13.1 Up Up And Away

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An Introduction to Chapter 13

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13.1 Up Up And Away

  1. 1. Matter Very Simple Up, Up and Away
  2. 2. Evidence for Gases… <ul><li>Write down what you know about the following: </li></ul><ul><li>What are gases? </li></ul><ul><li>What is pressure? </li></ul><ul><li>Why are gases so useful? Think tyres, balloons etc… </li></ul>
  3. 3. Evidence for Gases <ul><li>We will find out: </li></ul><ul><ul><li>Why gases behave as they do </li></ul></ul><ul><ul><li>How to measure the mass of a gas </li></ul></ul><ul><ul><li>How microscopic behaviour of a gas explains its macroscopic behaviour </li></ul></ul>
  4. 4. Gases <ul><li>Gases are: </li></ul><ul><ul><li>Made of molecules separated by empty space </li></ul></ul><ul><ul><li>About 1000 times less dense than solids </li></ul></ul><ul><ul><li>Compressible </li></ul></ul><ul><ul><li>No shape </li></ul></ul><ul><li>They are useful because </li></ul><ul><ul><li>They can provide cushioning (e.g. in a tyre) with a very low mass – compare this to solid rubber </li></ul></ul><ul><ul><li>They provide buoyancy – hot air balloon, rubber ring </li></ul></ul><ul><ul><li>But do gases have any mass? </li></ul></ul>
  5. 5. I can hardly stand up!
  6. 6. Mass of Air <ul><li>Air has a mass of about 1.2 kg m -3 </li></ul><ul><li>This mass is due to the molecules present in the volume of gas </li></ul><ul><li>What would the mass of 1cm 3 of air be? </li></ul>
  7. 7. Mass of Air <ul><li>Air has a mass of about 1.2 kg m -3 </li></ul><ul><li>This equals 1200 g m -3 </li></ul><ul><li>This equals 1200 g / 1 x 10 6 cm -3 </li></ul><ul><li>As there are 1 x 10 6 cm 3 in 1m 3 </li></ul><ul><li>This equals 0.012 g cm -3 </li></ul>
  8. 8. Under Pressure… <ul><li>What were your descriptions of pressure? </li></ul>
  9. 9. Under Pressure… <ul><li>Pressure in gases is due to collisions between the molecules of the gas and the walls of its container… </li></ul><ul><li>More Collisions = More Pressure! </li></ul>
  10. 10. Collisions cause pressure on container walls But what happens if the volume of the container is changed???
  11. 11. Boyle’s Law – Pressure is proportional to ‘crowding’ of molecules pV = constant x Number of molecules
  12. 12. Evidence for Gases <ul><li>We found out: </li></ul><ul><ul><li>Why gases behave as they do – they are made of molecules that collide with their container </li></ul></ul><ul><ul><li>How to measure the mass of a gas – all gases have mass </li></ul></ul><ul><ul><li>How microscopic behaviour of a gas explains its macroscopic behaviour – molecular collisions can account for changes in pressure </li></ul></ul>

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