Home School Subject: Physics Lesson: Changing state Questions: Copy and complete: a. A solid must be given latent heat before it can melt into a liquid and a liquid must be given latent heat before it can boil into a gas. The energy is needed to overcome the forces between the molecules so that they can move freely. b. The specific latent heat of fusion of a substance is the amount of energy needed in joules to melt 1 kg kg of solid to liquid without changing the temperature. Its unit is joules/ kg c. The formul for energy needed to change state : specific latent heat x mass to change state. d. Impurities or change of pressure affects the melting point of water. Increased pressure increase the melting point of water. e. Impurities or change of pressure affects the boiling point of water. Increased pressure increase the boiling point of water. f. Evaporation causes cooling because only the faster ( hotter ) molecules escape leaving the slower ( cooler ) molecules in the liquid. The specific latent heat of fusion of a substance is the amount of energy needed in joules to melt 1 kg of solid to liquid without changing the temperature. Its unit is joule/kg ice cubes need a lot of latent heat in order to melt. They need 340000 joules/kg of latent heat. The specific latent heat of vaporization or evaporation of a substance is the amount of energy needed in joules to boil 1 kg of liquid to gas without changing the temperature. Its unit is joule/kg. The specific latent heat of vaporization is 2300000 joules/kg. Properties of three states of matter: • Solids have fixed shape, size and volume. • Solids cannot be compressed or squashed easily. • Soilds have strong inter molecular forces and they are closely or tightly packed together. As a result they have no free space between the molecules. • Liquids have fixed volume but no fixed shape. They take the shape of their containers. • Liquids are very difficult to compress. • Liquids have less strong inter molecular forces than solids and they have little more space between their molecules so they can slip and slide over each other. • Gases have no fixed shape and volume. They take the shape of their containers. • Gases are very easy to compress. • Gases have a lots of space between them. They have have less strong inter molecular forces than solids and liquids between their molecules. Now copy and complete these characteristics in a table chart: Mathematical Problems: If the specific latent heat of ice is 340000 j/kg and the specific latent heat of steam is 2300000 j/kg, calculate: a. Energy needed to melt 2 kg of ice at o degree celsius. b. Energy needed to melt 500 gm of ice at o degree celsius. c. Energy needed to boil 3 kg of water at 100 degree celsius. d. Energy needed to boil 100 gm of water at 100 degree celsius
Posted on: Wed, 07 Aug 2013 18:06:50 +0000
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