General Characteristics of Gases

All matter exists in three states: gas, liquid and solid.
A molecular level representation of gaseous, liquid and solid states is shown in the following Fig:
A gas consists of molecules separated wide apart in empty space. The molecules are free to move about throughout the container.
A liquid has molecules touching each other. However, the intermolecular space, permit the movement of molecules throughout the liquid.
A solid has molecules, atoms or ions arranged in a certain order in fixed positions in the crystal lattice.
The particles in a solid are not free to move about but vibrate in their fixed positions. 
Of the three states of matter, the gaseous state is the one most studied and best understood. We shall consider it first.

General Characteristics of Gases

(1) Expansibility
Gases have limitless expansibility. They expand to fill the entire vessel they are placed in
(2) Compressibility
Gases are easily compressed by application of pressure to a movable piston fitted in the container
(3) Diffusibility
Gases can diffuse rapidly through each other to form a homogeneous mixture.
(4) Pressure
Gases exert pressure on the walls of the container in all directions.
(5) Effect of Heat
When a gas, confined in a vessel is heated, its pressure increases.
Upon heating in a vessel fitted with a piston, volume of the gas increases.
The above properties of gases can be easily explained by the Kinetic Molecular Theory.

Parameters of A gas

A gas sample can be described in terms of four parameters (measurable properties):
(1) the volume, V of the gas
(2) its pressure, P
(3) its temperature, T
(4) the number of moles, n, of gas in the container

(1) Volume of the gas, V

The volume of the container is the volume of the gas sample.
It is usually given in litre (l or L) or millilitres (ml or mL).

1 1itre(l) = 1000 ml and 1 ml = 10–3 

One millilitre is practically equal to one cubic centimetre (cc). Actually

1 litre(l) = 1000.028 cc

The SI unit for volume is cubic metre (m3) and the smaller unit is decimeter3 (dm3)..

(2) The pressure of a gas, P

The pressure of a gas is defined as the force exerted by the impacts of its molecules per unit surface area in contact.
The pressure of a gas sample can be measured with the help of a mercury manometer (Fig. 1) Similarly, the atmospheric pressure can be determined with a mercury barometer (Fig. 2).
The pressure of air that can support 760 mm Hg column at sea level, is called one atmosphere (1 atm).
The unit of pressure, millimetre of mercury, is also called torr. Thus,
1 atm = 760 mm Hg = 760 torr
The SI unit of pressure is the Pascal (Pa). The relation between atmosphere, torr and pascal is :
1 atm = 760 torr = 1.013 × 105 Pa
The unit of pressure (Pascal) is not in common use.

(3) Temperature, T

The temperature of a gas may be measured in Centigrade degrees (°C) or Celsius degrees.
The SI unit of temperature is Kelvin (K) or Absolute degree. The centigrade degrees can be converted to kelvins by using the equation.
K = °C + 273
The Kelvin temperature (or absolute temperature) is always used in calculations of other parameters of gases.
Remember that the degree sign (°) is not used with K.

(4) The Moles of a Gas Sample, n

The number of moles, n, of a sample of a gas in a container can be found by dividing the mass, m, of the sample by the molar mass, M (molecular mass).

The gas Laws


The volume of a given sample of gas depends on the temperature and pressure applied to it. Any change in temperature or pressure will affect the volume of the gas.
As results of experimental studies from 17th to 19th century, scientists derived the relationships among the pressure, temperature and volume of a given mass of gas. These relationships, which describe the general behavior of gases, are called the gas laws.

The Ideal Gas Equation

This is called the Universal Gas Law. It is also called Ideal Gas Law
Ideal Gas Law as it applies to all gases which exhibit ideal behaviour i.e., obey the gas laws perfectly.
The ideal gas law may be stated as : the volume of a given amount of gas is directly proportional to the number of moles of gas, directly proportional to the temperature, and inversely proportional to the pressure.


Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.


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