Page 178 - Chemistry ICSE Class IX

166 ICSE Chemistry – 9

Temperature (T)
Temperature is a measure of the degree of hotness. In gases, it is a measure
of kinetic energy possessed by the molecules. The instrument which
is commonly used to measure temperature of any substance is called
thermometer.
The temperature scale mostly used in laboratories and industries is
the Celsius scale (formerly called centigrade scale). The freezing point
of water corresponds to 0°C and the boiling point of water under 1 atm
pressure is 100°C on this scale. The value of 0°C has been accepted as the
standard temperature as per international agreement. Temperatures are
also expressed on the Kelvin scale. The zero of the Kelvin scale corresponds
to –273.15°C. Thus,
Temperature on the Kelvin scale = Temperature on Celsius scale + 273.15
or T(K) = t (°C) + 273.15 (7.3)

Boyle’s Law (Pressure-Volume Relationship)
Robert Boyle (1622) studied the effect of pressure on the volume of gases
when temperature is held constant. This relationship, known as the Boyle’s
law, is described below.
What is the Boyle’s law
At a constant temperature, the volume of a certain amount of a gas is
inversely proportional to the pressure exerted on it.
If V is the volume of a certain amount of gas under pressure P at any
xed temperature, then
V v 1 (T = constant)
P
or PV = Constant (7.4)
The value of the Constant in Eq. 7.4 depends upon the amount and
temperature of the gas.
Consider a given amount of a gas is occupying a volume V under
1
pressure P . Then, from Boyle’s law,
1
P V = Constant
1
1
Now, if temperature is kept constant and pressure is changed to P so
2
that the volume changes to V . Then, from the Boyle’s law,
2
P V = Constant
2
2
Now, since temperature and the amount of the gas remains unchanged
hence,
P V = P V 2
1
2
1
where, P is the original pressure, P is the new pressure
1
2
V is the original volume, V is the new volume
1
2
Thus, if the same amount of a gas is subjected to a change of pressure
at constant temperature, the product of each pressure-volume set of values
will be equal. Then, according to the Boyle’s law,
P V = P V = P V ...... = Constant (7.5)
3
1
3
2
2
1
This relationship has been found to be true for almost all gases under
ordinary conditions.