0.1 States of matter and the kinetic molecular theory

Introduction

In this chapter we will explore the states of matter and then look at the kinetic molecular theory. Matter exists in three states: solid, liquid and gas. We will also examine how the kinetic theory of matter helps explain boiling and melting points as well as other properties of matter.

States of matter

All matter is made up of particles. We can see this when we look at diffusion. Diffusion is the movement of particles from a high concentration to a low concentration. Diffusion can be seen as a spreading out of particles resulting in an even distribution of the particles. You can see diffusion when you place a drop of food colouring in water. The colour slowly spreads out through the water. If matter were not made of particles then we would only see a clump of colour when we put the food colouring in water, as there would be nothing that could move about and mix in with the water. The composition of matter will be looked at in What are the objects around us made of? .

Diffusion is a result of the constant thermal motion of particles. In [link] we will talk more about the thermal motion of particles.

In 1828 Robert Brown observed that pollen grains suspended in water moved about in a rapid, irregular motion. This motion has since become known as Brownian motion. Brownian motion is essentially diffusion of many particles.

Matter exists in one of three states, namely solid, liquid and gas. Matter can change between these states by either adding heat or removing heat. This is known as a change of state. As we heat an object (e.g. water) it goes from a solid to a liquid to a gas. As we cool an object it goes from a gas to a liquid to a solid. The changes of state that you should know are:

• Melting Melting is the process of going from solid to liquid.
• Boiling Boiling (or evaporation) is the process of going from liquid to gas.
• Freezing Freezing is the process of going from liquid to solid.
• Condensation Condensation is the process of going from gas to liquid.
• Sublimation Occasionally (e.g. for carbon dioxide) we can go directly from solid to gas in a process called sublimation.

If we know the melting and boiling point of a substance then we can say what state (solid, liquid or gas) it will be in at any temperature.

Experiment: states of matter

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