Everything Science Grade 10

CHAPTER 3. STATES OF MATTER AND THE KINETIC MOLECULAR THEORY 3.2

The kinetic molecular

theory

ESAAL

Thekinetic theory of matterhelps us to explain why matter exists in differentphases(i.e.

solid, liquid and gas), and how matter can change from one phase to the next. The kinetic

theory of matter also helps us to understand other properties of matter. It is important to

realise that what we will go on to describe is only atheory. It cannot be proved beyond

doubt, but the fact that it helps us to explain our observations of changes in phase, and

other properties of matter, suggests that it probably is more than just a theory.

Broadly, the kinetic theory of matter says that all matter is composed ofparticleswhich

have a certain amount ofenergywhich allows them to move at different speeds depending

on the temperature (energy). There arespacesbetween the particles and alsoattractive

forcesbetween particles when they come close together.

solid liquid gas

Figure 3.4: The three states of matter

Table 3.2 summarises the characteristics of the particles that are in each phase of matter.

Taking copper as an example we find that in the solid phase the copper atoms have little

energy. They vibrate in fixed positions. The atoms are held closely together in a regular

pattern called alattice. If the copper is heated, the energy of the atoms increases. This

means that some of the copper atoms are able to overcome the forces that are holding

them together, and they move away from each other to formliquid copper. This is why

liquid copper is able to flow, because the atoms are more free to move than when they

were in the solid lattice. If the liquid is heated further, it will become a gas. Gas particles

have lots of energy and are far away from each other. That is why it is difficult to keep a

gas in a specific area! The attractive forces between the particles are very weak. Gas atoms

will fill the container they are in. Figure 3.1 shows the changes in phase that may occur in

matter, and the names that describe these processes.

Activity: The three phases of water

Chemistry: Matter and Materials 59