Class 9: Notes

1. Matter in Our Surroundings
Chapter 1: NCERT

2. Agenda
1.1 Physical Nature of Matter
1.1.1 Matter is made up of particles
1.2 Characteristics of Particles of Matter
1.3 States of Matter
1.3.1 The Solid state
1.3.2 The Liquid state
1.3.3 The Gaseous State
1.4 Can matter change its state?
1.4.1 Effect of change of temperature
1.4.2 Effect of change of pressure
1.5 Evaporation
1.5.1 Factors affecting Evaporation
1.5.2 Evaporation causes cooling

3. Physical Nature of Matter
Q. What is a matter?
A. Anything that occupies space and has mass is called a matter. For example:- Chalk,
Milk, Sugar etc. Each matter is a collection of lots of tiny particles.
Q. What are the characteristics of the particles of matter?
A. Characteristics of the particles of matter:
– The particles of matter are very small.
– The particles of matter have space between them.
– The particles of matter are moving constantly.
This is called Brownian Motion.
They possess kinetic energy (which increases with increase in temperature)
– The particles of matter attract one another.
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.
Q. What is diffusion?
A. Intermixing of particles of two different types of matter on their own is called diffusion.
The rate of diffusion increases with increase in temperature.

4. Solid
• Retains a fixed volume
and shape
• Maximum force of
attraction between
particles
• Rigid - particles locked
into place or closely
packed
• Cannot be compressed
• Particles have
minimum kinetic
energy
• Little free space
between particles
• does not flow easily
• Highest density
Liquid
• Assumes the shape of the
part of the container which
it occupies
• Less force of attraction
between particles than
solid
• Particles are loosely
packed as compared to
solids
• Particles can move/ slide
past one another
• Not easily compressible
• Kinetic energy of particles
is more than that of solids
• Free space between
particles is more than
solids
• Flows easily
• Lower density than solids
Gas
• Assumes the shape
and volume of its
container
• Negligible force of
attraction between
particles
• Particles are loosely
packed
• Easily Compressible
• Particles have
maximum kinetic
energy
• Particles can move
past one another
• Lots of free space
between particles
• Flows easily
• Lowest density
Differentiate between Solid, Liquid and Gas

5. Change of State of Matter
• Latent heat of fusion: is the heat energy
required by the solid particles to overcome the force
of attraction and change into liquid state, without any
rise in temperature.
• Latent heat of vaporization: is the heat
required by the liquid particles to overcome the force
of attraction and change into gaseous state, without
any rise in temperature.
• Melting point : The temperature at which a solid
melts to become a liquid at the atmospheric
pressure.
• Boiling point : The temperature at which liquid
melts to become gas at the atmospheric pressure.
Increase
heat
Increase
heat
Increase
pressure
Increase
pressure
Solid Liquid Gas
Melting point Boiling point
Latent heat of fusion Latent heat of vaporizationSalt reduces the freezing
point of water

6. Change of State of Matter What is the
difference b/w gas
and vapour ?
Vapour is used to
denote the gaseous
state of fluids which
exists as liquids
under normal
conditions, while
gas is used to
denote the gaseous
state at normal
temperature.
The change of solid state
from liquid state on cooling
is also known as
Condensation
Vaporization is the
transition of matter from a
solid or liquid phase into a
gaseous (or vapor) phase
A change of state
directly from solid
to gas without
changing into
liquid state is
called sublimation
A change of vapour
to solid state without
undergoing the liquid
state is called
deposition
solidification
The process of changing solid
in liquid by heating it is called
melting
The process of changing
liquid into solid by cooling it is
called Freezing
A substance which is a liquid in normal condition, when changed
to solid to by cooling it under pressure is called solidification
Liquefaction
A substance which is gas in
normal condition, when changed to
liquid by cooling it under pressure
is called liquefaction
Endothermic
Absorbs heat
Exothermic
Releases heat

7. Evaporation
Q. What is evaporation?
A. Change of liquid into its vapour at any temperature below its boiling point is called
evaporation.
Q. What are the factors that affect evaporation?
A. The rate of evaporation increases with –
• An increase of surface area:
We know that evaporation is a surface phenomenon. If the surface area is
increased, the rate of evaporation increases. For example, while putting
clothes for drying up we spread them out.
• An increase of temperature:
With the increase of temperature, more number of particles get enough kinetic
energy to go into the vapour state.
• A 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.
• An increase in wind speed:
It is a common observation that clothes dry faster on a windy day. 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.

