Lecture No. 10 Properties of Liquids & Solids

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CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state • When a liquid or solid evaporates or a solid melts, there are changes in the distances between the particles of the substance. • Because there are strong attractive forces between the particles, there are changes in the potential energy that accompany these changes of state. • To study these energy changes, let’s begin by examining how the temperature of a substance varies as it is heated.

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CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• The above figure illustrates the way the temperature of a substance changes as we added heat to it at a constant rate, starting with the solid and finishing with the gaseous state of the substance, the graph is sometimes called a heating curve for the substance. 3 CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• As shown in the figure, as heat is added to the solid, the temperature rises until the solid begins to melt. The temperature then remains constant while both solid and liquid phases are present. 4

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• After all the solid has melted, additional heat raises the temperature of the liquid. This continues until the boiling point is reached, at which point the temperature levels off again. 5

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• The temperature of the boiling liquid stays the same until all of it has boiled away and changed to a gas. Then finally, more heat just raises the temperature of the gas.

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CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves • In the portions of the graph where the temperature raises, that occur where we are heating the solid, the liquid or the gas phase of the substance. • Since the temperature is related to average kinetic energy, the heat we add in these regions of the heating curve goes primarily to increasing the average kinetic energies of the particles. In other words, the added heat makes the particles go faster and collide with each other with more force.

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CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves • In the portions of the graph where the temperature remains constant, the average kinetic energy of the particles is not changing. • This means that all the heat being added must be increasing the potential energy of the particles. • During melting, the particles held rigidly in the solid begin to separate slightly as they form the mobile liquid phase. • The separation of particles gives rise to a potential energy increase, which is what we measure by following the heat input during the melting process. 8

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves • During boiling, an even greater increases occurs in the distances between the molecules. Here they go from the relatively tight packing in the liquid to the widely spaced distribution of molecules in the gas. This gives rise to an even larger increase in the potential energy, which we see as a longer region on the heating curve during the boiling of the liquid.

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CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• The opposite of a heating curve is a cooling curve, shown in the above figure. • Here, we start with a gas and gradually cool it until we reach a solid. 10

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• The cooling curve looks very much like the opposite of a heating curve, except for what often happens when the temperature of the liquid approaches the freezing point.

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CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• We might expect that when the freezing point is reached, removal of more heat would cause the immediate formation of some solid. • Often, however, the liquid continues to cool below its freezing point, a phenomenon called super cooling. 12

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• The reason super cooling happens is that the molecules of the liquid at the freezing point are in a jumbled disordered arrangement, not in the ordered structure that we will see is characteristic of a solid. 13

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• While, the molecules move about, waiting for a few to form the beginnings of a crystal, the temperature continues to drop because heat is continually begin removed. 14

CH102 Mahmoud A. A. Ibrahim 7. Energy changes during changes of state Heating curves and cooling curves

• Finally, a tiny crystal forms and other molecules quickly join it, losing potential energy which is changed to kinetic energy. This sudden increase in kinetic energy raises the temperature to the freezing point and then freezing continues normally as more heat is taken away. 15

CH102 Mahmoud A. A. Ibrahim End of Lecture 10

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CH102 Mahmoud A. A. Ibrahim