In gaseous state the particles are very loosely packed. The voids between the particles are quite large so much that the actual volume of the particles is negligible as compared to the total volume of gas. The attractive forces between the particles are negligibly small. The particle motion is very rapid and they move in all the directions in straight lines. Let us explain some of the characteristics of gases.

• Because of the random motion and weak interparticle forces, the particles do not have any bonding surface. As a result they do not have definite shape or definite volume. They tend to occupy each nook and comer of the space available to them. Hence they take up the shape and volume of container.

• They have very low densities because of large empty space between the molecules.

• Gases are highly compressible. This is because of the fact that the large voids between the gas molecules can be decreased by applying external pressure.

• Gases exert pressure on the walls of the container because of the bombardment of gas molecules against the walls of the container.

• Gases mix evenly and uniformly without any mechanical aid. This property is called diffusion. It occurs because of rapid movement of gas molecules and their tendency to enter into the voids of each other.

**SOME MEASURABLE PROPERTIES OF GASES**

The important measurable properties of gases are mass, volume, temperature and pressure because the state of the gas is described by the relationship between these variables.

**1. Mass**

The gases do possess mass. The amount of gas is generally expressed in terms of its number of moles. For a gas of molar mass (M), the mass in gram (w) is related to the number of moles (n) as

n = w / M

**2. Volume**

The volume of the gas is the space occupied by its molecules under given set of conditions. Since gases occupy the entire space available to them, therefore, the measurement of the gas volume simply requires the measurement of the volume of the container in which the gas is enclosed. Units of Volume

The SI unit of volume is Cubic meter (m^{3} ). Since it is too big a unit, therefore, volume of a gas is generally expressed in terms of litres (L) or cm3 or dml. The relationship between different units are:

1 M^{3} = 10^{3} L = 10 dM^{3} = 10^{6} cm^{3}

**3. Pressure**

Gases exert outward force on the walls of the container in which they are enclosed. The outward force experienced by the walls is due to bombardment of gas molecules on the walls. This outward force per unit area of the walls is termed as gas pressure.

Units of Gas Pressure

Pressure is force per unit area

Pressure = force / area

By definition,

Force = Mass x Acceleration

= mass x velocity / time = mass x distance / time x time

= kg x m / s x s

Thus, SI unit of force is kg ms-2 which is called Newton (N)

1 N = 1 kg ms^{-2}

Putting SI units of force (N) and area (m2) we get SI unit of pressure as N/m^{2} or Nm^{-2} . This SI unit of pressure is called pascal (Pa) in the honour of French mathematician and physicist Blaise Pascal

1 Pa = 1 N m^{-2}

Now, 1000 Pa = 1 kilo pascal abbreviated as kPa.

For gases, this unit of pressure is very small. Hence pressure is generally expressed in terms of bigger unit named as bar. One bar represents 100 kilo pascal. Thus

1 bar= 100 kPa = l03 Pa

It may be noted that the older unit of pressure of the gases was atmosphere which was abbreviated as atm. The relationship between these units is given as follows:

1 stm = 1.0135 har = 1.01325 x 105 pa

or bar = 0.987 atm = 105 pa

**4. Temperature**

In general, temperature may be defined as degree of hotness. Temperature can be measured in terms of the effect that its change produces on other measurable properties such as expansion of objects. Expansion of mercury is commonly used for the measurement of temperature. One of the common temperature-measuring device is thermometer. It has a mercury column in a thin capillary tube. The length of mercury column changes with the change in temperature.

**Various Temperature Scales**

• Celsius scale of temperature (^{0}C). This is most common scale of temperature and was earlier known as centigrade scale. In this scale, the melting point of ice is ^{0}C whereas the boiling point of water is 100^{0}C at one atmospheric pressure. The range between these two points has been divided into hundred equal parts. Each division corresponds to one degree in Celsius scale. As the zero in celsius scale is arbitrarily fixed, therefore, it is possible to have temperature below the freezing point. It may appear that celsius scale can be extended to negative temperature indefinitely, but experimental behaviour of gases shows that temperatures below – 273.15°C are impossible to be attained.

Kelvin scale of temperature (K). This scale of temperature is quite significant and is very useful for scientific work and can justified by thermodynamic arguments. It is therefore, also called thermodynamic scale of temperature. Kelvin is also SI unit of temperature. The zero point on kelvin scale is equal to lowest possible temperature (- 273.l5°C) and is known as absolute zero (0 K). The temperature in celsius scale can be converted into kelvin scale by adding 273.15 (or 273 for the sake of simplicity) to it.

K = ^{0}C + 273.15

**Standard Temperature and Pressure (S.T.P.)**

** **

Since volume of a given mass of a gas depends on temperature and pressure both, it is necessary to specify the values of P and T when the value of V is stated. In general, the comparison of the volumes of different gases is made with reference to standard temperature and pressure. The standard temperature is taken as ^{0}C (or 273.15 K). The standard pressure according to the latest recommendations is taken as 1 bar (or l05 pascal). The molar volume of ideal gas at S.T.P. conditions is 22.71098 L mol^{-1} (= 22.7 dm3 mol^{-1}). These conditions are quite often abbreviated as S. T.P. meaning standard temperature and pressure.

It is worth noting that previous S.T.P. conditions (0°C or 273.15 K) temperature and 1 atm (= 1.01325 bar) pressure are still used in many books quite often. At these conditions the molar volume of ideal gas is 22.413996 dm3 moi-1 (= 22.4 dm^{3} mol^{-1}).