Chemistry, Third edition

CHANGES IN THE STATE OF MATTER

from each other. This process is called evaporation, and the resulting gas consists of

particles which (at low gas pressure) move entirely independently of each other.

Similar explanations apply (but in reverse) to the other changes of state. These

‘phase changes’ are shown in Fig. 10.3.

The ‘molecular picture’ of the three states of matter is summarized in Fig. 10.2(b),

with the particles in a solid vibrating, the particles in a liquid sliding over each other

(explaining why liquids take up the shape of their container), and the particles in a

gas moving so rapidly that they take up the shape andvolume of their container.

The position of atoms (and ions) in solids and liquids may be studied using X-ray

diffraction. This technique confirms that atoms in a liquid are in a state of orderliness

which is intermediate between that of gases and solids.

Explanations based upon the kinetic molecular theory

The kinetic theory of matter explains many everyday observations including:

1. The relative density of solids, liquids and gases

The density of a material tells us how closely packed the particles are. Experiments

show that solids usually possess the greatest density, and gases (in which the

molecules are moving fastest) the least. Gases have low densities because the gaps

between the particles are relatively great. For this reason, gases have a volume which

may easily be reduced by applying pressure. This is what we mean when we say that

gases are easily compressed.

2. The variation of gas pressure with temperature

Gas pressure results from the bombardment of the container walls by molecules of

the gas (Fig. 10.2(b)). At high temperatures, the gas molecules are moving faster and

both the number of collisions per second, and the energy of the colliding molecules,

increases. This causes an increase in gas pressure.

3. The evaporation of liquids

A drop of alcohol in a dish slowly vanishes. This ‘vanishing act’ is no magic, because

the alcohol has simply become alcohol gas (alcohol vapour). Evaporation of liquid

molecules from the liquid surface occurs at all temperatures. Only fast-moving

molecules are able to escape from the attractive forces of the other molecules in the

liquid. Since the number of fast-moving molecules increases as the average kinetic

energy of the sample of molecules increases, the rate of evaporation of a liquid

increases with temperature.

4. Diffusion

The spreading out of particles is called diffusion. For example, a gas (or vapour) will

fill up its container because the gas molecules are in constant and random move-

ment, and a crystal of copper(II) sulfate will slowly dissolve in water (even without

agitation) because the water molecules are continually striking the crystal and the

individual copper and sulfate ions disperse into the water.

155

Fig. 10.3Changes of phase.

Changes of phase

What is the significance of

the double pointed arrows

( ) in Fig. 10.3?

Exercise 10A