Change in the State of Matter Class 9

Interconversion of States of Matter

Matter is a fascinating and versatile thing – it can transform from one state to another under different conditions including the transitions from solid to liquid to gas and vice versa. This process is known as the inter-conversion of states of matter, and it's a fundamental concept in the study of how substances change.

Key Points:

a) Changing States: At room temperature, water is a liquid. But if you lower the temperature enough, it turns into ice, a solid. And if you heat it up, it becomes steam, a gas. These changes in state occur because of the energy levels of the particles that make up matter.

b) Temperature's Role: Temperature plays a crucial role in these changes. As you heat a substance, its temperature increases, and so does the energy of its particles. At a certain point, the particles have enough energy to break free from their fixed positions, and the substance changes from a solid to a liquid. If you continue to add heat, the particles gain even more energy and start moving even more freely, resulting in a change to a gaseous state.

c) Cooling Down and Reversal: The reverse is true as well. When you cool a gas, its particles lose energy and slow down. As a result, the gas condenses into a liquid. Further cooling causes the particles to lose even more energy, and the liquid turns into a solid.

d) Common Examples: We see the interconversion of states of matter all around us. Steam rising from a hot cup of tea is water vapour changing into a gas due to the heat. Dew forming on the grass in the early morning is water vapour in the air condensing into tiny water droplets as it cools down.

Effect of Change of Temperature on States of Matter

Temperature plays a crucial role in determining the state of matter. As the temperature of a substance changes, its particles gain or lose energy, leading to transitions between solid, liquid, and gas states.

a) Solid to Liquid (Melting): When you heat a solid, such as ice, its temperature rises. The increase in temperature causes the particles within the solid to vibrate more vigorously. As the particles gain energy, the forces holding them in a rigid structure start to weaken. Eventually, the particles overcome these forces and start moving more freely, resulting in the solid turning into a liquid. This transition is known as melting.

b) Liquid to Gas (Evaporation): As you heat a liquid, like water, its temperature increases. The added energy causes the liquid particles to move faster and collide more frequently. Some particles near the surface gain enough energy to overcome the attractive forces holding them in the liquid phase. They escape into the air as gas particles, creating water vapour. This process is called evaporation.

c) Liquid to Solid (Freezing): When you cool down a liquid, its temperature decreases. The decrease in temperature causes the particles to lose energy and slow down. As the particles move more slowly, the attractive forces between them become stronger. Eventually, the particles arrange themselves into a more ordered structure, forming a solid. This transition is known as freezing.

d) Gas to Liquid (Condensation): As you cool a gas, like steam, its temperature decreases. The gas particles lose energy and slow down. The decrease in kinetic energy weakens the repulsive forces between particles and allows attractive forces to dominate. The gas particles come closer together and form liquid droplets. This change from a gas to a liquid is called condensation.

Boiling Point, Melting Point and Freezing Point

Boiling point, melting point, and freezing point are important temperature values that characterise the transitions between different states of matter.

Boiling Point

1. The boiling point of a substance is the temperature at which its liquid state changes into a gaseous state. It's the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure.
2. Vapour pressure is the pressure created by some of the liquid turning into gas. When a liquid is in a closed container, some of its particles escape and become gas. These gas particles push on the container walls, creating pressure. Higher temperatures make more particles turn into gas, so the vapour pressure increases with heat. When the pressure from the gas matches the air pressure, the liquid starts boiling. Vapour pressure is important in processes like boiling and evaporation.
3. Different substances have different boiling points due to variations in their intermolecular forces.
4. For example, the boiling point of water is 100^oC (212^oF) at standard atmospheric pressure. When the water reaches this temperature, it starts to boil and transform into steam, which is its gaseous state.

Melting Point

1. The melting point of a substance is the temperature at which it changes from a solid to a liquid. At the melting point, the solid gains enough thermal energy to overcome the forces of attraction between its particles, causing them to move more freely and transform into a liquid state.
2. For example, the melting point of ice (solid water) is 0^oC (32^oF). When you heat ice to this temperature, it melts and becomes liquid water.

Freezing Point

1. The freezing point of a substance is the temperature at which its liquid state changes into a solid state. It's essentially the reverse of the melting point. At the freezing point, the particles in the liquid lose enough thermal energy to form a solid structure, and the liquid solidifies.
2. The freezing point of a substance is the same as its melting point. For example, the freezing point of water is also 0^oC (32^oF). When liquid water is cooled to this temperature, it freezes and turns into ice.

It's important to note that the boiling point, melting point, and freezing point of a substance can change with variations in pressure.

Effect of Change of Pressure on States of Matter

Pressure is another crucial factor that can influence the state of matter. Changes in pressure can lead to transitions between solid, liquid, and gas states.

