Chemistry, Third edition

GAS LAWS

When the liquid boils, its temperature does not rise – all the added heat is used to

boil the liquid. Both the liquid and gas (vapour) are at the same temperature. This

constant temperature is called the boiling point(orboiling temperature) of the sub-

stance. The boiling point of water at an air pressure of one atmosphere is 100C. (At

this temperature, the vapour pressure of water equals one atmosphere, see page 166.)

At E, all the substance is present as a gas. If we contained the gas in a sealed vessel

and then applied more heat, its temperature would continue to rise. The slope of EF

depends upon the specific heat capacity of the gas.

If a substance is cooled (e.g. by putting it in a freezer) the resulting cooling curve

is the mirror image of Fig. 10.4. The energy changes of Fig. 10.4 are also reversed. For

example, steam condensing to liquid water and liquid water that is freezing both give

outheat. Now try Exercise 10C.

Gas laws

Think of a gas in a sealed container. We can measure the number of moles nof gas

inside, the pressure Pof the gas (Box 10.2), the volume Vof the gas (which equals the

volume of the container) and the temperature Tof the gas. The observed relationships

betweenP,V,nandTare summarized by four laws: Charles’ law, Boyle’s law, Avo-

gadro’s law and Dalton’s law. Gases that obey these laws perfectly are known as ideal

gases. Real gases behave approximately as ideal gases at room temperature and at

atmospheric pressure. Gases become less ideal at higher pressures ( 1 atm) and at low

temperatures (especially close to their boiling point).

If a gas is at 25C and at a pressure of exactly 100 kPa (1 bar), the gas is said to be

atstandard ambient temperature and pressure(symbolized SATP). An earlier set of

conditions, known as standard temperature and pressure(STP) refers to a gas at

0 C and exactly 1 atm pressure.

10.3

157

Bubbles

of vapour

in boiling

water

Liquid

water

100 °C

Heat

Water vapour at 100°C

Fig. 10.5Boiling water.

Pressure

Use the information in

Box 10.2 to convert the

following to Pascals:

(i) 20 torr

(ii)10 atmospheres

(iii)107 000 N m^2

(iv)1 kPa.

Exercise 10D

BOX 10.2

Gas pressure

Pressure is defined as force per unit area:

Pressure

force

area

The SI unit of force is the newton (N), and

the SI unit of area is the square metre (m^2 ).

So, the SI unit of pressure is newton per

square metre (N m^2 ) also known as the

pascal(Pa).

The pressure of air at sea level is said to be

one ‘standard atmosphere’ (often simply

called ‘one atmosphere’). The symbol for

(standard) atmosphere is atm:

1 atm 101325Nm^2 101.325 kPa

1 atm 

1  105 Nm^2 101 kPa

Exactly 100 kPa is called 1 bar:

100 kPa 1 bar

The ‘torr’ is still used as a unit of pressure

in some laboratories. One torr is the

pressure of air which supports

1 mm of mercury in a barometer at sea level.

There are 760 torr in 1 atmosphere:

1 atm 760 torr 101.325 kPa

Boiling

When cooking vegetables by

boiling them in water on a

cooker, does the speed at

which the vegetables cook

depend upon the rate at

which the water is boiled?

Exercise 10C