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CBSE NOTES CLASS 9 SCIENCE

CHAPTER 1 MATTER IN OUR SURROUNDINGS

Matter

Physical Nature of Matter

1. Matter is made up of particles

2. Particles of matter are very small

Characteristics of particles of matter

1. Particles of matter have space between them

2. Particles are in continuous motion

3. Particles of matter attract each other

States of matter

Solid Liquid Gas

Plasma and Bose-Einstein condensate

Explanation for Properties of Solids

Explanation for Properties of Liquids

Explanation for Properties of Gases

Interchange of states of matter

Effect of Change of Temperature on Solids

Melting Point

Latent Heat of Fusion

Effect of Change of Temperature on Liquids

Boiling Point

Latent Heat of Vaporization

Effect of Change of Pressure

Sublimation

Evaporation

Factors Affecting Evaporation

Evaporation Causes Cooling

Evaporation Vs Boiling (difference)

Different Scales of Temperature

Pressure and Its Units

CBSE NOTES CLASS 9 SCIENCE

CHAPTER 1 MATTER IN OUR SURROUNDINGS

Matter

Anything that occupies space and has mass is called matter.

Physical Nature of Matter

1. Matter is made up of particles

If we dissolve some sugar/salt in water, we see that there is no change in water level and sugar/salt simply disappears.

We can explain this only if we assume that the matter (sugar/salt) is made up of particles.

The particles of sugar/salt have disappeared in the space between particles of water.

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2. Particles of matter are very small

The particles of matter are very small. We cannot see them by naked eye.

Activity

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(a) Take 2-3 crystals of potassium permanganate and dissolve them in 100 mL of water.

(b) Take out approximately 10 mL of this solution and put it into 90 mL of clear water.

(c) Take out 10 mL of this solution and put it into another 90 mL of clear water.

(d) If we keep dividing the solution like this 5 to 8 times, we observe that the water is still coloured.

  • There are millions of tiny particles in just one crystal of potassium permanganate, hence it can colour a large amount of water (1000 L).

  • We can thus conclude that the particles of matter are very small.

Characteristics of particles of matter

1. Particles of matter have space between them

In the above activity we observe that the potassium permanganate got evenly distributed in water. We can say that the particles of potassium permanganate have occupied the space between particles of water.

2. Particles are in continuous motion

  • When we put burning incense stick in a corner of the room, we can smell it from another corner of the room.

  • We can smell the food being cooked in the kitchen from another part of the house.

  • The particles of potassium permanganate spread throughout the volume of water and we can see the particles moving through water.

3. Particles of matter attract each other

  • Particles of matter have force acting between them, just like the force between Bihu dancers. 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 can exist in five states,

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Solid

Liquid

Gas

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Definite shape, distinct boundaries and fixed volumes

Shape not definite but definite volume.

Neither definite shape nor definite volume.

Inter particle distances are smallest.

Inter particle distances are larger than in solids but smaller than in gases.

Inter particle distances are largest.

Incompressible.

Slightly compressible

Highly compressible.

High density

Density is lower than solids, higher than gases.

Density of gases is least.

Inter particle forces of attraction in solids are strongest.

Inter particle forces of attraction in liquids are weaker than solids, but stronger than gases.

Inter particle forces of attraction in gases are the weakest.

They are rigid due to fixed shape and do not flow.

Liquids flow and change shape, so they are not rigid but are fluids.

They are fluids

Particle of solids diffuse slowly

Particles of liquids diffuse faster than solids, but slower than gases.

Particles of gases diffuse fastest.

Some solids are brittle, some are malleable and ductile.

Not malleable and ductile

Not malleable and ductile

  • A rubber band changes its shape on stretching, why is it still called solid?

    A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks. Hence it is solid.


  • When kept in different jars sugar and salt take the shape of the jar. Are they solid?

    The shape of each individual sugar or salt crystal remains fixed whether we take it in our hands, put it in a plate or in a jar. Hence it is solid.


  • Even though a sponge is solid, we are able to compress it. How?

    A sponge has minute holes, in which air is trapped. When we press the sponge, the air is expelled out and we are able to compress it. When it is released, the air gets into it again.


  • 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 compared to particles in the solid state.

  • In the gaseous state, the particles move about randomly at high speed and have large space between each other. Hence they diffuse fastest.

  • Due to the random movement, the particles of gases hit each other and also on the walls of the container. The striking particles exert force on the walls of the container. The force exerted by gas particles per unit area on the walls of the container is called pressure.
Plasma (non–evaluative)
  • Plasma is an ionized gas.

  • Plasma is a very good conductor of electricity and is affected by electric and magnetic fields.

  • Plasma, like gases have an indefinite shape and an indefinite volume.
Bose-Einstein condensate (non–evaluative)
  • In a BEC, the molecules are really close to each other (even closer than atoms in a solid).

  • BEC is a state of matter that can arise at very low temperatures.

  • The scientists who worked with the Bose-Einstein condensate received a Nobel Prize for their work in 1995.

Explanation for Properties of Solids

  • Solids have a definite shape and a definite volume because the particles are locked (fixed) in their place.

  • Solids do not flow because the particles cannot move/slide past one another.

  • Solids are not easily compressible because there is very little free space between particles.

Explanation for Properties of Liquids

  • Liquids are not easily compressible and have a definite volume because there is little free space between particles.

  • Liquids have indefinite shape because the particles can slide past one another.

  • Liquids flow easily because the particles of liquids can move/slide past one another easily, due to weaker attraction force compared to solids. Hence they are fluids.

Explanation for Properties of Gases

  • Gases are highly compressible because there is large free space between the particles of a gas.

  • Gases flow very easily because the particles randomly move past one another. Hence they are fluids.

