BAMA. Aerosols and pressure Notes for teachers

Transcription

1 Contents There are five chapters place ideas about particle models of solids, liquids and gases in the topical context of aerosols What are aerosols? looks at the construction of an aerosol and the need for a propellant. Particle matters describes the difference between solids, liquids and gases in terms of the behaviour of particles. It considers how the behaviour of the particles changes as the material is heated and draws on ideas about relating the kinetic energy of the particles to the temperature. Pressure looks at the idea of pressure as the force per unit area and develops ideas about atmospheric pressure. How do gases behave? describes how gases can cause pressure, in terms of movement of particles, relating microscopic behaviour to the gas laws. What s in an aerosol? explains how an aerosol releases its contents. 1

2 Curriculum links (using the COL keyword scheme) Scientific enquiry Application of science - generally Contexts for science Using science to explain Scientific prediction Fairness of test/comparison SI units Solids, liquids & gases Properties of materials Changes of state Particle theory Changing materials Solubility Earth science Atmosphere & oceans: biosphere Forces & motion Forces - generally Pressure Energy Conduction, convection & evaporation Web links Materials and properties Hot metal heating and cooling 2

3 Using the resource 1 - Using the particle model to explain the behaviour of gases Linking the behaviour of particles in a gas to the macroscopic behaviour of gases provides an example of the way scientists work building a model and testing it with experiments to see if the theory correctly predicts the outcome of experiments. It is useful to begin with the marbles in a tray model of the particles in a gas. As you shake the tray there are two distinct sounds the sound of marbles colliding with each other and the sound of marbles colliding with the walls of the tray. The pressure of the gas is due to these collisions. If you have an internet connection to your lab, display page 3 on the screen or whiteboard (if not, save the page as a web archive). Picture 3.5 illustrates the collisions of the particles with the walls of the container. What would happen if the container had only half the volume? You can model this by putting a divider into the marble tray and confining the marbles in half the area. The number of collisions with the sides increases, in fact the marbles only have to travel half as far before making a collision with the sides of the container, so the pressure doubles. Boyle s Law apparatus demonstrates the relationship between pressure of a gas and the volume it occupies. If you have an internet connection to your lab, display page 9 on the screen or whiteboard (if not, save the page as a web archive). Picture 4.1 demonstrates the relationship between pressure and volume inside a bicycle pump. What happens if the gas is given more energy? Shaking the marble tray more rapidly certainly makes more noise the particles are moving more rapidly, so they hit the sides of the tray more quickly with more momentum and also more frequently. These two factors mean that the pressure increases with an increase in kinetic energy of the particles. As temperature is related to kinetic energy, in increase in temperature results in an increase in pressure. Heating a flask of air will show in increase in pressure if the flask is connected to a pressure meter. This is best done by immersing a round bottomed glass flask of air in a water bath and using a fine tube to connect the flask to a pressure gauge. The temperature and pressure can be recorded and a graph plotted. Extrapolating the graph back to find the temperature at which the pressure would be zero gives a way of determining absolute zero. Pupils often find the idea of absolute zero fascinating and have difficulty believing that the particles would be stationary. These ideas about the relationship between pressure and temperature are illustrated on page 10, picture 4.4 illustrates the particle model. 3

4 Using the resource 2 - Using the particle model to explain evaporation by cooling Pupils should already have a clear idea of the particle model for solids, liquids and gases. Evaporation by cooling is easily experienced by putting a drop of a volatile liquid on the back of pupils hands. As the liquid evaporates it carries energy away with it, leaving the skin cooler. The same principal is used by the body sweat evaporating from the skin leaves the skin cooler. The effect can be explained in terms of the particle model of a liquid. The temperature of the liquid (and skin) is determined by the average kinetic energy of the particles in the liquid. To evaporate the particles need energy, which they gain by collisions with particles in the skin. The evaporating particles leave the surface, with their energy. So the average energy of the particles left behind is lower the temperature has fallen. Another alternative demonstration of the effect is to put a thermometer bulb into a volatile liquids and then remove it. The temperature falls as the liquid evaporates from the bulb. If you have an internet connection to your lab, display page 4 on the screen or whiteboard (if not, save the page as a web archive). Picture 2.6 uses the analogy of escaping prisoners to explain the process. 4

5 Student worksheet 1 Explaining evaporation by cooling 1. Complete the sentences: The particles in a liquid move around freely within the liquid, they have energy. The average kinetic energy of the particles determines the of the liquid. A particle with a lot of energy that is close to the surface may have sufficient energy to from the surface. This will leave the particles behind with a average kinetic energy and a temperature. 2. Explain and describe a situation where the process of evaporation is used to cool a hot body. S1

6 Age Student worksheet 2 The Gas Laws 1. Complete these sentences to explain how the pressure of a gas depends on the energy of its particles. The particles in a gas are in all directions. They with the walls of the container. Each time a particle bounces off the wall it exerts a very small on the wall. The effect of millions of particles hitting the walls each second produces a on the walls. If the volume of the container is reduced, the particles travel before colliding with a wall and so make collisions and the pressure. If more energy is supplied to the gas the particles will move and the temperature will. The particles will make collisions with the walls of the container and each collision will exert a force so the pressure will. 2. Use ideas about particles in a gas to explain why a pressurized can, such as an aerosol canister, should not be put on a fire, even when empty. S2