1. Gases can be compressed, exert pressure in all directions, mix on their own without aid, and are least dense compared to liquids and solids. 2. Charles' law states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature. Boyle's law states that at constant temperature, the volume of a gas is inversely proportional to its pressure. 3. Graham's law of diffusion states that the rate of diffusion of a gas is inversely proportional to the square root of its density.

1. BEHAVIOUR OF GASES
CLASS X
By
Yasmin Sayed, M.Sc., B.Ed.
G.U.H.S PB Dargah, Kudchi (R). DIST: Belgaum.

2. 2
Behaviour of gases
Now, we are going to discuss about behaviour of gases. Before that, let us
review what a gas is and how gases are different from solids and liquids.
Solids Liquids Gases
Fixed volume Fixed volume No fixed volume
Fixed shape Takes the shape of the
container
No fixed shape
Now let us see how gases behave in certain conditions.
Main Features Of Gases:-
 Gases can be compressed.
Gases can be compressed because the particles in gases are very far apart, so
they are easy to compress, whereas in solids, the
particles are very close together that they are difficult to
be compressed. So, more the distance between the
particles of a substance, the easier it is to compress.
Compression of gas
 Gases exert pressure in all directions.
The particles in a gas move quickly in all directions. When the gas particles
collide with each other or with the wall of the container, they cause pressure.
The more particles hit the wall, the higher is the
pressure.

3. 3
 Gases mix on their own without any mechanical aid.
We don’t have to mix the gases by our hand or any external aid; gases mixes
on its own. For example, when the perfume smell is let loose in a room, the
particles mix with the air particles and move quickly in all directions by
themselves. Gas particles move quickly; it happens more quickly in hot gases.
 Gases are least dense than solids and liquids.
Density means mass per volume or in simple terms “relative heaviness” per
volume.
Calculating Density:-How do we actually measure density of a gas???
An easy way to understand gas density is to observe its behavior compared to
air. For example, think of helium-filled balloons. These balloons rise because
they are less dense than surrounding air. Rising balloon tells us that helium is
less dense than air.
Gas laws
 Charle’s law
 Boyle’s law
 Graham’s law of diffusion
Hot Air Balloon Technology:-
As you may recall, heating a gas will increase the speed of the gas particles.
Higher the temperature of the gas, faster will be the moving particles.
What the balloon burner flame really doing is heating the air inside the
balloon causing the air particles to move
faster and spread out which makes the
balloon less dense than the air around it
which causes the balloon to rise.
This phenomenon (hot air rising) is
because of decreased density.

4. 4
Jacques Charles in the year 1787 researched the relationship between
the temperature of a gas and its volume. He showed that if the pressure
of the gas is kept constant and the gas is heated, its volume will increase.
Cooling a gas will decrease its volume. This relationship is known as
Charles law.
Charles Law- Law states that “At constant pressure, the
volume of a fixed mass of gas is directly proportional to its
absolute temperature.”
Jacques Charles OR
When pressure of the gas remains constant, the volume of the gas will increase
as the temperature increases.
Mathematically, V ∝ T (pressure constant)
V= KT (K is Charles constant)
Suppose the gas has temperature T1 & volume V1
At the same pressure, T1 is raised to T2
V1 changes to V2
By Charles law, V1 =k T1
∴ V1 / T1 = k------ 1
Similarly, V2 = k T2
V2 / T2 = k------ 2

5. 5
According to equations 1 & 2
V1 / T1 = V2 / T2 = k[at constant p] Charles law equation
APPLICATION OF CHARLES LAW IN DAY-TO-DAY LIFE
 A balloon blown up in warm room shrinks when it is taken to a
colder area like outdoors.
 Football shrinks when it is taken outdoors.
 Soda bottle explodes when it is exposed to heat.
 Human lung’s capacity is reduced in colder weather; runners and
other athletes may find it harder to perform in cold weather for
this reason.
 Rising of bread and cakes in the oven; tiny pockets of air from
yeast or other ingredients are heated and expand causing the
dough to expand which results in bigger/bulged baked food.
Question Time
1. Balloon floats in air, why?
Ans: Because heating of a gas will increase its
volume and they tend to spread out, this creates a
difference in density between the air in the balloon
and the air outside. Thus, balloon floats in air.
2. Soda bottle on exposure to heat explodes, why?
Ans: When the temperature increases, the gas
inside the bottle expands which increases volume,
thus it explodes.

