CBSE Class 9 Science Matter in our Surrounding Exam Notes

CBSE Class 9 Science Matter in our Surrounding Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Matter is all around us, from the food we eat to the air we breathe. This CBSE Class 9 Science exam notes page on Matter in Our Surroundings covers the definition and particle nature of matter, the physical states of matter and their properties, the comparison of solids, liquids and gases, the change of state (melting, boiling, condensation, freezing and sublimation), the effects of temperature and pressure, latent heat of fusion and vaporisation, evaporation and its factors, and diffusion in gases, liquids and solids.

MATTER

Anything which occupies space and has mass is called matter. Food, water, air, clothes, a table, a chair, plants and trees are all matter.

Indian philosophers said that all matter, living or non-living, was made up of five basic elements: air, earth, fire, sky and water.

Matter can be classified on the basis of its physical properties and on the basis of its chemical properties. On the basis of chemical properties, matter is classified as elements, compounds and mixtures.

Everything around us is made of tiny pieces or particles. The particles that make up matter are atoms or molecules.

Characteristics of particles of matter:
1. The particles of matter are very, very small.
2. The particles of matter have spaces between them.
3. The particles of matter are constantly moving.
4. The particles of matter attract each other.

CLASSIFICATION OF MATTER

On the basis of physical states, all matter can be classified into three groups:
1. Solids
2. Liquids
3. Gases

Properties of solids
1. Solids have a fixed shape and a fixed volume.
2. Solids cannot be compressed much.
3. Solids have high densities. They are heavy.
4. Solids do not fill their container completely.
5. Solids do not flow.
Examples: Ice, wood, coal, stone, iron, brick.

Properties of liquids
1. Liquids have a fixed volume but no fixed shape. Liquids take the shape of the vessel in which they are placed.
2. Like solids, liquids cannot be compressed much.
3. Liquids have moderate to high densities. They are usually less dense than solids.
4. Liquids do not fill their container completely.
5. Liquids generally flow easily.
Examples: Water, milk, fruit juice, ink, groundnut oil, kerosene, etc.

Properties of gases
1. Gases have neither a fixed shape nor a fixed volume. Gases acquire the shape and volume of the vessel in which they are kept.
2. Gases can be compressed easily.
3. Gases have very low densities. They are very, very light.
4. Gases fill their container completely.
5. Gases flow easily.
Examples: Air, oxygen, hydrogen, nitrogen.

Comparison of characteristic properties of solids, liquids and gases

CHANGE OF STATE OF MATTER

A substance may exist in any of the three states of matter (solid, liquid or gas), depending upon the conditions of temperature and pressure. By changing these conditions, a substance can be made to exist as a solid, a liquid or a gas.

A solid on heating usually changes into a liquid, which on further heating changes into a gas. Similarly, a gas on cooling condenses into a liquid, which on further cooling changes into a solid.

[Figure: Diagram of the change of state: SOLID changes to LIQUID by melting and LIQUID changes back to SOLID by freezing; the arrows are labelled Gas, Melting and Freezing, showing that the gas state is reached from the liquid state]

The most familiar example is water, which exists in all three states:
(a) Solid: ice
(b) Liquid: water
(c) Gas: water vapour

Ice, a solid, may be melted to form water (liquid), which on further heating changes into steam (gas). These changes can be reversed on cooling.

[Figure: Ice (solid) changes on heating to water (liquid) and then to steam (gas); on cooling, steam changes to water and water changes to ice]

Effect of temperature change: By increasing the temperature (by heating), a solid can be converted into a liquid, and a liquid can be converted into a gas (or vapour). By decreasing the temperature (by cooling), a gas can be converted into a liquid, and a liquid can be converted into a solid.

Solid to liquid change: Melting
Definition: The process in which a solid substance changes into a liquid on heating is called melting (or fusion).
Melting point: The temperature at which a solid substance melts and changes into a liquid at atmospheric pressure is called the melting point of the substance.

In solids, the particles are tightly packed together. When a solid is heated, its particles become more energetic, and their kinetic energy increases. The particles start vibrating more strongly with greater speed. The energy supplied by heat overcomes the intermolecular forces of attraction between the particles. As a result, the particles leave their mean positions and break away from each other. When this happens, the solid melts and a liquid is formed.

Examples:
Melting point of ice = 0 ºC
Melting point of wax = 63 ºC
Melting point of iron = 1535 ºC

The melting point of a solid is a measure of the force of attraction between its particles. The higher the melting point of a solid, the greater the force of attraction between its particles.

Liquid to gas change: Boiling (or vaporisation)
Definition: The process in which a liquid changes rapidly into a gas on heating is called boiling.
Boiling point: The temperature at which a liquid boils and changes rapidly into a gas at atmospheric pressure is called the boiling point of the liquid.

In a liquid, most of the particles are close together. When heat energy is supplied to the liquid, its particles vibrate even faster. Some particles become so energetic that they can overcome the attractive forces of the particles around them. They become free to move and escape from the liquid. When this happens, the liquid evaporates, that is, it starts changing into a gas.

Examples:
Boiling point of water = 100 ºC
Boiling point of alcohol = 78 ºC
Boiling point of mercury = 357 ºC

The boiling point of a liquid is a measure of the force of attraction between its particles. The higher the boiling point of a liquid, the greater the force of attraction between its particles.

When a liquid is heated, the heat energy makes its particles move even faster. At the boiling point, the particles of the liquid have enough kinetic energy to overcome the forces of attraction holding them together, and they separate into individual particles. The liquid then boils to form a gas.

Gas to liquid change: Condensation
The process of changing a gas into a liquid by cooling is called condensation. Condensation is the reverse of boiling.

