The Liquid State

�Before You Begin:

To master this material you will need to understand Kinetic Molecular Theory and Intermolecular Attractions.

drop of water

Properties of Liquids:

Compressibility

A liquid has a definite volume but not a definite shape. Like a gas, a liquid takes the shape of its container. Unlike a gas, the volume of a liquid doesn’t change much as pressure increases. We can explain the properties of a liquid using kinetic molecular theory. A liquid is similar to an ideal gas because the molecules that make up the liquid are moving enough to prevent the substance from having a fixed shape. A liquid is different from an ideal gas because the molecules that make up the liquid are so close together that they resist changes in volume even at high pressure.

diagram of the difference between a gas and a liquid--in a gas the molecules are very few and far apart; in a liquid the molecules are more densely packed

Surface Tension

The surface tension is the force that makes a liquid assume a shape that has the least amount of surface area. This force can be thought of as an elastic ‘skin’ stretched over the surface of a liquid. Liquids form spherical droplets because a sphere is the solid shape with the least surface area per unit volume. The liquid will roll itself into a droplet so as to stretch the skin as little as possible. Surface tension is caused by unbalanced van der Waals forces. Molecule in the bulk of the liquid are attracted to neighbors on all sides. The molecules on the surface are attracted to molecules below but not above. This results in a net force pulling the surface molecules inward.

diagram of the forces responsible for surface tension

Cohesive forces are the attractions between a particle and others of the same kind, such as the hydrogen bonds among a collection water molecules. Adhesive forces are attractions between a particle and others of a different kind, such as the attraction of a water molecule to the glass of its container. These take place at surfaces and interfaces.

Surface tension and adhesive force cause a liquid to form a meniscus in a glass test tube. A liquid with strong cohesive forces, such as water, tries to roll itself into a ball to minimize surface tension. But water is also attracted to the walls of the test tube by adhesive dipole-dipole forces. The best compromise is to form a concave surface.

water in glass forms a concave meniscus due to strong adhesive forces

Liquid mercury will form a meniscus that is convex, because it is non-polar. It has induced dipole adhesive forces, but it does not form strong intermolecular attractions with the glass.

mercury in glass forms a convex meniscus due to weak adhesive forces

So, how can you predict if a liquid will have strong or weak adhesive forces? In general, the adhesive forces will be strong if the liquid has the same types and strengths of intermolecular attractions for the container as its cohesive forces. Glass is silicon dioxide, a polar compound capable of fairly strong dipole-dipole attractions. So is water. Mercury, on the other hand, is not polar. It is composed of individual mercury atoms loosely bound by metallic bonds. Its only adhesive force is induced dipole and the metallic bonds themselves. Water sticks to the glass because if forms dipole-dipole attractions to the glass that are similar to those that it forms with itself. The mercury, in contrast, is not attracted to the glass at all. We will discuss attractions between different substances in greater detail when we examine mixtures.

Capillary Action

When water is in a tube, it has dipole-dipole interactions with the glass. Since these attractions are strong, the water will flow upwards clinging to the wall of the tube until the adhesive force equals the gravity pulling on the mass of suspended water. Paper products are made from cellulose, a fiber found in plant material. Water soaks into a paper towel, in part because adhesive force helps draw the water into spaces around the paper fibers, kind of like capillary action but on the outside of the tube!

Viscosity

Viscosity is the amount of resistance to flow that a particular liquid has. In other words, viscosity is a measure of how thick or sticky a liquid is. The higher the viscosity of a liquid is, the harder the liquid is to pour. Liquids with the strongest intermolecular attractions will have the highest viscosity. As the temperature of a liquid increases, the viscosity decreases. At higher temperatures, the particles of the liquid have higher kinetic energy to overcome their intermolecular attractions. This is why hot maple syrup pours faster than cold maple syrup.

This site was last updated 05/13/05