BEHAVIOUR OF GASES

CLASS X

By

Yasmin Sayed, M.Sc., B.Ed.

G.U.H.S PB Dargah, Kudchi (R). DIST: Belgaum.

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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.

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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.

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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

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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.

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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)

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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.

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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

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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.