The Liquid State

Skills to develop

Define liquid
Explain viscosity, surface tension, and capillary action

The Liquid State

The liquid state of a material has a definite volume, but it does not have a definite shape and takes the shape of the container, unlike that of the solid state. Unlike the gas state, a liquid does not occupy the entire volume of the container if its volume is larger than the volume of the liquid.

At the molecular level, the arrangement of the molecules is random, unlike that of the solid state in which the molecules are regular and periodic. The molecules in the liquid state have translational motions as those in a gas state. There is short order intermolecular ordering or structure, however.

The most familar liquid states at room temperature are water, alcohol, benzene, carbon tetrachloride, corn oil, Castor oil, and gasoline. Name a liquid not listed here, please.

Glasses, although solids, are often called frozen liquids, because the arrangements of molecules in glasses are very similar to those in liquid states.

Viscosity

Viscosity of a liquid is a measure of its inability to flow, and this is measured in N s m^-2 (SI Units) or poise (P) or centipoise (cP).
1 P = 0.1 N s m^-2
1 cP = 0.001 N s m^-2

Viscometers are used to measure viscosity today, and the animation shown here is from the web site of Norcross, Ltd, which manufactures viscometers. For chemical engineering applications, the Handbook of Viscosity lists values for the chemical processing and petroleum refining industries.

The viscosity of water depends on temperature. At 293 K (20°C), the viscosity of water is 1.002 cP. Its viscosity decreases as temperature increases:

Viscosity of Water as a Function of Temperature

Temperature /°C	Viscosity /cP
20	1.002
40	0.653
60	0.467
80	0.355
100	0.282

In the following table, the viscosities of some common liquids at 293 K are given. Viscosity is an important property, not just numbers. Take a look of the value, and recall the handling of some of these liquids. For example, what is common between Castor and glycerol? They both have very high viscosity. So does honey, but I do not have a number for it, because it is not a pure compound, and its viscosity varies with composition and temperature too.

Viscosity and Surface Tension of Varoius Liquids at 293 K

Common liquid	Viscosity /cP	Surface tension /N m^-1
Diethyl ether	0.233	0.0728
Chloroform	0.58	0.0271
Benzene	0.652	0.0289
Carbon tetracholoride	0.969	0.0270
Water	1.002	0.0728
Ethanol	1.200	0.0228
Mercury	1.554	0.436
Olive oil	84	-
Castor oil	986	-
Glycerol	1490	0.0634
Glasses	very large	-

You need not to memorize this table, but the table is presented for you to correlate the numbers with the properties of the substances you encounter from time to time. An interesting thing to do is to coorelate the viscousity with molecular structures of liquids. In general, liquids of large molecules have high viscousity.

Surface Tension

Surface tension is the energy required to stretch a unit change of the surface area. Thus its units are N * m m^-2 = N/m. The surface tensions of some common liquids are also listed above. As the data shows, there is no direct correlation between viscosity and surface tension. These two properties are independent of each other.

The surface tension is due to the unbalanced force experience by molecules at the surface of a liquid. As a result of surface tension, a drop of liquid tends to form a sphere, because a sphere offers the smallest area for a definite volume.

Substances with low surface tension have a tendency to form films. When detergent is added to water, it lowers the surface tension. Blowing soap water with a straw form bubbles, due to the low surface tension.

Like density, viscosity, and boiling point, surface tension is a bulk property that affects everyday phenomena such as the food we drink and eat. This link let you see the importance of this property in food science.

Capillary Action and Wetting

Cohesion is the intermolecular attraction between like molecules, and adhesion is that between unlike molecules. Liquids with high surface tensions have strong cohesion forces, and they are poor wetting liquid due to low adhesion forces.

A detergent or wetting agent is a substance that increases the adhesion force between two different materials. Molecules of detergent usually have a polar and an non-polar portion. When added, the wetting agent increases the wetting action of water with the non-polar material. By this action, dirt is removed when washed with water.

When a small tube is dipped into a liquid, the level in the tube is usually higher or lower than that of the bulk liquid. If adhesion force between the tube material and the liquid is stonger than the cohesion force, the level is higher. Otherwise, the level is lower. Such phenomena are called capillary action.

Capillary action is one of the factors responsible for transport of liquid and nutrients in plants, and sometimes in animals. Next time when you dip a straw into your drink, watch the levels and explain the phenomenon.

Capillary action is often a topic of study in grade schools, and this link connects you with this action in everyday life.

Confidence Building Questions

A drop of water is placed in a large flask which is under vacuum. The water soon evaporated, why?

Skill -
Use a phase diagram to explain the phase transition.
At 274 K, what pressure is required for water to be a stable phase?

Skill -
Know where to find the value, not to memorize it.
Molecules at which state possesses the highest kinetic energy?

Skill -
Molecules in the solid state possess only vibrational and rotational kinetic energy, not translational kinetic energy.
Molecules at which state possesses the highest order?

Discussion -
Because of regularity and periodicity, arrangements of molecules in solid is said to be orderly.
How does viscosity of water change as the temperature increases?

Skill -
Explain the reduction of viscosity in terms of cohesion.
What is the action by with plants transport water and nutrients from the root to leaves?

Skill -
Correlate phenomena to principles.
In an attempt to clean up oil spills, what properties of material shall be considere?

Further interest -
For further insight, see The Environmental Solution for Oil and Chemical Spills
What happens when two cheerios are placed near each other in a bowl of milk (and why)?

Further interest -
Click the question to see more on this question.