## Class 9 - Physics

#### 4. Liquid to Solid Change : Freezing

When water is cooled (by lowering its temperature by keeping in the freezer compartment of a refrigerator), it changes into solid 'ice'. The process of changing a liquid into a solid by cooling, is called freezing. For example, when water is cooled, it gets converted into a solid called "ice". This is called freezing of water. Freezing means solidification. When water freezes to form ice, then there is a change from liquid state to solid state. Please note that freezing is the reverse of melting. So, the freezing point of a liquid is the same as the melting point of its solid form. For example, the melting point of ice is 0°C, so the freezing point of water is also 0°C. A change of state (liquid to solid) occurs during freezing. This happens as follows.
When a liquid is cooled (by lowering its temperature), its particles lose energy due to which they move slowly. If the liquid is cooled enough (up to its freezing point), its each particle stops moving and vibrates about a fixed position. At this stage the liquid freezes and becomes a solid.
From the above discussion we conclude that the state of matter can be changed by changing the temperature (by heating or cooling). For example, when a solid is heated, it melts to form a liquid. And when the liquid is heated further, it boils to form a gas. These changes can be easily reversed. For example, when a gas is cooled, it condenses to form a liquid. And when a liquid is cooled further, it freezes (or solidifies) to form a solid. The conversion of matter from one physical state to another can be shown as follows :

## Notes

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