation occurs when a solid’s vapor pressure exceeds the surrounding atmospheric pressure at a given temperature, and the solid molecules escape directly into the gas phase without first melting into a liquid. This process is endothermic, meaning that energy is absorbed by the solid during the phase change. Examples of substances that can undergo sublimation include dry ice (solid carbon dioxide), mothballs (naphthalene), and freeze-dried foods.

Deposition occurs when a gas’s vapor pressure is reduced below its saturation vapor pressure, and the gas molecules condense directly into a solid without first forming a liquid. This process is exothermic, meaning that energy is released by the gas during the phase change. Examples of substances that can undergo deposition include frost forming on a cold surface and snowflakes forming in the atmosphere.

The conditions at which sublimation and deposition occur depend on the properties of the substance, such as its molecular structure and intermolecular forces, as well as the temperature and pressure conditions. Some substances may not undergo sublimation or deposition under normal conditions, while others may do so readily.

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