# Definition of Terms used to Describe the Behaviour of Gases

The following terms are generally used to describe the behaviour of gases.

### 1. Volume

The volume of a gas is the space it occupies. The SI unit for volume is cubic metre, m^{3}, Cubic decimetre dm3, and cubic centimetre cm^{3} are commonly used. For conversion: 1.0dm^{3} = 1000cm^{3}.

### 2. Temperature

Temperature is a measure of the degree of hotness or coldness of a body. It is measured by the use of a thermometer. Temperature is measured in Celsius or centigrade scale, and also Kelvin temperature.

The Celsius or centigrade temperature scale uses the freezing point of water 0^{0}C as its starting point. The Kelvin temperature scale has -273^{0}C as the starting point. Kelvin temperature also known a absolute temperature is higher than Celsius temperature by 273 degrees. Thus, 0^{0}C on Celsius or centigrade scale is 273k on the absolute or Kelvin temperature scale.

**Temperature Conversion**

0^{0}C = 273k, -273^{0}C = 0k

To convert a Celsius temperature (0^{0}C) to a Kelvin temperature (K), add 273

i.e k = 0^{0}C +273

to convert Kelvin temperature (k) to Celsius temperature (0^{0}C), subtract 273 from Kelvin temperature

0^{0}C = k â€“ 273

**Examples**

**Convert the following Celsius temperatures to Kelvin temperatures**

(a) 122^{0}C (b) -132^{0}C (c) 0^{0}C

Formula K = ^{0}C + 273

(a) 122^{0}C = (122 +273)k = 395k

(b) -132^{0}C = (-132 +273)k = 141k

(c) 0^{0}C = (0 +273)k = 273k

**Convert the following Kelvin temperature to Celsius temperatures**

(a) 155k (b) 294k (c) 525k

Formula: 0^{0}C = k – 273

(a) 155k = (155 â€“ 273)^{0}C = -118^{0}C

(b) 294k = (294 – 273)^{0}C = 21^{0}C

(c ) 525k = (525 – 273)^{0}C = 252^{0}C

### 3. Pressure

Pressure is defined as force per unit area. The SI unit of pressure is Newton per metre square.

Gas pressures are also expressed in atmosphere (atm). Gas pressures are also expressed in Pascals (Pa) and also in millimetre mercury (mmHg).

### Standard Temperature and Pressure s.t.p

A reference temperature and pressure has been arbitrarily adopted as standard. Standard temperature is 0^{0}C or 273k.

Standard pressure is 760 millimetres of mercury (760mmHg) or 1.01 x 10^{5} Newton per metre square (1.01 x 10^{5} Nm^{-2}) or 1 atmospheric pressure (1atm) or 101325 Pascals (101325Pa).

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