Substances and Reactions

Classifying Substances

A substance is a pure form of matter. Substances can be classifi ed or put into groups with similar substances.

Chemists have elaborated common ways of classifying:

• Solids, liquids and gases,
• Elements, mixtures, compounds, and
• Metals, non-metals, semi-metals.

Solids, Liquids and Gases

Solids have shape and volume. Liquids have volume but adopt the shape of their container. Gases occupy the shape and volume of their container.

Solids, Liquids and Gases. Changes of State

The three states of matter, solid, liquid and gas exist because each one has a diff erent balance of kinetic and potential energy. If energy is added or removed from the substance, it changes its state.

Kinetic Theory of matter

All matter is composed of particles (atoms, molecules, ions). Th e particles attract each other via inter atomic forces. All the particles have kinetic energy (K.E.) which varies with temperature. Th e state of matter depends on balance between:

• K.E. of particles (tending to separate), and
• Attraction between particles (tending to pull together).


In a solid the atoms are close together, held in position by the strong inter atomic electrostatic forces. They have some K.E., which makes them vibrate about an equilibrium position, but they cannot change places. Th is means that solids have a fixed shape and a fixed volume i.e. they are rigid.


In a liquid the atoms are not much farther apart than in a solid. Th ere are still quite strong inter atomic bonds between the atoms. Th e atoms have more kinetic energy and are able to move randomly, changing places with their neighbors. There is less order than in a solid. Th us a liquid has no fixed shape but does occupy a fixed volume.


In a gas the atoms move randomly at high speeds. They are much further apart than in solids or liquids. Th ere are no inter atomic forces between the atoms. Interaction only occurs when the atoms (or molecules) collide. Th e internal energy of a gas is entirely kinetic.

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