Water is the only substance we routinely encounter as a solid, a liquid, and a gas. At low temperatures, it is a solid in which the individual molecules are locked into a rigid structure. As we raise the temperature, the average kinetic energy of the molecules increases, which increases the rate at which these molecules move.
There are three ways in which a water molecule move: (1) vibration, (2) rotation, and (3) translation. Water molecules vibrate when H--O bonds are stretched or bent. Rotation involves the motion of a molecule around its center of gravity. Translation literally means to change from one place to another. It therefore describes the motion of molecules through space.
To understand the effect of this motion, we need to differentiate between intramolecular and intermolecular bonds. The covalent bonds between the hydrogen and oxygen atoms in a water molecule are called intramolecular bonds. (The prefix intra- comes from the Latin stem meaning "within or inside." Thus, intramural sports match teams from the same institution.) The bonds between the neighboring water molecules in ice are called intermolecular bonds, from the Latin stem meaning "between." (This far more common prefix is used in words such as interface, intercollegiate, and international.)
The intramolecular bonds that hold the atoms in H2O molecules together are almost 25 times as strong as the intermolecular bonds between water molecules. (It takes 464 kJ/mol to break the H--O bonds within a water molecule and only 19 kJ/mol to break the bonds between water molecules.)
All three modes of motion disrupt the bonds between water molecules. As the system becomes warmer, the thermal energy of the water molecules eventually becomes too large to allow these molecules to be locked into the rigid structure of ice. At this point, the solid melts to form a liquid in which intermolecular bonds are constantly broken and reformed as the molecules move through the liquid. Eventually, the thermal energy of the water molecules becomes so large that they move too rapidly to form intermolecular bonds and the liquid boils to form a gas in which each particle moves more or less randomly through space.
The difference between solids and liquids, or liquids and gases, is therefore based on a competition between the strength of intermolecular bonds and the thermal energy of the system. At a given temperature, substances that contain strong intermolecular bonds are more likely to be solids. For a given intermolecular bond strength, the higher the temperature, the more likely the substance will be a gas.
The kinetic theory assumes that there is no force of attraction between the particles in a gas. If this assumption were correct, gases would never condense to form liquids and solids at low temperatures. In 1873 the Dutch physicist Johannes van der Waals derived an equation that not only included the force of attraction between gas particles but also corrected for the fact that the volume of these particles becomes a significant fraction of the total volume of the gas at high pressures.
The van der Waals equation is used today to give a better fit to the experimental data of real gases than can be obtained with the ideal gas equation. But that wasn't van der Waals's goal. He was trying to develop a model that would explain the behavior of liquids by including terms that reflected the size of the atoms or molecules in the liquid and the strength of the bonds between these atoms or molecules. The weak intermolecular bonds in liquids and solids are therefore often called van der Waals forces. These forces can be divided into three categories: (1) dipole-dipole, (2) dipole-induced dipole, and (3) induced dipole-induced dipole.
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