Week 1 · Unit 1 Matter and Energy, opener; Lesson 1 States of Matter: solids, liquids and gases and their properties, the particle model, plasma · 3 lessons
2
The particle model explains the differences
Science Prep 2 Term 1, Unit 1: pp. 1–9
GoalUse the particle model to explain why solids, liquids and gases behave differently, and why gases compress and substances diffuse.
New words
particle · an atom or a molecule that a substance is made ofdiffusion · the spreading of particles from where there are many to where there are fewBrownian motion · the jiggling of dust or pollen grains caused by fluid molecules hitting them
Explanation
The particle theory says that every substance is made of particles, which are atoms or molecules. They are too small to see. All particles of one substance are alike and differ from the particles of other substances. Between the particles there are attractions, which store potential energy, and spaces. The particles also have kinetic energy and never stop moving. In a solid the attractions are strong and the spaces are small, so the particles only vibrate about fixed places. In a liquid the spaces are a little larger, and the particles slide past each other, so a liquid flows. In a gas the spaces are very large and the attractions very weak, so the particles move freely and far apart and fill the container. A gas compresses because there is so much empty space to squeeze out; the particles themselves do not shrink. Diffusion happens because moving particles spread out, fast in gases, slowly in liquids and very slowly in solids, and faster when warmer. Robert Brown, a Scottish botanist, saw pollen grains in water jiggle in 1827. Today we know that fast water molecules keep hitting them from every side.
Worked examples
Why does perfume smell across a room? Its particles move, bump into air particles, and spread from where there are many to where there are few. This is diffusion in a gas, and the smell reaches you in seconds.
A drop of food colour is put in two glasses of the same size, one with cold water and one with water at room temperature. It spreads faster in the warmer glass, because warmer particles move faster. In both glasses it spreads far more slowly than a smell in air.
Why is a bicycle pump hard to push when its outlet is blocked? The air particles are squeezed into a smaller volume, so they hit the walls more often. The particles are not smaller, the spaces between them are.
Class activity
Particle tray. Pour a layer of rice or lentils into a tray, level and tightly packed: this is a solid. Tilt and shake it gently so the grains slide: a liquid. Shake hard and let some grains jump apart: a gas. Draw all three. Then write one way in which your tray is a poor model of real particles (for example, real particles are never still).
Practice
1
How do the particles of a solid move?
They vibrate about fixed places.
2
Why can a gas be compressed much more than a liquid?
The spaces between gas particles are very large, so they can be pushed closer together.
3
Which picture shows a liquid? Say how you know.
The middle one: the particles touch but are not in neat rows, and they have a surface.
4
Under a microscope, a pollen grain in water jiggles. Which particles are really moving it?
The invisible water molecules that keep hitting the pollen grain.
