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Thursday, September 4, 2008
Chapter 9: Thermal Properties of Matter
Internal Energy Total energy of the particles. It comprises of two componenets - kinetic energy & potential energy. The kinetic component of internal energy is due to the vibration of the particles. It is directly related to temperature. The higher the temperature, the more vigorous the vibrations of the particles. In liquids and gases, the particles are able to move freely. Thus, the kinetic energy is due to their movement instead of vibrations. The potential component of internal energy is due to the stretching and compressing of the intermolecule bonds as the particles vibrate. The amount of potential energy stored on the bonds depends on the force between the particles and how far apart the particles are. Therefore, we can see that if the temperature of the substance rises, it is due to and increase in the average kinetic energy of its particles only. Melting and Solidification Melting When a solid changes to a liquid upon heating, this change of state is called melting. For a pure substance, melting occus at a definite or constant temperature. This particular temperature is known as the melting point of the substance. Solidification The reverse process of melting is called solidification - changing from a liquid to a sold. A pure substance will solidify or freeze at a temperature equal to its melting point. For example, water freezes to form ice at 0 degree celsius. We call this temperature of 0 degree celsius the freezing point of water. Boiling and condensation When a pure liquid is heated and it changes to a vapour at a fixed or constant temperature, we call this change of state boiling. This particular temperature is known as the boiling point of the substance.The reverse of boiling is condensation. It is the change of state from vapour to liquid when a substance is cooled at the same constant temperature as in boiling. -During boiling, the temperature remains constant at its boiling point. thermal energy is being absorbed by the substance. Evaporation Evaporation is the change of state from liquid to gas. The different between boiling and evaporation is that evaporation can occur at any temperature. Boiling 1. Occurs at fixed temperature 2. quick process 3.Takes place throughout the liquid 4.Bubbles are formed in the liquid 5.Temperature remains constant 6.Thermal energy supplied by an energy source Evaporation 1. Occurs at any temperature 2. Slow process 3. Takes place only at liquid surface 4. No bubbles formed in the liquid 5. Temperature may change 6. Thermal energy supplied by the surroundings Factors affecting the rate of evaporation: 1) Temperature Raising the temperature of the liquid will increase the rate of evaporation. A warmer liquid means that a greater number of molecules at the surface layer are energetic enough to escape. 2) Humidity of the surrounding air The rate of evaporation decreases with increasing humidity. Wet clothes do not dry easily if the surrounding air is damp. Wet floors take a longer time to dry on a humid day. Similarly, the rate of evaporation increases with lower humidity. 3) Surface area of the liquid The rate of evaporation increase when there is more exposed surface are of the liquid. This is because evaporation only takes place at the exposed surface of a liquid. A larger exposed surface area means more molecules can escape from the liquid. 4) Movement of air The rate of evaporation increases when the surrounding air is moving. 5) Pressure Reducing the atmospheric pressure increases the rate of evaporation. 6) Boiling point of liquid Liquids with lower boling points will evaporate faster. END OF CHAPTER NINE :D TADAAAAAAAAA |