8. Evaporation
Q. How does evaporation cause cooling?
A. 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.
Q. What is the difference between boiling and vaporisation?
Q. What are the applications of inter conversion of states of matter?
A. The inter conversion of states of matter is used to :
(i) generate electricity in thermal power plant.
(ii) separate nitrogen and oxygen from air by liquefaction.
(iii) prepare machine parts
(iv) prepare room fresheners
(v) prepare ice-creams.
Boiling Vaporisation
(i) It is a process of changing liquids into vapour. (i) It is a process of changing liquids into vapour.
(ii) Molecules of liquids escape from all parts of the liquids
into atmosphere.
Molecules of liquids escape from upper surface of the liquid
into atmosphere.
(iii) It is a fast process. (iii) It is a slow process.
(iv) It takes place only at a fixed temperature called boiling
point.
(iv) It takes place at all temperature, even at melting point.
(v) In boiling, the vapour pressure of liquids is equal to the
atmospheric pressure
(v) In evaporation the vapour pressure of liquid is less than the
atmospheric pressure

9. Effects of pressure on gas:
Liquefaction and Solidification
• The liquefaction of gas by increasing of pressure:
• In general condition, the particles of gas are far from each other. They move independently and rapidly in
all directions. There is lot of space between the particles of gas. The force of attraction is negligible
between the particles of gas. Because of these characters gas can be compressed a lot.
• When pressure is applied on the gas, its particles come closer to each other. After applying high
pressure, the particles of gas come very close to each other that they start attracting each other. The
space between the particles of gas also decreases with increase in pressure. While applying high
pressure a lot of heat is evolved, which was using by the particles of gas in movement. So, while
applying high pressure to compress the gas, temperature is keeping low to overcome with the heat
released. As they come closer, they lose kinetic energy. Finally they get closely packed at a certain high
pressure, at this stage they get bonded with each other and turned into liquid.
• This process is called liquidation or liquefaction of gas.
• Petroleum Gas is compressed to small volumes. At more pressure it turns into liquid, after that it is
packed into cylinders. This is used widely as fuel to cook food in households. Compressed petroleum gas
is commonly known as LPG (Liquefied Petroleum Gas) since it turns into liquid form.
• Oxygen, hydrogen, nitrogen, helium, etc. are compressed to liquefy and used for many purposes.
Liquefied oxygen is used as propellant in spacecraft rockets. Liquefied oxygen is used for industrial and
medical purposes also. Liquefied oxygen is of blue color.
• On further increase in pressure, gas turns into solid – Solidification
• Carbon dioxide is gas turns into solid on high pressure. Solid carbon dioxide is known as dry-ice. It is
also known as ‘Cardice’ or ‘Card ice’.
• Dry ice is used in the storage of food.
• On decrease in pressure liquefied gas turns into gas. Solid carbon dioxide (dry ice) turns into gas without
changing into liquid on decrease in pressure and increase of temperature.

10. Q&A
• Question: Why should we wear cotton clothes in summer?
• Answer: During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that
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.
• Question: Give reasons for the following observation: The smell of hot sizzling food reaches you several meters away,
but to get the smell from cold food you have to go close.
• Answer: Smell of anything comes because of gases emanating from the given thing. The smell reaches to us because
of diffusion of gas. The rate of diffusion increases with increase in temperature. This happens because of higher kinetic
energy due to higher temperature. That is why smell of hot sizzling food reaches to us from several feet. On the other
hand, the kinetic energy of gases emanating from cold food is low because of lower temperature. Due to this, we need
to move closer to a cold food to take its smell.
• Question: A diver is able to cut through water in a swimming pool. Which property of mater does this observation
show?
• Answer: Water is also a matter. We know that particles of matter have space between them. In case of fluids, the space
between particles is large enough and due to this it becomes easier for a diver to cut through water in a swimming pool.
• Question: Why do we see water droplets on outer surface of glass containing ice-cold water?
• Answer: The water vapour present in the air, on coming in contact with the cold surface of the glass, looses energy and
gets converted into liquid in the form of small water droplets.