6. 6
Boyle’s law (relationship between volume and pressure at
the constant temperature)
It states that “at constant temperature, volume of
a given mass of ideal gas is inversely related to its
pressure.”
Pressure goes up as volume goes down if
temperature remains constant. Pressure goes
down as volume goes up if temperature remains
constant.
According to Boyle’s law,
V ∝
𝟏
𝐏
V= k ×
𝟏
𝐏
(k is constant)
By cross multiplying PV = K where V= volume
P=pressure
K=constant
Thus, we conclude that “product of pressure and volume remains
constant.”
∴ P1 V1 = P2 V2 p1 = pressure before some change,
V1 = volume before some change
P2 and V2 are the new values after change
(at fixed temperature)

7. 7
APPLICATION OF BOYLE’S LAW:-
 When you squeeze a balloon, you try to decrease its volume which
increases the pressure and the balloon pops.
 When you pump air in a tyre, you are increasing
pressure on the air pump thus decreasing the volume
inside the air pump.
 Deep sea fish die when they are brought to the
surface due to low pressure.
 Scuba diver’s body would burst if he suddenly comes
to the surface.
 We breathe because of Boyle's Law.
 Balloons work because of Boyle's Law.
 We feel pain in ears while in a plane during ascending
or descending due to imbalance in pressure inside and outside of ear
drums.
When the plane starts to rise, it is going from an area of high pressure
(where your ears are used to) to an area of low pressure. As the inside
pressure of your ears increases with altitude compared to the
surrounding, the air inside increases in volume, thus straining your
eardrum.
Diffusion:- Diffusion is a physical process of movement of a gas molecule from
a region of higher concentration to a region of lower
concentration.
Examples-
If someone is cooking in the kitchen, it doesn't take long
for the smell to travel around the house to other rooms.

8. 8
You can smell perfume because it diffuses into the air and makes its way into
your nose.
1. Cigarette smoke diffuses into the air.
2. The smell of hot food reaches faster than the smell of
cold food .
Question time
1. The smell of hot food reaches faster than smell of cold food?
Because particles of matter are continuously moving, so they possess the
kinetic energy. As the temperature rises, particles move faster. Thus, particles
that carry smell of hot food move faster than the smell of the cold food.
2. Burning kerosene doesn’t produce much smell but when its ceased
produces strong smell?
With burning of kerosene, few molecules diffuse in air. On putting off the
fire, the burnt hot vapour enters into air and strong smell is felt.
Rate of diffusion -The rate of diffusion of a gas is equal to the volume of a gas
diffusing per unit time.
Graham’s law of diffusion It states that “the rate
of diffusion of a gas is inversely proportional to
the square root of its density.”
Suppose if the temperature and pressure remain constant, molecules with a
lower density will effuse faster than molecules with a higher density. Graham
even went further by finding out how much faster one molecule would be over
another.
Mathematically,
r ∝
𝟏
𝐝
(at const temp) r= rate of diffusion

9. 9
r =K
𝟏
𝐝
d= density
k= proportionality constant
k= r 𝐝
Usually, this formula is used to compare the rates of two different gases at
equal temperatures and pressures. The ratio of the rate (or speed) of gas A
over gas B is equal to the square root of the mass of gas B over the mass of gas
A.
Relationship between diffusion and mass
r =
𝑲
𝒎
𝑽
where d =
𝒎𝒂𝒔𝒔
𝒗𝒐𝒍𝒖𝒎𝒆
∴ r = k
𝑽
𝒎
this shows that rate of diffusion of a gas is inversely
related to mass of the gas.