Liquids

Molecular crystals are held together by weak intermolecular forces. They can be held together by hydrogen bonds or other intermolecular forces that are much weaker than ionic and covalent network crystalline forms. They lack free electrons and so do not conduct electricity well. They have much lower boiling and melting points when compared to other crystal types.

LIQUIDS

So far, we’ve discussed phase changes as properties of different substances. Liquids in particular have unique properties that depend on the nature of the intermolecular relationships. In this section, we will talk about these things, including the surface tension, capillary action, viscosity, and boiling point.

Surface tension is described as the energy required to increase the surface area of a liquid by a specific amount with the SI units for this being in joules per meter squared. The bonding of water involves hydrogen bonding, yielding a high surface tension of water compared to other liquids. Mercury is even higher with a surface tension six-times that of water. With water, a glass of water can actually be overfilled with the level of water higher than the rim of the glass. Paper clips can float on the surface of water, despite the density, because of the surface tension of water, which prevents the “breakage” of the surface of the water.

There are other ways to measure the surface tension besides as joules per square meter. Another is dyne per centimeter where one dyne is 1 x 10-5 Newtons. The stronger the intermolecular forces, the higher the surface tension. High surface tension leads to high boiling points. Mercury is the main anomaly to this in that it has a high surface tension but not a very boiling point . In the case of mercury, the surface tension is high because of the metallic forces between the molecules.

Soaps and detergents can disrupt the normal intermolecular attractions seen in water molecules so these can decrease the surface tension. These are also referred to as surfactants. Surfactants are used to fight fires in large-scale firefighting because they can spread across burning surfaces.

Intermolecular forces also play a role in determining the capillary action of a liquid. Capillaries are small tubes, like straws, that have forces causing water to rise to higher than the level expected when a tube is put into a larger bath of water. The smaller the diameter of the tube, the higher the liquid rises. It involves both the cohesive forces of the liquid molecules with each other and the adhesive forces that bind the liquid to the surface of the tube.

The molecular makeup of the capillary makes a difference as well. Glass contains silicon hydroxide, which is polar and attracts water molecules (that are also polar). If the adhesive forces are greater than the cohesive forces, the liquid will rise in the tube against the force of gravity. Mercury has a negative capillary action, in part because of its lack of adhesive force with glass. The meniscus (the shape of the upper surface of a column of liquid) is concave with water but is convex with mercury because of the different phenomena involved. Figure 41 shows the capillary action in water and mercury:

Figure 41.

Viscosity is the resistance of liquid to flow. Certain liquids have a low viscosity (like water and gasoline), while others (like molasses) have a high viscosity. It can be