CBSE Science Notes Class 9 | Matter in our surroundings

Introduction

There are a large number of things around us that we see and feel. For example, you can see the book in front of you. The book occupies some space. Space occupied by the book is called its volume. If you pick up the book, you can also feel its mass. You cannot see the air around you, yet if you feel a balloon with air and weigh it carefully,you will find that not only does air occupy space, but it also has mass. Things like a book, air, water, wood, cloth, etc., are examples of matter. Anything that occupies space and has mass, is called matter.

Substance

A substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. For example, sugar dissolve in water can be separated from water by simply evaporating the water. Here sugar is a substance that cannot be broken into its components by any physical process. Similarly, sodium chloride, lime (calcium oxide), etc., are all substances.

Physical nature of matter.

The matter is made of particles
The particles of matter are too small

1. The matter is made of particles

The matter is made of discrete particles. This can be easily verified by performing the following activity.

Solid dissolves in water, the volume of the water does not change.

Add some sugar to the water and dissolve it with the help of a glass rod. You will see that the sugar has disappeared, but there is no change in the level of water. The particles of sugar dissolved in water occupy the space between the particles of water. That is why the water level in the beaker did not rise. Had sugar been continuous like a block of wood, the water level in the beaker would have risen.

2. The particles of matter are too small

Experiments

Take 2-3 crystals of potassium permanganate and dissolve them in 100 ml of water. Take out approximately 10 ml of this solution and put it into 90 ml of clear water. Take out 10 ml of this solution and put it into another 90 ml of clear water. Keep diluting the solution like this 5 to 8 times. Is the water still coloured ?

With every dilution, though the colour becomes light, it is still visible

This experiment shows that just a few crystals of potassium permanganate can colour a large volume of water (about 1000 L). So we conclude that there must be millions of tiny particles in just one crystal of potassium permanganate, which keeps on dividing itself into smaller and smaller particles.

The same activity can be done using 2 mL of Dettol instead of potassium permanganate. The smell can be detected even on repeated dilution

Characteristics of Particles of Matter

Particles of matter have space between them.
Particles of matter are continuously moving.
Particles of matter attract each other.

1) Particles of matter have space between them

Particles of sugar, salt, Dettol, or potassium permanganate got evenly distributed in the water. Similarly, when we make tea, coffee or lemonade (nimbu paani ), particles of one type of matter get into the spaces between particles of the other. This shows that there is enough space between particles of matter.

Activity 1

➠ Put an unlit incense stick in a corner of your class. How close do you have to go near it so as to get its smell?

➠ Light the incense stick. What happens? Do you get the smell sitting at a distance?

Activity 2

➠ Take two glasses/beakers filled with water.

➠ Put a drop of blue or red ink slowly and carefully along the sides of the first beaker and honey in the same way in the second beaker.

➠ What do you observe immediately after adding the ink drop?

➠ What do you observe immediately after adding a drop of honey?

Activity 3

➠ Drop a crystal of copper sulphate or potassium permanganate into a glass of hot water and another containing cold water. Do not stir the solution.

➠ What do you observe just above the solid crystal in the glass?

➠ Does the rate of mixing change with temperature? Why and how?

2. Particles of matter are continuously moving

Particles of matter are continuously moving, that is, they possess what we call kinetic energy. As the temperature rises, particles move faster. So, we can say that with the 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.

Diffusion👉 The intermixing of particles of two different types of matter on their own is called diffusion.

We also observe that on heating, diffusion becomes faster.

States of Matter

👉Matter around us 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

Properties of these three states of matter

Change of State of Matter

1. By changing the temperature

2. By changing the pressure

EFFECT OF CHANGE OF TEMPERATURE

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

When a solid melts, its temperature remains the same, so where does the heat energy go?

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.

Latent Heat of Fusion

👉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.

👉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 ℃ = 273 + 100 = 373 K)`.

Sublimation

👉Sublimation is the change of solid state directly to gaseous state without going through liquid state.

Deposition

👉Deposition is the change of gaseous state directly to solid state without going through liquid state.

EFFECT OF CHANGE OF PRESSURE

Applying pressure and reducing temperature can liquefy gases.

Solid carbon dioxide `(CO_2 )` is stored under high pressure. Solid `CO_2` gets converted directly to 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

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.

Evaporation is a surface phenomenon.

👉 The process of a liquid changing into vapour even below its boiling point is called evaporation.

Factors Affecting Evaporation

1. Temperature

2. Surface area

3. Humidity

4. Wind speed

COOLING CAUSED BY EVAPORATION

The cooling caused by evaporation is based on the fact that when a liquid evaporates, it draws the latent heat of vaporisation from anything that it touches. By losing heat, anything gets cooled.

The particles of liquid absorb energy from the surroundings. This absorption of energy from the surroundings makes the surroundings cold.

When you pour some acetone (nail polish remover) on your palm. The particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool

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.

References

Foundation Science Chemistry Class IX
NCERT Science Class IX
Chemistry Class IX (S. Chand)