Physics

Sound


1. Latent Heat of Fusion (Solid to Liquid Change)


The latent heat of fusion can be studied by performing an experiment as follows : We take some crushed ice in a beaker and suspend a thermometer in it (see Figure 40). We note the temperature of ice. It is found to be 0°C. We now heat the ice gently by using a small flame of the burner. On heating, ice starts melting to form water. We keep on recording the temperature of melting ice on the thermometer every minute. As more heat is given, more ice melts to form water but the thermometer reading remains at 0°C. As long as there remains even a little of ice in the beaker, the temperature does not rise, it remains constant at 0°C. This shows that there is no rise in temperature during the melting of ice. It is only when all the ice has melted that the temperature of water (formed from ice) starts rising on further heating. The heat which is going into ice but not increasing its temperature, is the energy required to change the state of ice from solid to liquid (water). This is known as the latent heat of fusion of ice (or latent heat of melting of ice). We will now give a definition of the latent heat of fusion of a solid substance.
The latent heat of fusion (or melting) of a solid is the quantity of heat in joules required to convert 1 kilogram of the solid (at its melting point) to liquid, without any change in temperature. It has been found by experiments that 3.34X105 joules of heat has to be supplied to change 1 kilogram of ice (at its melting point, 0°C) to water at the same temperature of 0°C. So, the latent heat of fusion of ice is 3.34X105 joules per kilogram (or 3.34X105 J/kg).
We have just seen that when a solid melts (on heating), its temperature remains the same. Now, an important question arises : Where does the heat energy go ? Actually, this heat energy is used up in changing the state of the solid substance by overcoming the force of attraction between its particles.This point will become more clear from the following example of melting of ice.
We will now discuss why the temperature of melting ice does not rise even though heat is being supplied continuously. This can be explained as follows: Ice is a solid substance, so the particles of ice attract one another with strong forces. These forces of attraction hold the particles closely packed in solid ice. The heat which we supply to ice during melting is all used up to overcome the forces of attraction between ice particles so that they become somewhat loose and form liquid water. This heat does not increase the kinetic energy of particles and hence no rise in temperature takes place during the melting of ice. But then all the ice has melted to form water, further heating increases the kinetic energy of water particles due to which the temperature of water starts rising sharply.
The latent heat of fusion of ice is 3.34X105 J/kg. By saying that the latent heat of fusion of ice is 3.34X105 joules per kilogram, we mean that 3.34X105 joules of heat is required to change 1 kilogram of ice at its melting point of 0°C into water at the same temperature (of 0°C). This means that 1 kilogram of ice at 0°C has 3.34X105 joules of less heat than 1 kilogram of water at the same temperature of 0°C. It has been found that ice at 0°C is more effective in cooling a substance than water at 0°C. This is due to the fact that for melting, each kilogram of ice takes its latent heat of 3.34X105 joules from the substance and hence cools the substance more effectively. On the other hand, water at 0°C cannot take any such latent heat from the substance.
When we hold a piece of ice in our hand, it feels very cold. This can be explained as follows : The piece of ice held in our hand starts melting slowly. This ice takes the latent heat (required for melting) from our hand. Our hand loses heat to ice and hence we feel it to becold.
When a solid melts, it absorbs heat to form liquid. The reverse of this is also true. That is, when a liquid freezes to form a solid, an equal amount of heat is given out. For example, when ice at 0°C melts, it absorbs latent heat of fusion to form water at the same temperature, 0°C. Now, when water at 0°C freezes to form ice at 0°C, then it gives out an equal amount of heat.

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Notes


Sound - Notes
1. 3. Humidity of Air
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2. 2. Liquid to Gas Change : Boiling (or Vaporisation)
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3. 4. Wind Speed
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4. Effect of 'Heating' and 'Cooling' on a Saturated Solution
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5. 2. Latent Heat of Vaporisation (Liquid to Gas Change)
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6. 2. Surface Area of the Liquid
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7. Classification Of Matter As Solids, Liquids And Gases
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8. Matter is made of particles
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9. Properties of Liquids
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10. Why Solids, Liquids and Gases Have Different Properties
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11. 2. The Particles of Matter have Spaces Between Them
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12. Liquids
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13. Properties of Solids
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14. Sound - Study Points
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15. 1. Solid to Liquid Change : Melting
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16. 3. Gas to Liquid Change : Condensation
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17. Evaporation
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18. 1. Latent Heat of Fusion (Solid to Liquid Change)
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19. Diffusion in Liquids
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20. 1. Temperature
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21. Effect of Change of Temperture
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22. Latent heat
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23. 1. The Particles of Matter are Very, Very Small
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24. Gases
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25. 4. The Particles of Matter Attract Each Other
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26. Change of State of matter
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27. Effect of Change pressure
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28. 3. The Particles of Matter are Constantly Moving
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29. Characteristics of Particles of Matter
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30. 4. Liquid to Solid Change : Freezing
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31. Two More States of Matter : Plasma and Bose-Einstein Condensate
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32. Factors Affecting Evaporation
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33. Diffusion
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34. Cooling Caused by Evaporation
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35. Solids
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36. Dissolving a Solid in a Liquid
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37. Matter In Our Surroundings
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38. The Common Unit Of Temperature and Si Unit Of Temperatre
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39. Mixing of Two Gases
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40. To Show That Solids and Liquids Cannot be Compressed but Gases Can be Compressed Easily
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41. Evidence for Particles in Matter
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42. Sublimation
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43. Rigid and Fluid
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44. To Show that Liquids do not have a Fixed Shape but they have a Fixed Volume
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45. To Show the Presence of Water Vapour in Air
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46. Movement of Pollen Grains in Water
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47. Diffusion in Solids
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48. Gases Can be Liquefied by Applying Pressure and Lowering Temperature
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49. Diffusion in Gases
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50. To Show that Gases do not have a Fixed Shape or a Fixed Volume
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