  • Gases have indefinite shape and indefinite volume because the particles of a gas have large spaces between them and they can move past one another easily.

Interchange of states of matter

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Matter can change from one state to another by changing temperature or pressure.

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For example water can exist in three states of matter, solid as ice, liquid as water, and gas as water vapours.

Effect of Change of Temperature on Solids

Particles of solid are vibrating around mean position. 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 speeds. 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 particles move sufficiently away from each other, the solid melts and gets converted to a liquid.

Melting Point

  • When a solid melts, its temperature remains constant. The temperature, at which a solid melts to become a liquid at atmospheric pressure, is called its melting point.

  • The melting point of ice is 0oC or 273.16 K.

  • The process of melting, that is, change of solid state into liquid state is also known as fusion.

Latent Heat of Fusion - The amount of heat energy required to change 1 kg of a solid into liquid at atmospheric pressure and at its melting point, is known as the latent heat of fusion.

Effect of Change of Temperature on Liquids

When we supply heat energy to a liquid, its temperature rises and particles start moving faster, due to increase in kinetic energy. At a certain temperature, a stage 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.

Boiling Point

  • When a liquid vaporizes, its temperature remains constant. The temperature, at which a liquid starts changing into gas (boiling) at atmospheric pressure, is known as its boiling point.

  • Boiling point of water is 100 oC or 373.16 K.

  • Boiling is a bulk phenomenon. Particles from the bulk of the liquid gain enough energy to change into the vapour state.

Latent Heat of Vaporization - The amount of heat energy required to change 1 kg of a liquid into gas at atmospheric pressure and at its boiling point, is known as the latent heat of vaporization.

  • Particles in water at 0 0C (273 K) have more energy as compared to particles in ice at the same temperature. This is because the particles of water have absorbed extra energy in the form of latent heat of fusion.

  • Particles in steam, that is, water vapour at 373.6 K (100 0C) have more energy than those of water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporization.

Sublimation

The changing of solid, directly into vapours on heating & vapours, into solid on cooling, is called sublimation. Examples of sublimable substances are ammonium chloride, camphor, naphthalene & iodine.

Effect of Change of Pressure

Increasing or decreasing the pressure can change the state of matter. Applying pressure and reducing temperature can liquefy gases.

Solid carbon dioxide (CO2) is stored under high pressure. Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. Solid carbon dioxide is also known as dry ice.

Evaporation

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

Explanation

Particles of matter are always moving and are never at rest. At a given temperature, in any gas, liquid or solid, there are particles with different amounts of kinetic energy.

In case of liquids, a small fraction of particles at the surface, having higher kinetic energy, are able to break away from the forces of attraction of other particles and get converted into vapour.

  • Evaporation is a surface phenomenon as only the particles at the surface, which have enough kinetic energy, are evaporated.

Factors Affecting Evaporation

The rate of evaporation increases,

  • With an increase in surface area.

    Evaporation is a surface phenomenon as only the particles at the surface, which have enough energy, are evaporated.

    For example, while putting clothes for drying up, we spread them out.


  • With the increase in temperature.

    With the increase of temperature more number of particles acquire enough kinetic energy to go into the vapour state.

    For example, on a hot summer day, the clothes dry up faster.


  • Decrease in humidity.

    Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases.

    For example the clothes dry slowly on a rainy day.


  • Increase in wind speed.

    With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding, thus causing more evaporation.

    For example, clothes dry faster on a windy day.

Evaporation Causes Cooling

The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surroundings cold.

Examples

  • When we pour some acetone (nail polish remover) on our palm, it causes cool sensation.

    The particles of acetone gain energy from our palm or surroundings and evaporate causing the palm to feel cool.


  • After a hot sunny day, people sprinkle water on the roof or on open ground because the large latent heat of vaporization of water helps to cool the hot surface.

  • 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 vaporization is absorbed from the body leaving the body cool.

    Cotton is a good absorber of water. It 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 loses energy on coming in contact with the cold glass of water, and gets converted to liquid state, which we see as water droplets.


  • Why does a desert cooler cool better on a hot dry day?

    A desert cooler, cools better on a hot dry day because the temperature is high and humidity is less, which helps in better evaporation. Due to the higher rate of evaporation the desert cooler gives better cooling effect.


  • How does the water kept in an earthen pot (matka) become cool during summer?

    In an earthen pot, water gets evaporated quickly through the pores of the earthern pot. Heat energy is utilized during the process of evaporation. This energy is taken from the remaining particles of water, which leads to a lowering of temperature in the remaining amount of water.


  • Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

    A saucer has larger surface area than the cup. Hence rate of evaporation is faster due to which tea or milk cools rapidly. As a result we are able to sip tea or milk faster from the saucer than from a cup.

Evaporation Vs Boiling (difference)

Boiling occours only at boiling point. On the other hand evaporation can occour at any temperature below the boiling point.

Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into vapour state.

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

Different Scales of Temperature

SI unit of temperature is Kelvin.

0 0C = 273.16 K. We take 0 0C = 273 K.

T K = t oC + 273

Kelvin scale of temperature has always positive sign, hence regarded as better scale than Celsius.

Conversion formulae

F-329=C5   and   K=C+273

Pressure and Its Units

Due to the random movement, the particles of gases hit each other and also on the walls of the container. The striking particles exert force on the walls of the container. The force exerted by gas particles per unit area on the walls of the container is called pressure.

The pressure of air in the atmosphere is called atmospheric pressure.

The atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric pressure.

Atmosphere (atm) is a unit of measuring pressure exerted by a gas.

The SI unit of pressure is Pascal (Pa)
1 atmosphere = 1.01×105 Pa.