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Chemistry LibreTexts

12.1: Interactions between Molecules

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  • In the winter, many people find the snow and ice beautiful. They enjoy getting out to ski or ice-skate. Others don't find that time of year to be so much fun. When the snow melts, the roads get very sloppy and messy. Those people look forward to spring when all the ice and snow are gone and the weather is warmer.

    Melting Point

    Solids are similar to liquids in that both are condensed states, with particles that are far closer together than those of a gas. However, while liquids are fluid, solids are not. The particles of most solids are packed tightly together in an orderly arrangement. The motion of individual atoms, ions, or molecules in a solid is restricted to vibrational motion about a fixed point. Solids are almost completely incompressible and are the densest of the three states of matter.

    As a solid is heated, its particles vibrate more rapidly as the solid absorbs kinetic energy. Eventually, the organization of the particles within the solid structure begins to break down and the solid starts to melt. The melting point is the temperature at which a solid changes into a liquid. At its melting point, the disruptive vibrations of the particles of the solid overcome the attractive forces operating within the solid. As with boiling points, the melting point of a solid is dependent on the strength of those attractive forces. Sodium chloride \(\left( \ce{NaCl} \right)\) is an ionic compound that consists of a multitude of strong ionic bonds. Sodium chloride melts at \(801^\text{o} \text{C}\). Ice (solid \(\ce{H_2O}\)) is a molecular compound whose molecules are held together by hydrogen bonds. Though hydrogen bonds are the strongest of the intermolecular forces, the strength of hydrogen bonds is much less than that of ionic bonds. The melting point of ice is 0 °C.

    220px-Melting_icecubes.gif

    Figure \(\PageIndex{1}\): Melting ice cubes illustrate the process of this phase transition. Image used with permission (Public Domain; Moussa).

    The melting point of a solid is the same as the freezing point of the liquid. At that temperature, the solid and liquid states of the substance are in equilibrium. For water, this equilibrium occurs at \(0^\text{o} \text{C}\).

    \[\ce{H_2O} \left( s \right) \rightleftharpoons \ce{H_2O} \left( l \right)\]

    We tend to think of solids as those materials that are solid at room temperature. However, all materials have melting points of some sort. Gases become solids at extremely low temperatures, and liquids will also become solid if the temperature is low enough. The table below gives the melting points of some common materials.

    Materials Melting Point (°C)
    Table \(\PageIndex{1}\): Melting Points of Common Materials
    Hydrogen -259
    Oxygen -219
    Diethyl ether -116
    Ethanol -114
    Water 0
    Pure silver 961
    Pure gold 1063
    Iron 1538

    Summary

    The melting point is the temperature at which a solid changes into a liquid. Intermolecular forces have a strong influence on melting point.

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