Phase+Changes



The six phase changes of matter are vaporization, condensation, melting, freezing, sublimation, and deposition. Vaporization is the change from liquid to gas, and condensation is its opposite (gas to liquid). Melting, or fusion, is the change from solid to liquid, and freezing is its opposite. Sublimation is the change of a solid directly into a gas without becoming a liquid, and deposition is its opposite. The exothermic phase changes are condensation, freezing, and deposition. The endothermic phase changes are vaporization, melting, and sublimation.



Every time matter changes phases, it is only because of a change in energy. These changes in energy can be measured in kJ/mol. For example, the heat of fusion for water is 6.01 kJ/mol and its heat of vaporization is 40.7 kJ/mol. This difference in energy changes is a trend in all substances. This is because changing from a solid to a liquid all requires enough energy to overcome some attractive forces, while becoming a gas requires even more energy to virtually overcome all intermolecular forces. These two measurements are important to finding the heat of sublimation. The heat of sublimation is actually the sum of the heat of fusion and the heat of vaporization. This makes logical sense because the substance is reaching the same end result, but it is taking a "pit stop" as a liquid. Therefore the heat of sublimation for ice is 47 kJ/mol. Another important aspect of phase change is that every change has its opposite (condensation's opposite is vaporization, etc.). The same thing applies to energy changes among opposite changes. This means that a change and its opposite will have the same magnitude, but different sign. For example, since the heat of vaporization of water is 40.7 kJ/mol, then the heat of condensation is -40.7 kJ/mol. This graphic organizer below shows all the phase changes, their opposite, and the sign (exothermic, negative, is blue, and endothermic, positive, is red).



A plot of temperature verses time is a heating curve.



In the graph above the solid starts at -40° C. It will stay a solid until the temperature rises to 0° C, where it begins to melt. When the entire solid is melted into a liquid, the temperature begins to rise again. Then the temperature reaches 100° C the liquid begins to boil and turn into a gas. The temperature remains 100° C until all the liquid is turned to gas, then the temperature begins to rise again. The amount of heat needed to raise the temperature is found by multiplying the specific heat by the mass by the temperature change. The greater the specific heat the more heat will be needed to raise the temperature. This occurs when the substance is in its solid liquid or gas phase. When the substance is in the melting or boiling phase, the temperature is constant.

A great website that helps explain the heating curve of water is:http://netcamp.prn.bc.ca/nuggets/heatingcurve.swf



The critical temperature is the highest temperature in which a substance will exist in a distinct liquid phase. Each critical temperature has a corresponding critical pressure, which is the pressure required to liquefy a substance at its critical temperature. The reason that this happens is become at some temperature different for every substance, the heat provides enough energy for the gas to overcome any and every intermolecular forces. Water's critical temperature is 374 degrees Celsius and its critical pressure is 165,500 torr or 217.7 atm. This is a very high critical temperature and pressure because polar water must overcome its hydrogen-bonding forces. Trends in critical temperature and pressure are that the lower values belong to substances that are nonpolar and smaller molecular weights. This is because these molecules have less intermolecular forces. The picture of liquid nitrogen below illustrates how high pressures and low temperatures are required to keep it in liquid phase. When the sample of nitrogen was placed on the table in normal conditions, the temperature to above the critical temperature of nitrogen. Therefore, the liquid started to revert to its gaseous phase, as can be seen in the picture.

Sources: Brown, LeMay, and Bursten. //Chemistry, the Central Science//.

Images: Brown, LeMay, and Bursten. //Chemistry, the Central Science//. http://www.istockphoto.com/file_closeup/?id=1816408&refnum=468603 http://netcamp.prn.bc.ca/nuggets/heatingcurve.swf http://www.bbc.co.uk/schools/ks3bitesize/science/images/sci_dia_21.gif

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