States and Changes of State of Matter
Senior 1 · Chemistry
States and Changes of State of Matter
Lesson 2 of 5
22 min
Umutoni pumps up a football at seven in the morning. At lunchtime, after the ball has lain in the hot sun, it feels much harder, although nobody has added any air. And if you push the plunger of a sealed syringe full of air, it moves in a long way, but a sealed syringe full of water will not budge. To explain these everyday puzzles, scientists use one simple model of what matter is made of.
Take three plastic syringes without needles. Fill one with air, one with water and one with dry sand. Seal each tip firmly with a finger or a cap and push each plunger as hard as you can (Figure 2.1).
Figure 2.1: Only the gas can be compressed.
Warning
Safety. Use syringes with no needles, and point the tips down at the bench: if the seal slips, water or sand can spray out. Do not push so hard that a syringe cracks.
The word kinetic means "to do with movement". The kinetic theory of matter explains the properties of solids, liquids and gases with a few simple ideas about particles:
Definition
Kinetic theory of matter: the model which states that all matter is made of tiny particles that are always moving, move faster when heated, attract one another, and have spaces between them.
Definition
Kinetic energy: the energy an object or particle has because it is moving. Faster particles have more kinetic energy; temperature is a measure of the average kinetic energy of the particles.
Table 2.1: Particles in the three states
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | regular pattern, tightly packed | close together, no pattern | far apart, no pattern |
| Movement | vibrate about fixed positions | slide past one another | move fast in straight lines in all directions |
| Spaces between particles | very small | small | very large |
| Forces between particles | very strong | fairly strong | very weak |
| Energy of particles | lowest | medium | highest |
Solids keep their shape because strong forces hold the particles in fixed positions; they can only vibrate. Liquids flow because the particles, though close together, can slide past one another. Gases spread out to fill any container because the forces between their widely spaced particles are very weak, and the particles move freely in every direction.
In the activity, air could be squashed because there is a great deal of empty space between gas particles; the plunger simply pushed the particles closer together. In water and sand the particles are already touching, so there is no space to squash them into.
Worked example
Problem. Use the kinetic theory to explain why a gas can be compressed but a liquid cannot.
Solution.
Answer: a gas has large spaces between its particles and a liquid does not.
Gas particles move very fast and keep hitting the walls of their container. Each collision gives the wall a tiny push; millions of these pushes every second produce the pressure of the gas. When a gas is heated, its particles move faster, so they hit the walls harder and more often, and the pressure rises (Figure 2.2).
Definition
Gas pressure: the push that a gas exerts on the walls of its container, caused by its particles colliding with the walls.
Figure 2.2: Heating a gas in a closed container raises its pressure.
Worked example
Problem. Explain why Umutoni's football is harder at lunchtime than at seven in the morning.
Solution.
When a solid or liquid is heated, its particles vibrate or move more and take up a little more room, so the substance expands. That is why the liquid in a thermometer rises, why bridges are built with small gaps at their joints, and why electricity cables sag more on a hot afternoon than on a cold morning.
Warning
Never heat a sealed container or throw an aerosol can on a fire. As the gas inside is heated its pressure rises until the container bursts, and flammable contents can explode.
Warning
Common mistake: writing that "the particles get bigger" when a substance expands. The particles stay the same size; it is the spaces between them that get bigger as they move faster.
Tip
Remember the three Ms for particles: they are minute, always moving, and they move more when heated.
FuseSchool: States Of Matter - Solids, Liquids & Gases (3 min). Watch how the particle movement changes as each state is heated.
Practice
Sources and credits
