CBSE Class 9 Chemistry Matter in Our Surroundings Notes Set 09

Changes of State

1. Melting (fusion): solid to liquid at the melting point. Freezing: liquid to solid. The melting point of ice is 273.15 K (0 degrees C).
2. Boiling (vaporisation): liquid to vapour at the boiling point, 373 K (100 degrees C) for water at normal pressure.
3. Latent heat is the heat absorbed or released at constant temperature during a change of state. Latent heat of fusion for ice is 3.34 x 10 to the power 5 J per kg and of vaporisation for water is 22.5 x 10 to the power 5 J per kg. Steam at 100 degrees C has more energy than water at 100 degrees C because of this hidden heat.
4. Sublimation: direct change from solid to gas, as in camphor, naphthalene and ammonium chloride. Deposition is the reverse.
5. Temperature scales: K = degrees C + 273.

Evaporation

1. Evaporation is the change of a liquid to vapour below its boiling point, taking place at the surface only. It causes cooling because the fastest particles escape and the liquid loses energy.
2. The rate increases with larger surface area, higher temperature and wind speed. It decreases with higher humidity.
3. Examples: wet clothes dry faster in sun and wind, sweat cools us, water in an earthen pot stays cool, and dogs pant to cool themselves.
4. A change of pressure also changes the state: a gas can be liquefied by cooling and compressing it. Solid carbon dioxide is called dry ice because it goes directly to gas at normal pressure.