Changes of state

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Changes of state

  1. 1. MELTING AND BOILING SEC 3 PHYSICS
  2. 2. On the Molecular Level <ul><li>We have learnt previously that when molecules absorb heat energy, they will start to move faster. </li></ul><ul><li>When the molecules have reached a certain speed, the energy that they absorb will now be used to break the attractive forces between them. </li></ul>
  3. 3. On a Molecular Level (cont’d) <ul><li>When the energy absorbed is used to break the attractive forces between them, there is no increase in speed. </li></ul><ul><li>Which means energy absorbed can only be used for either one of the purpose (break attractive forces OR increase in speed). Only one change is taking place at any time. </li></ul>
  4. 4. So what do we see? <ul><li>When the energy is used to increase the speed of the molecule, we see a rise in temperature of the substance. </li></ul><ul><li>When the energy is used to break attractive forces between molecules, we see the substance changing in state. </li></ul><ul><li>This energy which brings about a change in state is known as LATENT HEAT </li></ul>
  5. 5. Changes in States melting Boiling CONDENSATION FREEZING Gain in heat Loss in heat Solid Gas Liquid
  6. 6. MELTING AND SOLIDIFICATION <ul><li>When a solid changes to a liquid on heating, we call it melting </li></ul><ul><li>In this instance, heat is ABSORBED by the substance </li></ul><ul><li>When a liquid changes to a solid on cooling, we call it solidification (freezing) </li></ul><ul><li>In this instance, heat is LOST by the substance </li></ul>
  7. 7. MELTING AND SOLIDIFICATION <ul><li>In both cases, the heat that is absorbed or lost is known as the latent heat of fusion. </li></ul><ul><li>When this latent heat is absorbed or lost, there is no change in temperature. </li></ul><ul><li>This means when a substance is undergoing a change in state, the temperature will remain constant. </li></ul>
  8. 8. The Temperature-time Graph temperature time This part of the graph shows a constant temperature which means this is when the substance is changing state
  9. 9. Effect of Impurities on Freezing Point <ul><li>When impurities are added to water, it would lower the freezing point of water. This means that water now freezes at a lower temperature. </li></ul>
  10. 10. Effect of Impurities on Freezing Point <ul><li>Example: </li></ul><ul><li>I have 2 cups of water, A and B. </li></ul><ul><li>I add salt to B. When I try to freeze the water in both cups, A would freeze at 0 o C but B would freeze maybe at –10 o C. </li></ul><ul><li>This means B would stay as a liquid even at 0 o C or lower until the temperature reaches –10 o C. </li></ul><ul><li>When A becomes solid ice at 0 o C, B is still liquid. </li></ul>
  11. 11. Effect of Impurities on Freezing Point The reverse situation would be true. (When melting.) A (no salt added) B (salt added) Ice cube A would melt at 0 o C. But B would melt(turn into liquid) at a temperature even lower.
  12. 12. Effect of Pressure on Melting Point <ul><li>Melting point of ice is lowered by an increase in pressure </li></ul><ul><li>At higher pressure, an ice cube would melt at temperatures lower than 0 o C. </li></ul><ul><li>At lower pressure (on the mountain), ice would only melt at temperatures higher than 0 o C. Which is why there can be snow on the mountain top even when surrounding temperatures are above 0 o C. </li></ul>
  13. 13. <ul><li>Example of pressing two ice cubes together. </li></ul><ul><li>When pressed together, the pressure they experience is higher, so the melting point is lowered causing the ice to melt. </li></ul><ul><li>When hand releases, the pressure they experience is lowered, so the water will now freeze at higher temperature. </li></ul><ul><li>Another example is walking on ice. </li></ul><ul><li>And ice skating. </li></ul>Effect of Pressure on Melting Point
  14. 14. boiling AND condensation <ul><li>When a liquid changes to a gas on heating, we call it boiling </li></ul><ul><li>In this instance, heat is ABSORBED by the substance </li></ul><ul><li>When a gas changes to a liquid on cooling, we call it condensation. </li></ul><ul><li>In this instance, heat is LOST by the substance </li></ul>
  15. 15. Boiling AND condensation <ul><li>In both cases, the heat that is absorbed or lost is known as the latent heat of vaporisation. </li></ul><ul><li>When this latent heat is absorbed or lost, there is no change in temperature. </li></ul><ul><li>This means when a substance is undergoing a change in state, the temperature will remain constant. </li></ul>
  16. 16. Effect of Impurities on Boiling Point <ul><li>Adding impurities will raise the boiling point of an object. </li></ul><ul><li>That means to say the liquid will now boil at a higher temperature. </li></ul>
  17. 17. Effect of Pressure On Boiling Point <ul><li>Lower pressure would lower the boiling point of water. </li></ul><ul><li>Water will boil very quickly on the mountain top but the temperature reached is lower than 100 o C. </li></ul><ul><li>Increasing pressure would raise the boiling point of water. </li></ul><ul><li>Water will boil at a higher temperature above 100 o C. </li></ul>
  18. 18. Specific Latent Heat <ul><li>Recall that the energy absorbed or released causing a change in state is known as latent heat. </li></ul><ul><li>Example: latent heat of fusion </li></ul><ul><li>latent heat of vaporisation </li></ul>
  19. 19. Specific Latent Heat <ul><li>This is the heat absorbed or released when 1 kg of the substance undergoes a change in state. </li></ul><ul><li>There are 2 types of specific latent heat: </li></ul><ul><li>Specific latent heat of fusion </li></ul><ul><li>Specific latent heat of vaporisation </li></ul>
  20. 20. Specific Latent Heat of Fusion <ul><li>This is the heat required by 1 kg of a substance to change from solid to liquid or vice versa without a change in temperature </li></ul><ul><li>Formula: L f = m l f </li></ul><ul><li>Unit: Jkg -1 </li></ul>
  21. 21. Specific Latent Heat of Vaporisation <ul><li>This is the heat required by 1 kg of a substance to change from liquid to gas or vice versa without a change in temperature </li></ul><ul><li>Formula: L v = m l v </li></ul><ul><li>Unit: Jkg -1 </li></ul>
  22. 22. Example: <ul><li>4400 J of heat was supplied to 2kg of water at 100 o C. All of the water turned into steam. What is the specific latent heat of vaporisation of water? </li></ul>Solution: L v = m l v 4400 = 2 ( l v ) l v = 4400 / 2 l v =2200 Jkg -1
  23. 23. Evaporation <ul><li>Liquid changing into gas. </li></ul><ul><li>Seems similar to boiling but there are differences. </li></ul>
  24. 24. Differences <ul><li>Heat supplied by surroundings </li></ul><ul><li>Heat supply by energy source </li></ul><ul><li>Temperature may change </li></ul><ul><li>Temperature remains constant </li></ul><ul><li>No bubbles are seen </li></ul><ul><li>Bubbles are formed </li></ul><ul><li>Takes places only on surface </li></ul><ul><li>Takes places within liquid </li></ul><ul><li>Slow process </li></ul><ul><li>Quick process </li></ul><ul><li>Occurs at any temperature </li></ul><ul><li>Occurs at fixed temperature </li></ul>Evaporation Boiling
  25. 25. Factors Affecting rate of Evaporation <ul><li>Temperature </li></ul><ul><li>Temperature of liquid and surroundings. </li></ul>

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