Gases, typical properties, effect of change in temperature and pressure on volume of gas

Typical properties of gases

Following are common properties of gases.

Diffusion:

The movement of molecules from a higher concentration to a lower concentration is called diffusion. OR

The spreading tendency of gas molecules from one place to another is called diffusion.

For example, spreading of characteristics smell of rotten egg throughout the room is due diffusion. Molecules of one gas can move easily between the molecules of other gas because they are widely separated and don't have any attractive or repulsive forces.

Effusion:

The escape of gas molecules through a tiny hole one after another without collision is known as effusion.

Example. Leakage of helium gas through a gas balloon.

Pressure

Force exerted by gas molecules per unit area is called pressure of the gas.

Mathmatically,

P = Force / Area

All gases molecules exert pressure in all directions.

Unit: atm, pascal,

SI Units: Pascal or N/m2, as pascal is very small unit, kilopascal (kPa) is used for atmospheric pressure under ordinary conditions.

1 atm= At sea level at 0oC, atmospheric pressure is 760 mmHg or 760 torr. This pressure is known as one atm.

1 atm = 760 mmHg

= 760 torr

Relationship between atm and pascal

1atm = 101.325 kPa

= 1.01325 ×105Pa

Compressibility ( سکڑاؤ)

Gases can easily be compressed. The property of gases to be compressed is called compressibility. This property is common in all gases.

Why are gases highly compressible?

Answer. Gases are highly compressible because there are large empty spaces among gas molecules. On applying pressure, distance between gas molecules decreases, therefore volume also decreases.

Mobility

The ability of gas molecules move freely is called mobility. Due to this property, gases can be transported through pipes over long distances.

Density

Clossness or compactne (قربت) of gas molecules is known as density.

Mass per unit volume of gas is called density of gas.

Mathmatically

d = mass /volume

Gases have relatively low density under normal conditions. This is because gas molecules are widely separated from each other and occupy more volume.

When we decrease the temperature, volume decreases, so density increases.

For example, density of oxygen at 20 oc is 1.4g/dm3 and at 0oC is 1.5g/dm3.

Question: Breifly describe the effect on the volume of gas by change in pressure and temperature.

Answer.

Effect on the volume of gas by change in pressure of gas.

There are large empty spaces between gas molecules. When temperature is kept constant and external pressure on the gas is increased, the distance between gas molecules decreases and hence volume of gas decreases. However, this change pressure will not change the kinetic energy of molecules at same temperature.

Effect on the volume of gas by change in temperature

When pressure is kept constant and temperature of gas is increased, the average kinetic energy of gas molecules increases. Due to increase in increased kinetic energy, gas molecules hit the wall of container more energetically and frequently. This results in increased internal pressure. As the result, volume of gas increases to maintain constant pressure.

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