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Heating and Cooling Curve of a Pure Substance

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A heating curve is a plot of temperature verses time for a substance where energy is added at a constant rate. (http://www.biology-online.org/dictionary/Heating_curve). A cooling curve is defined as a curve obtained by plotting time against temperature for a solid-liquid mixture cooling under constant conditions. (http://www.answers.com/topic/cooling-curve) When a substance is going through a physical change such as melting or vaporization, it can be found as a plateau because there is no temperature change occurring. Heat is never stopped being added to the substance, but instead of the substance taking the heat and increasing in temperature, it takes the heat and changes its molecules bonding from a solid to a liquid. The experiment shows that heat is seen to be lost as the liquid cools down and then the temperature averts going down as the liquid turns to a solid with no change in temperature. Then the temperature begins to fall again after all of the liquid has solidified and the solid cools. The flat part of the graph demonstrates that invisible heat, Latent, that is given off when liquids turn to solids but we cannot show this using a thermometer as the temperature does not change even though a lot of heat is being lost. The kinetic energy of the molecules is slightest in the solid phase because the molecules can't move much. When temperature is raised causing the kinetic energy to increase the material turns liquid from solid which is called the melting point. In a liquid the molecules can move, but with a lot of restrictions. If you increase the temperature further, the kinetic energy increases. At one position, the molecules can move with great speed and minimum restrictions. Such a state is called gaseous state and such a point of temperature is called the boiling point. (http://answers.yahoo.com/question/index?qid=20090901014650AAbizRN)

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