1. Understanding physical change
Module 7.2

2. The particle model

3. The particle model is a simplified representation of solids, liquids and gases.
In the particle model, solids, liquids and gases are made up of invisible, ball-like particles that are:
1. Hard, incompressible
2. Attracted to each other.
3. Constantly moving.
The particle model
Solid Liquid Gas

4. SOLIDS – Are incompressible and hold their shape
SOLIDS – Are incompressible and hold their shape. (The particles are held strongly in a fixed position, but they vibrate on the spot. These vibrations increase with temperature.) LIQUIDS – Are incompressible and flow to take the shape of their container. (The particles are held weakly together and can move freely throughout the liquid. The particles move faster as the temperature increases.) GASES – Are compressible and fill their container. (No attraction between particles, Large spaces between particles. These particles move in straight lines.)
The particle model

5. Expansion and Contraction
As the particles are heated they gain more kinetic energy.
This makes them move and vibrate more violently, causing the substance to take up more space (expand).
When the substance is cooled, the reverse happens and it contracts.
Expansion and Contraction

6. Expansion and Contraction

7. Expansion and Contraction in Solids
In solids, as the temperature increases, so do the vibrations of the particles. This pushes the particles further apart and causes the solid to expand (get larger).
When the solid is cooled, the particles vibrate less, allowing them to be packed more tightly and the solid contracts (shrinks).
Expansion and Contraction in Solids

8. Expansion and Contraction in Liquids
In liquids, as the temperature increases, so do the vibrations of the particles. This causes liquids to expand when heated and contract when cooled.
Liquids tend to expand and contract more than solids because the particles are free to move.
Expansion and Contraction in Liquids

9. Expansion and Contraction in Gases
In gases, as the temperature increases, the particles travel faster. This means they hit the sides of their container more often and with more force. In a flexible container (such as a balloon) it will expand. In a rigid container (such as a glass bottle) the pressure will increase.
If a gas is cooled, the particles travel slower and collide less frequently and with less force, which results in lower pressure. In a flexible container, it will contract. In a rigid container, the pressure is reduced.
Expansion and Contraction in Gases

10. Changes of state and the particle model: Melting and Freezing
Melting - Increasing the temperature of a solid gives its particles enough energy to overcome the attraction between them and they form a liquid.
Freezing - By cooling particles in a liquid, the particles become slow and less energetic until the attraction between the particles is able to fix them in position, forming a solid.
Changes of state and the particle model: Melting and Freezing

11. Changes of state and the particle model: Evaporation and Condensation
Evaporation – Increasing the temperature of a liquid gives the particles enough energy to overcome the forces of attraction holding them together and they form a gas.
Condensation – As a gas is cooled, the particles move slower until the forces of attraction between the particles can hold them together to form liquid droplets.
Changes of state and the particle model: Evaporation and Condensation

12. Changes of state and the particle model summary

13. Mixtures are formed when two or more pure substances are mixed together.
Solutions are an example of a mixture. When a solute dissolves, its particles are spread evenly throughout the solvent. The particles are so small that they are invisible – even with the most powerful microscope.
Mixing and Solutions

14. Diffusion

15. Diffusion and the particle model
Diffusion is when the particles of two gases or two liquids mix evenly (diffuse) without stirring.
For example, when perfume is sprayed it will diffuse through the air particles and fill the room.
A substance moves from a region of high concentration to a region of low concentration (to even things up).
During diffusion the particles move in straight lines until they collide with another particle and then change direction.
Increasing the temperature speeds up diffusion because the particles will travel faster.
Gases diffuse quicker than liquids because the particles are further apart.
Diffusion and the particle model

16. Diffusion

17. Activities  States of Matter  Diffusion in the lab
Into Science Activity
Activities  States of Matter  Diffusion in the lab

18. Module 7.2
Review Questions
(page 303)
Q1,2,4,5,6,8,13,14,17