Matter in Our Surroundings
Class 09 ScienceMatter is made up of small particles. The matter exists in three states - solid, liquid and gas. The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases.
Characteristics of Particles of Matter
There is enough space between particles of matter.
Particles of matter are continuously moving, that is, they possess kinetic energy. As the temperature rises, particles move faster. So, with increase in temperature the kinetic energy of the particles also increases.
Particles of matter intermix on their own with each other. They do so by getting into the spaces between the particles. This intermixing of particles of two different types of matter on their own is called diffusion.
Particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another.
States of Matter
Matter exists in three different states - solid, liquid and gas. These states of matter arise due to the variation in the characteristics of the particles of matter.
Solid State
Solids have a definite shape, distinct boundaries and fixed volumes, that is, have negligible compressibility. Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
Liquid State
Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid.
Gaseous State
Gases are highly compressible as compared to solids and liquids.
In the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.
Change of State
Water can exist in three states of matter:
- Solid, as ice
- Liquid, as water
- Gas, as water vapour
Solid to Liquid
On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely.
A stage is reached when the solid melts and is converted to a liquid. The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of ice is 273.15 K. The process of melting, that is, change of solid state into liquid state is also known as fusion.
During melting, the temperature of the system does not change after the melting point is reached. This happens even though we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles.
As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat. The word latent means hidden. The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion. So, particles in water at 0°C (273 K) have more energy as compared to particles in ice at the same temperature.
Liquid to Gas
When we supply heat energy to water, particles start moving even faster. At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.
For water, this temperature is 373 K (100°C = 273 + 100 = 373 K).
Particles in steam, that is, water vapour at 373 K (100° C) have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.
Solid to Gas / Gas to Solid
A change of state directly from solid to gas without changing into liquid state is called sublimation and the direct change of gas to solid without changing into liquid is called deposition.
Effect of Change of Pressure
Applying pressure and reducing temperature can liquefy gases.
For example, solid carbon dioxide (CO2) is stored under high pressure. It gets converted directly into gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice.
Evaporation
At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of liquid into vapours at any temperature below its boiling point is called evaporation.
Factors Affecting Evaporation
The rate of evaporation increases with:
- an increase of surface area
- an increase of temperature
- a decrease in humidity
- an increase in wind speed
Why should we wear cotton clothes in summer?
During summer, we perspire more because of the mechanism of our body which keeps us cool. During evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.
Why do we see water droplets on the outer surface of a glass containing ice-cold water?
The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.