# Cambridge IGCSE™ Chemistry 4th Edition

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## Book Preface

FOCUS POINTS

★ What is the structure of matter?
★ What are the three states of matter?
★ How does kinetic particle theory help us understand how matter behaves?

In this first chapter, you will look at the three states of matter: solids, liquids and gases. The structure of these states of matter and how the structures can be changed from one to another is key to understanding the states of matter.
You will use the kinetic particle theory to help explain how matter behaves, so you can understand the difference in the properties of the three states of matter and how the properties are linked to the strength of bonds between the particles they contain. Why, for example, can you compress gases but cannot compress a solid? By the end of this chapter, you should be able to answer this question and use the ideas involved to help you to understand many everyday observations, such as why car windows mist up on a cold morning or why dew forms on grass at night.

1.1 Solids, liquids and gases
Chemistry is about what matter is like and how it behaves, and our explanations and predictions of its behaviour. What is matter? This word is used to cover all the substances and materials from which the physical universe is composed. There are many millions of different substances known, and all of them can be categorised as solids, liquids or gases (Figure 1.1). These are what we call the three states of matter.

1 StateS of matter
A solid, at a given temperature, has a definite volume and shape which may be affected by changes in temperature. Solids usually increase slightly in size when heated, called expansion (Figure 1.2),
and usually decrease in size if cooled, called contraction.
A liquid, at a given temperature, has a
fixed volume and will take up the shape of any container into which it is poured. Like a solid, a liquid’s volume is slightly affected by changes in temperature.
A gas, at a given temperature, has neither a definite shape nor a definite volume. It will take up the shape of any container into which it is placed and will spread out evenly within it. Unlike solids and liquids, the volumes of gases are affected greatly by changes in temperature.
Liquids and gases, unlike solids, are compressible. This means that their volume can be reduced by
the application of pressure. Gases are much more compressible than liquids.

1.2 The kinetic particle
theory of matter
The kinetic particle theory helps to explain the way that matter behaves. It is based on the idea that all matter is made up of tiny particles. This theory explains the physical properties of matter in terms of the movement of the particles from which it is made.

The main points of the theory are:

» All matter is made up of tiny, moving particles, invisible to your eye. Different substances have different types of particles (atoms, molecules or ions) of varying sizes.
» The particles move all the time. The higher the temperature, the faster they move on average.
» Heavier particles move more slowly than lighter ones at a given temperature.

The kinetic particle theory can be used as a scientific model to explain how the arrangement of particles relates to the properties of the three states of matter.
Explaining the states of matter
In a solid, the particles attract one another. There are attractive forces between the particles which hold them close together. The particles have little freedom of movement and can only vibrate about a fixed position. They are arranged in a regular manner, which explains why many solids form crystals.
It is possible to model such crystals by using spheres to represent the particles. For example, Figure 1.3a shows spheres built up in a regular way to represent the structure of a chrome alum crystal. The shape is very similar to that of a part of an actual chrome alum crystal (Figure 1.3b).

Studies using X-ray crystallography (Figure 1.4) have confirmed how particles are arranged in crystal structures. When crystals of a pure substance
form under a given set of conditions, the particles are always arranged (or packed) in the same way. However, the particles may be packed in different ways in crystals of different substances. For example, common salt (sodium chloride) has its particles arranged to give cubic crystals as shown in Figure 1.5.

In a liquid, the particles are still close together but they move around in a random way and often collide with one another. The forces of attraction between the particles in a liquid are weaker than those in
a solid. Particles in the liquid form of a substance have more energy on average than the particles in the solid form of the same substance.
In a gas, the particles are relatively far apart. They are free to move anywhere within the container in which they are held. They move randomly at very high velocities, much more rapidly than those in a liquid. They collide with each other, but less often than in a liquid, and they also collide with the walls of the container. They exert virtually no forces of attraction on each other because they are relatively far apart. Such forces, however, are very significant. If they did not exist, we could not have solids or liquids (see Changes of state, p. 4).
The arrangement of particles in solids, liquids and gases is shown in Figure 1.6.