Page 194 - Chemistry ICSE Class IX

182 ICSE Chemistry – 9

Let us say it Again

— Boyle’s law. At a constant temperature, the volume of a certain amount of gas is inversely proportional to the pressure
exerted on it.
 For a gas, under constant temperature conditions,

PV = Constant
or P V = P V = P V ....
1 1 2 2 3 3
— Charles’ law. At a constant pressure, the volume of certain amount of a gas varies linearly with temperature on the
Celsius scale.
 The volume of a gas increases or decreases by 1/273th part of its volume at 0°C for each degree rise or fall in
temperature. The plot of V vs t (°C) is a straight line not passing through the origin.
 When the pressure is kept constant, the volume of a certain amount of a gas varies directly with the absolute
temperature of the gas. The plot of V vs T (K) is a straight line passing through the origin.
 For a gas, under constant pressure conditions,
= Constant
V
T
or V 1 = V 2 = V 3 ....
T 1 T 2 T 3
— Absolute zero. The temperature of –273.15°C, the lowest temperature to have any significance, is called absolute zero.
— Ideal gas equation. A mathematical expression describing the simultaneous effect of the changes in temperature and
pressure on the volume of a given amount of a gas is called ideal gas equation.
Various forms of the ideal gas equation are,
PV = nRT
PV = RT
m
2
1
or P V 1 = P V 2
1 2
T T
–1
–1
The constant R is called universal gas constant and the value of this constant equals to 8.314 J mol K .
— Standard temperature and pressure. Temperature of 0°C (or 273 K) and pressure equal to 1 bar are called standard
temperature and pressure (or STP) conditions.
— Normal temperature and pressure. Temperature of 0°C (or 273K) and pressure equal to 1 atm (or 76 cm Hg, or 760 mm
Hg) are called normal temperature and pressure (NTP) conditions.