Let's explore how pressure affects different states of matter:

a) Gas to Liquid (Condensation under Pressure): When you increase the pressure on a gas, its particles are forced to come closer together. As the gas particles are compressed, their kinetic energy decreases, and the attractive forces between them become more significant. Eventually, the gas particles condense and form liquid droplets. This transition from a gas to a liquid due to increased pressure is known as condensation.

b) Liquid to Solid (Solidification under Pressure): When pressure is applied to a liquid, its particles are pushed closer together. The attractive forces between the particles become stronger, and they arrange themselves in a more ordered manner. This leads to a decrease in the kinetic energy of the particles, and the liquid transforms into a solid. This process is called solidification.

c) Gas to Solid (Deposition): When a gas is subjected to high pressure, its particles lose kinetic energy and slow down. The decrease in energy causes the gas particles to come closer together and stick to surfaces. Instead of transitioning through the liquid state, the gas particles directly form a solid on the surfaces. This process is called deposition.

d) Solid to Gas (Sublimation under Pressure): Sublimation is the process in which a solid directly transforms into a gas without passing through the liquid state. When pressure is applied to a solid, its particles gain energy and start moving more rapidly. The increased kinetic energy allows the particles to overcome the attractive forces that hold them in a solid structure. This results in the solid changing into a gas.

Evaporation

Evaporation is a natural process that occurs when a liquid substance, like water, changes into a gaseous state, typically as a result of heating. It's a crucial part of the water cycle and plays a significant role in cooling, weather patterns, and everyday experiences like drying clothes.

Key Points:

a) Particle Movement: In a liquid, like water, the particles are in constant motion due to their kinetic energy. While most particles stay close to the surface of the liquid due to intermolecular forces, some particles at the surface have higher energy and can break free from these forces.

b) Energy Absorption: When heat is applied to the liquid, it increases the kinetic energy of the particles. This extra energy gives them a greater chance of escaping the intermolecular forces that hold them in the liquid state.

c) Escape to Vapour: The particles with the highest energy at the liquid's surface gain enough energy to overcome these forces and transition to the gaseous state. These particles become water vapour molecules, which are individual water particles floating in the air.

d) Cooling Effect: As particles leave the liquid and become vapour, they take away energy from the remaining liquid. This leads to a cooling effect on the liquid's surface. You might have experienced this cooling sensation when water evaporates from your skin after a swim.

Factors Affecting Evaporation

Several factors influence the rate of evaporation:

a) Temperature: Higher temperatures provide more energy to the particles, increasing their chance of escaping from the liquid. Thus, evaporation is faster at higher temperatures.

b) Surface Area: Larger surface areas expose more liquid particles to the air, allowing more particles to escape. This is why wet clothes dry faster on a clothesline.

c) Humidity: Humidity refers to the amount of moisture already present in the air. When the air is already humid (high moisture content), it becomes more difficult for water vapour to escape the liquid and evaporate.

d) Wind Speed: Wind blows away the water vapour molecules from the liquid's surface, creating a lower concentration of vapour near the liquid. This concentration difference encourages more particles to evaporate, increasing the rate of evaporation.

Real-Life Applications

Evaporation has practical applications in our daily lives. It's the reason wet clothes dry after washing, why we feel cool when we're wet in front of a fan, and why our bodies sweat to regulate temperature. It's also a crucial process in nature, powering the water cycle, which includes processes like condensation and precipitation that maintain Earth's water balance.

Latent Heat

Latent heat is a concept in thermodynamics that refers to the heat energy absorbed or released during a change of state of a substance without a change in temperature. It's the energy required to bring about a transition between different states of matter (solid, liquid, gas) while keeping the temperature constant. The term "latent" here means hidden, as this heat energy doesn't cause a temperature change but is involved in the change of state itself.

There are two main types of latent heat:

a) Latent Heat of Fusion: This is the amount of heat energy required to change a substance from a solid state to a liquid state or vice versa, at its melting or freezing point, respectively. During this process, the temperature remains constant while the solid melts or the liquid freezes. Heat energy is used to weaken the intermolecular forces holding the particles together in a solid or to strengthen them in a liquid.

b) Latent Heat of Vaporisation: This is the amount of heat energy needed to change a substance from a liquid state to a gaseous state or vice versa, at its boiling or condensation point, respectively. Similar to fusion, the temperature remains constant during this transition. Heat energy is used to overcome the intermolecular forces in the liquid or to create them in the gas.

Real-Life Examples: When ice melts to water, it absorbs the latent heat of fusion from its surroundings, causing the temperature to remain constant during the melting process.
When water vapour condenses into liquid water, it releases latent heat of vaporisation into the surroundings, causing the temperature to remain constant during condensation.

Frequently Asked Questions

1. How does the interconversion of states of matter differ from a chemical change?

In the interconversion of states of matter, only the physical state changes, but the chemical composition of the substance remains the same. For example, when ice melts into water, both are chemically H?O. In a chemical change, the composition changes, resulting in the formation of new substances.

2. Why do solids melt when heated and liquids evaporate when further heated?

Solids melt when heated because the heat energy causes the particles to vibrate more vigorously, weakening the bonds between them, and turning the solid into a liquid. Liquids evaporate when further heated because the particles gain enough energy to overcome the intermolecular forces and escape into the air as gas.

3. What is the melting point, and how does it relate to the freezing point of a substance?

The melting point is the temperature at which a solid turns into a liquid, while the freezing point is the temperature at which a liquid turns into a solid. For most substances, the melting point and freezing point are the same.

4. How does pressure influence the liquefaction of gases?

Under high pressure, gases can be compressed into liquids. This is because increased pressure reduces the space between gas particles, bringing them close enough to form intermolecular attractions, resulting in the gas condensing into a liquid.

5. What is latent heat, and why is it important in the phase change of matter?

Latent heat is the amount of heat energy absorbed or released by a substance during a phase change without changing its temperature. It is important because it provides the energy required to overcome intermolecular forces and change the state of matter.