States of Matter

Matter is anything that has mass and occupies space. Scientists explain the properties and behaviour of matter using the particulate theory of matter, which states that:

• all matter is made of particles
• the particles are in constant, random motion
• there are spaces between the particles
• there are forces of attraction between the particles

This theory is useful because it lets us explain both the physical properties of matter and the differences between the three states of matter.

Last lesson, a potassium manganate(VII) crystal dropped into water spread its purple colour throughout the beaker — evidence that particles move and that there are spaces between them. Today we test whether gases diffuse the same way.

The ammonia and hydrogen chloride particles diffuse through the air in the tube towards each other and react on contact to form solid ammonium chloride. Ammonia particles are lighter than hydrogen chloride particles, so they move faster through the air. This means the ammonia particles cover more distance before meeting the hydrogen chloride particles, so the white ring forms closer to the hydrochloric acid end of the tube.

Physical properties — shape, volume, density, compressibility, solubility, melting point and boiling point — can all be explained using the particulate theory:

• Solids have a fixed shape and fixed volume because their particles are packed closely together in a regular pattern and cannot move out of their fixed positions; they are very difficult to compress. Forces of attraction between the particles are strong, and particles have very little energy — they simply vibrate in place.
• Liquids flow and take the shape of their container, but keep a fixed volume, because their particles have small spaces between them and can move past one another. Liquids can be compressed slightly. Forces of attraction are weaker than in a solid, and particles have moderate energy — they move about slowly.
• Gases take the shape and volume of their container because their particles move freely and rapidly, with large spaces between them and only weak forces of attraction. Gases are easy to compress and generally have a much lower density than solids or liquids.

A change of state is a physical change caused by a change in temperature, which changes the kinetic energy of the particles:

• Melting — a solid changes to a liquid at its melting point, as particles gain enough energy to overcome the forces holding them in fixed positions.
• Evaporation — particles near the surface of a liquid gain enough energy to escape as a gas; this can happen at any temperature and cools the remaining liquid.
• Boiling — a liquid changes to a gas throughout the liquid at a fixed temperature, the boiling point.
• Condensation — a gas changes to a liquid as particles lose energy and move closer together.
• Freezing — a liquid changes to a solid at its freezing point (the same temperature as its melting point for a pure substance).
• Sublimation — a solid changes directly to a gas (or a gas directly to a solid) without passing through the liquid state, because the forces of attraction between particles are weak.

During melting and boiling, the temperature stays constant while heat is still being supplied, because all the added energy is being used to overcome the forces of attraction between particles rather than to increase their speed.