Class 9 Science Chapter 1 Matter in our Surroundings NCERT Notes

Chapter 1 Matter in our Surroundings Class 9 Science CBSE NCERT Notes

The matter is the material of which everything in this universe, (in and around us) is made up of. It is anything that occupies space and has mass and offers resistance to any applied force.

Classification of matter

Early Indian philosophers classified in the form of five basic elements as air, earth, fire, sky and water called Panch Tatva.

On the basis of the physical state matter is classified as solids, liquids and gases.

On the basis of chemical composition matter is classified as pure substances and mixtures.

Physical nature of matter

Matter is made up of particles.
The particles of matter are very tiny.
The particles of matter have space between them.
The particles of matter are continuously moving.
The particles of matter attract each other.

Matter is made up particles

Matter is made up of small particles. It can be proved with the help of following experiment.

Experiment: To show that matter is made up of small particles.

Materials Required: 100 ml beaker, water, salt, glass rod.

Procedure: Take a 100 ml beaker and fill it with water and mark the level of water.
Dissolve the given salt with the help of glass rod.
Observe the change in the water level and record your observations.

Observations: The salt gets dissolved in water. The particles of salt have entered the space between water molecules, therefore, the level of water does not change.

Conclusion: The salt consists of large number of small particles.

Size of Particles of Matter

The particles of matter are extremely small in size which cannot be seen even with powerful microscope. Their size can be observed with the help of following experiment.

Experiment: To show that matter is made up of very small particles.

Materials Required: Crystals of KMnO4 (potassium permanganate), water, beaker.

Procedure: Take 2-3 crystals of KMnO₄ and dissolve them in 50 ml of water.
Take 5 ml of solution from above beaker and put it into 50 ml of water and observe the colour of solution.
Take 5 ml of solution from above beaker and put it into 50 ml of water in third beaker and observe the colour of solution.

Observations: The colour of solution remains purple in all the beakers.

Conclusion: It shows that even 2-3 crystals of KMnO₄ consist of millions of small particles which dissolve in water giving purple colour to the solution.

Space between Particles of Matter

When we dissolve sugar, salt or KMnO₄ in water, particles get evenly distributed in water. Similarly, when we prepare tea, coffee the particles of one type of matter get into space between particles of the other. This shows that there is enough space between particles of matter.

Continuous Movement of Particles

Experiment: To show the particles of matter are continuously moving. Materials Required: Incense stick agarbati, match box.

Procedure:
Put an unlit incense stick in a corner of your class.
Go close to the incense stick to get its smell.
Now light the incense stick. Now try to get the smell from a distance.

Observations: The smell of unlit incense stick can be observed only by going close to it whereas the smell of lighted incense stick can be observed from a distance.

Conclusion : The particles of matter are continuously moving but the speed of particles is very slow. The speed of particles increases with the increase in temperature.

Attraction between Particles of Matter

There is force of attraction between particles of matter. It can be explained with the help of following game in the field.

Make four groups and form human chains as follows.
The first group should hold each other from back and lock arms like Bihu dancers.
The second group should hold hands to form human chain.
The third group should form a chain by touching each other with only their finger tips.
The fourth group should run around and try to break three human chains one by one into groups as small as possible.

Observations

The third group is easiest to break because of least force of attraction. It is similar to particles in gaseous state.

The first group is most difficult to break due to maximum force of attraction. It represents particles present in solid state.

The second group requires little force to break which shows it has force of attraction less than first group but more than third group. It represents particles in the liquid state. Even in solids force of attraction differs from one substance to another. There is maximum force of attraction between particles of iron nail, less in a piece of chalk and least in rubber band.

Due to the force of attraction between liquid particles, cutting a stream of water with your fingertips is difficult. As a result, there is a force of attraction between matter particles that binds them together. The strength of forces vary depending on the kind of substance.

States of matter

Matter exists in three different states. They are

Solid
Liquid
Gas

Properties of solids

Solids have definite shapes and fixed volume.
The space between the particle is minimum.
The force of attraction between the particles is maximum.
The movement of the particles is minimum.
They are least compressible.
Their rate of diffusion is least.

Properties of liquids

Liquids have no definite shape but have fixed volume. Liquids take the shape of the container.
The space between the particles is intermediate.
The force of attraction between the particles is intermediate.
The movement of the particles is intermediate.
They are less compressible.
Their rate of diffusion is more than solids.

Properties of gases

Gases have no definite shape or fixed volume. Gases occupy the whole space of the container.
The space between the particles is maximum.
The force of attraction between the particles is minimum.
The movement of the particles is maximum.
They are most compressible.
Their rate of diffusion is more than solids and liquids.

Change of state

When a solid is heated it changes into liquid. When a liquid is heated it changes into gas.

When a gas is cooled it changes to liquid. When a liquid is cooled it changes into solid.

For example, If ice is heated it changes into water. If water is heated it changes into steam. If steam is cooled it changes into water. If water is cooled it changes into ice.

Melting (Fusion)

When a solid is heated, the particles begin to vibrate with greater speed and begin to move more freely. Then at a particular temperature the solid melts and changes into liquid. The process of melting is also known as fusion.

The temperature at which a solid melts is called its melting point. The melting point of ice is 0°C or 273 K.

Latent heat of fusion

The amount of heat energy required to change 1kg of a solid into liquid at atmospheric pressure at its melting point is called the latent heat of fusion.

Boiling

When a liquid is heated, its particles begin to move even faster. Then at a particular temperature the liquid begins to boil and changes into gas (vapour).

Boiling is a bulk phenomenon. When a liquid boils the bulk of the liquid changes into vapour.

The temperature at which a liquid starts boiling is called its boiling point. The boiling point of water is 100°C or 373K (273 + 100).

Latent heat of vaporisation

The amount of heat energy required to change 1kg of a liquid into gas at atmospheric pressure at its boiling point is called the latent heat of vaporisation.

Sublimation

The change of state directly from solid to gas or from gas to solid is called sublimation.

For example, If solid camphor or ammonium chloride is heated, it changes into vapour. If the vapours are cooled it changes into solid.

Effect of pressure on gases

When pressure is applied on gas the particles come closer and the gas changes into liquid.

We can liquefy gases by applying pressure and reducing the temperature.

Compressed solid carbon dioxide is called dry ice. If the pressure is reduced it changes directly to gas without coming into liquid state. So solid carbon dioxide is known as dry ice.

Interconversion of the three states of matter

The states of matter are inter convertible. The state of matter can be changed by changing the temperature or pressure.

Evaporation

The change of a liquid into vapour at any temperature below its boiling point is called evaporation.

Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction and changes to vapour state.

Factors affecting evaporation

The rate of evaporation depends upon surface area, temperature, humidity and wind speed.

Increase in the surface area increases the rate of evaporation. Increase in temperature increases the rate of evaporation. Increase in humidity decreases the rate of evaporation. Increase in wind speed increases the rate of evaporation.

Evaporation causes cooling

When a liquid evaporates, the particles of the liquid absorb heat from the surroundings and evaporates. So the surroundings become cold.

For example, people sprinkle water on the roof or open ground because during evaporation water absorbs heat makes the hot surface cool. During summer we sweat more because during evaporation the sweat absorbs heat from our body making the body cool.

Wearing cotton clothes in summer keeps us cool because cotton absorbs sweat and when the sweat evaporates it absorbs heat from our body making the body cool.