Matter in Our Surroundings: Class 9 Science NCERT Chapter 1 Note

Everything in this universe is made up of material which scientists have named “matter”. The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand – every thing is matter.

Early Indian philosophers classified matter in the form of five basic elements – the “Panch Tatva”– air, earth, fire, sky and water.

1.1 Physical Nature of Matter

1.1.1 MATTER IS MADE UP OF PARTICLES

There are two schools of thought prevailed regarding the nature of matter. One school believed matter to be continuous like a block of wood, whereas, the other thought that matter was made up of particles like sand.

1.2 Characteristics of Particles of Matter

1.2.1 PARTICLES OF MATTER HAVE SPACE BETWEEN THEM

The particles of sugar, salt, Dettol, or potassium permanganate got evenly distributed in water.

Example, 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.

1.2.2 PARTICLES OF MATTER ARE CONTINUOUSLY MOVING

• Particles of matter are continuously moving, that is, they possess what we call the kinetic energy.
• As the temperature rises, particles move faster.
• When it increases in temperature the kinetic energy of the particles also increases.
• The intermixing of particles of two different types of matter on their own is called diffusion.
• On heating, diffusion becomes faster.

1.2.3 PARTICLES OF MATTER ATTRACT EACH OTHER

The 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.

1.3 States of Matter

The 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.

1.3.1 THE SOLID STATE

All the 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.

All the above are solids as:

• A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks.
• The shape of each individual sugar or salt crystal remains fixed, whether we take it in our hand, put it in a plate or in a jar.
• A sponge has minute holes, in which air is trapped, when we press it, the air is expelled out and we are able to compress it.

1.3.2 THE LIQUID STATE

• The 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.
• The solids and liquids can diffuse into liquids. The gases from the atmosphere diffuse and dissolve in water.
• These gases, especially oxygen and carbon dioxide, are essential for the survival of aquatic animals and plants.
• All living creatures need to breathe for survival.
• The aquatic animals can breathe under water due to the presence of dissolved oxygen in water.
• The solids, liquids and gases can diffuse into liquids.
• The rate of diffusion of liquids is higher than that of solids. This is due to the fact that in the liquid state, particles move freely and have greater space between each other as ompared to particles in the solid state.

1.3.3 THE GASEOUS STATE

• The gases are highly compressible as compared to solids and liquids.
• The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking
• or the oxygen supplied to hospitals in cylinders is compressed gas.
• Compressed natural gas (CNG) is used as fuel these days in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.
• With the rate of diffusion of solids and liquids, high speed of particles and large space
• between them, gases show the property of diffusing very fast into other gases.
• 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.

1.4 Can Matter Change its State?

• The water can exist in three states of matter–
• Solid, as ice,
• Liquid, as the familiar water, and
• Gas, as water vapour

1.4.1 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.

Latent heat: The 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.

1.4.2 EFFECT OF CHANGE OF PRESSURE

Applying pressure and reducing temperature can liquefy gases.

The solid carbon dioxide CO_{2} stored under high pressure. Solid carbon dioxide 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 CO_{2} is also known as dry ice.

1.5 Evaporation

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

1.5.1 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:

1.5.2 HOW DOES EVAPORATION CAUSE COOLING?

In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold.

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?

When take some ice-cold water in a tumbler. Soon we will see water the water droplets on the outer surface of the tumbler. 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.