- Gases maintain neither the volume nor the shape. They completely fill the container in which they are placed.

- They expand appreciably on heating.

- Gases are highly compressible. The volume of the gas decreases when the pressure increases.

- They diffuse rapidly into space.

- Gases exert equal pressure in all directions.

- All gases are colourless except a few e.g. chlorine (greenish yellow) bromine (reddish brown), nitrogen dioxide (reddish brown)

The behaviour of gases can be described by certain quantitative relationships called gas laws. They give the relationship between mass, pressure, volume and temperature.

## Measurable Properties of Gases

The important fundamental properties of gases are mass, volume, pressure and temperature. These are discussed below:### Measurement of mass

The mass of a gas can be measured by direct weighing. The container in which the gas is enclosed is first weighed. Then the weight of the container, after removing all the gas, is measured again. The difference between the two weights gives the mass of the gas.

### Units of mass

The most common unit for mass is expressed in grams (g) or kilogram (kg) (1 kg = 10^{3}g).

### Measurement of volume

The volume of a substance is the space occupied by it. A gas occupies the entire volume of the container available to it. The measurement of the volume of a gas only requires the measurement of the volume of the container enclosing it.

### Units of volume

Generally, volume is expressed in units of litres (L), millilitre (mL) or cubic centimetres (cm^{3}). The different units are related as

^{3}

1 L = 10^{3} mL or 1 dm^{3}

^{3}. As this unit is too large, volume is generally expressed in smaller units of cubic decimetre (dm

^{3}) or cubic centimetre (cm

^{3}). These are related to each other as:

1 m^{3} = 10^{3} dm^{3} = 10^{6} cm^{3}

## Measurement of Pressure

Gases exert uniform pressure in all the directions on the walls of the container in which they are confined. As pressure is force per unit area, the pressure of the gas is the force exerted by the gas per unit area on the walls of the container.

### Atmospheric pressure

Air is pulled towards the surface by gravity and this exerts pressure on the Earth's surface. The pressure exerted by the gases of the atmosphere on the surface of the Earth is called atmospheric pressure.

### Measurement of atmospheric pressure

Atmospheric pressure is measured by an instrument known as barometer. This consists of a glass tube closed at one end and filled with pure mercury. The tube is then inverted into an open vessel of mercury. The mercury level in the tube drops until the pressure due to the column of mercury in the tube becomes equal to the atmospheric pressure acting outside the tube. The region above the mercury in the tube is almost a perfect vacuum, therefore, the pressure due to mercury column is equal to atmospheric pressure.

The pressure (P) is expressed as:P = h.r.g

where 'h' is the height of mercury column in the barometer'r' is the density of mercury

'g' is the acceleration due to gravity.##### Fig: 2.1 - Mercury barometer

### Units of atmospheric pressure

The maximum height of mercury, which can be supported by the atmospheric pressure provides a measure of that pressure.

A standard pressure of one atmosphere (1 atm) is defined as the pressure that will support a column of mercury of 76 cm height at 0°C (density of mercury (13.5951 gcm^{-}

^{3}) and at standard gravity

= 980.665 cm^{-}^{2}.

1 atm = 76.0 cm of mercury (cm Hg)

= 760 mm of mercury (mm Hg)= 760 torr.

The S.I. unit of pressure is pascal (Pa), which is the pressure exerted when a force of Newton (1 N) acts on 1m^{2}area. Pascal is related to atmosphere as:

1 atm = 101.326 x 10^{3} N m^{-}^{2}

However, for approximate work, one atmosphere is taken to be equal to 10^{2} kPa or 10^{5} Pa or Nm^{-}^{2}.

## Measurement of Temperature

Temperature is a measure of hotness or coldness of a body. It is a measure of kinetic energy possessed by the molecules. A hot body is said to be at a higher temperature and a cold body is said to be at a lower temperature. The devices used to measure temperature are called Thermometers. The substance commonly used in the thermometers is mercury.

Temperature is commonly measured in Celsius scale (centigrade scale). In this scale, the freezing point of water (0°C) and the boiling point of water (100°C) at one atmospheric pressure are taken as reference points. The range between 0° to 100° is divided into one hundred equal parts, each division corresponding to 1°C.### Unit of temperature

The S.I. unit of temperature is degrees Kelvin (K) and this scale is known as Kelvin scale. The zero point on the Kelvin scale is known as absolute zero, which is equal to -273.15°C.

The temperature on the Celsius scale is converted to temperature on Kelvin scale by the addition of 273.15°. Thus,°C + 273.15 = K

### Standard temperature and pressure

The properties of a gas depends upon the temperature and pressure. Hence, it is convenient to specify a particular temperature and pressure for comparison of different gases. The standard conditions of temperature and pressure are abbreviated as S.T.P. (Standard Temperature and Pressure) and N.T.P. (Normal Temperature and Pressure).

Standard Temperature and Pressure (S.T.P. or N.T.P.) values are:Temperature = 0°C or 273.15 K

Pressure = 1 atm or 760 mm Hg or 760 torr or 101.325 kPa (S.I. units)
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