Liquid to solid change: Freezing
The process of changing a liquid into a solid by cooling is called freezing, or solidification. Freezing is the reverse of melting. So, the freezing point of a liquid is the same as the melting point of its solid form.

Examples:
Melting point of ice = 0 ºC
Freezing point of water = 0 ºC

EFFECT OF CHANGE OF PRESSURE

The three states of matter differ in the intermolecular forces and the intermolecular distances between their constituent particles.

1. Gases are compressible because, on applying pressure, the space between the gaseous particles decreases. Therefore, gases can be compressed readily.
2. When pressure is applied and the temperature is reduced, a gas can be converted into a liquid, that is, the gas is liquefied.
3. The process of converting a gas into a liquid by increasing the pressure or decreasing the temperature is called liquefaction.

A substance may exist in any of the three states of matter depending upon the conditions of temperature and pressure:
(1) If the melting point of a substance is above room temperature at atmospheric pressure, it is said to be a solid.
(2) If the boiling point of a substance is above room temperature under atmospheric pressure, it is classified as a liquid.
(3) If the boiling point of a substance is below room temperature at atmospheric pressure, it is called a gas.

LATENT HEAT

Definition: The heat energy which has to be supplied to change the state of a substance is called its latent heat. Latent heat does not raise the temperature, but it must always be supplied to change the state of a substance. The word 'latent' means 'hidden'.

Every substance has forces of attraction between its particles, which hold them together. To change the state of a substance, these forces of attraction must be broken. The latent heat does not increase the kinetic energy of the particles, so the temperature of a substance does not rise during a change of state.

Latent heat is of two types:

Latent heat of fusion: The heat required to convert a solid into the liquid state is called the latent heat of fusion. In other words, the latent heat of fusion of a solid is the quantity of heat in joules required to convert 1 kilogram of the solid into liquid, without any change in temperature.
Example: The latent heat of fusion of ice = 3.34 × 10⁵ J/kg.

Latent heat of vaporisation: The heat required to convert a liquid into the vapour state is called the latent heat of vaporisation. In other words, the latent heat of vaporisation of a liquid is the quantity of heat in joules required to convert 1 kilogram of the liquid into vapour or gas, without any change in temperature.
Example: The latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.

SUBLIMATION

Definition: The change of a solid directly into vapour on heating, and of vapour directly into solid on cooling, is known as sublimation.

Solid → (on heating) Vapour or gas; Vapour or gas → (on cooling) Solid

The solid substance which undergoes sublimation is said to 'sublime'. The solid obtained by cooling the vapours of the solid is called a 'sublimate'.

Examples: When solid ammonium chloride is heated, it directly changes into ammonium chloride vapour. When the hot ammonium chloride vapour is cooled, it directly changes into solid ammonium chloride. Other substances that sublime are iodine, camphor, naphthalene and anthracene.

EVAPORATION

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

Factors affecting evaporation:

1. Temperature: The rate of evaporation increases with an increase in temperature. This is because, with the increase in temperature, more particles get enough kinetic energy to go into the vapour state.
Example: Clothes dry more rapidly in summer than in winter.

2. Surface area: The rate of evaporation increases with an increase in the surface area of the liquid.
Example: If the same liquid is kept in a test tube and in a china dish, the liquid in the china dish evaporates more rapidly, because more of its surface area is exposed to air.

3. Humidity: Humidity is the amount of water vapour present in air. Air cannot hold more than a definite quantity of water vapour at a given temperature. If the amount of water vapour in air is already large (that is, the humidity is high), the rate of evaporation decreases. Thus, the rate of evaporation increases with a decrease in humidity.
Example: Clothes take longer to dry on a humid day.

4. Wind speed: The rate of evaporation increases with an increase in the speed of the wind. This is because the particles of water vapour are carried away by the wind, which decreases the amount of vapour in the atmosphere.
Example: Clothes dry faster on a windy day.

DIFFUSION

Definition: The spreading out and mixing of a substance with another substance, due to the motion of its particles, is called diffusion. Diffusion is a property of matter which is based on the motion of its particles.

Diffusion is fastest in gases, because the particles in gases move very rapidly. Diffusion is slowest in solids, because the particles in solids do not move much.

The rate of diffusion increases with an increase in the temperature of the diffusing substance. When the temperature is increased by heating, the particles gain kinetic energy and move more rapidly, which increases the rate of diffusion.

Diffusion in gases
Diffusion in gases is very fast, because the particles of gases move very quickly in all directions.
Example: When an incense stick (agarbatti) is lit in a corner of a room, its fragrance spreads through the whole room very quickly, due to the diffusion of its smoke into the air.
Example: When a bottle of perfume is opened in one corner of a room, its smell spreads through the room quickly, due to the diffusion of perfume vapours into the air.

Diffusion in liquids
Diffusion in liquids is slower than in gases, because the particles in liquids move more slowly than the particles in gases.
Example: The spreading of the purple colour of potassium permanganate in water, on its own, is due to the diffusion of potassium permanganate particles into the water.
Example: The spreading of the blue colour of copper sulphate in water, on its own, is due to the diffusion of copper sulphate particles into the water.
The rate of diffusion in liquids is much faster than in solids, because the particles in a liquid move much more freely and have greater spaces between them than the particles in a solid.

Diffusion in solids
Diffusion in solids is a very, very slow process.
Example: If we write something on a blackboard and leave it uncleaned for a long time, it becomes quite difficult to clean the blackboard afterwards. Some particles of chalk have diffused into the surface of the blackboard.
Example: If two metal blocks are bound together tightly and kept undisturbed for a few years, the particles of one metal are found to have diffused into the other metal.

Ø Effect of temperature change

♦ When we apply pressure and reduce temperature the gases can be converted into liquids i.e., gases will be liquefied.

= 22.5 × 105 J/kg