LSS_Physical Properties of Liquids Lesson

Physical Properties of Liquids

Most physical properties of liquids and solids depend on the strengths of intermolecular attractions. Even their ability to maintain their own volume or how solids maintain their shape is caused by IMAs. Here are some other physical properties of liquids and solids and their relationship to IMA.

Boiling Point

The boiling point of a compound is the temperature at which a compound turns from a liquid to a gas or a gas to a liquid. This temperature is a true measure of the forces of attractions between molecules as molecules separate from one another when they turn from a liquid to a gas. The stronger the attractions between particles, the more difficult it will be to separate the particles. So, the stronger the IMA, the higher the boiling point.

Surface Tension

Surface tension is that property of a liquid that gives it a "skin-like" surface. Some bugs can even walk on water due to the surface tension. Technically, surface tension is the property of liquids surface to bring the surface area to a minimum. The shape that results is a sphere. This is what causes raindrops to be shaped like spheres, why glass edges are smooth when heated, and why you can fill a cup of water slightly above the rim. Surface tension is caused by the difference between surface attractions and the attractions beneath the surface.

A molecule within the liquid is surrounded by like molecules on all sides, feeling the pull of attraction from all sides. Molecules at the surface are only pulled toward the molecules underneath it and beside it. This uneven distribution of attractions causes an overall pull toward the center, like a sphere. The magnitude of the surface tension is directly proportional to the strength of IMA. Water has a very high surface tension because of the hydrogen bonding between the molecules.

Ability of a Liquid to Wet a Surface

The ability of a liquid to wet a surface refers to the spreading of a liquid across a surface to form a thin film. To wet a surface, the intermolecular forces between the liquid and the surface must be about the same strength as within the liquid itself. For example, water won't wet a greasy surface, like the hood of a newly waxed car, but instead forms beads of water. On the waxy surface, the H bonding between water molecules is much stronger than the attraction between water and the grease. Therefore, no wetting occurs.

Viscosity

Viscosity is the resistance to flow. You can think of it as a kind of internal friction caused by intermolecular attractions. Generally speaking, the stronger the attractions the more viscous. But even though water has strong IMA, is not considered to be very viscous. The other factor that is important here is the ability of the molecules to tangle with each other. Molecules that are long chains tend to be very viscous. Viscosity is also variable with temperature. The viscosity increases as the temperature decreases.

Tendency to Evaporate

Have you ever wondered how a puddle of water can evaporate (change from a liquid to a gas) even though the temperature outside is far below the boiling point?

Molecules are constantly moving. Though they have an average kinetic energy, some are slower and some are faster.  

If a higher velocity molecule reaches the surface it can escape the attractions of neighbors and enter the vapor state. As these molecules of higher kinetic energy leave, the molecules with lower kinetic energy are left. As evaporation occurs the temperature decreases because the average kinetic energy decreases.

Several factors affect the rate of evaporation. They are:

1. Surface area - The greater the surface area, the greater the rate of evaporation.
2. Temperature - As temperature increases, the rate of evaporation increases. It is important to note that the amount of kinetic energy needed to escape is independent of temperature, but at higher temperatures more molecules have that required amount of energy.
3. Strength of IMA - The weaker the IMA, the greater the rate of evaporation. The weaker these forces holding the molecules in the liquid state, the lower the kinetic energy required to escape. An example of this is shown in the graphs below. Both graphs show liquids A and B at same temperature.

On the interactive below you can explore both temperature and strength of IMA (shown below as Van der Waals forces) and their relationship to the number of molecules with enough energy to evaporate. Pressure refers to atmospheric pressure. We will discuss its relationship to the liquid/gas phase change later. Right now, keep that value constant.

• Adjust the temperature by sliding the bar. Notice how the shape of the curve changes. What does this shape mean?
• Adjust the Van der Waals Force. Notice how the minimum amount of energy required to escape changes. What is the relationship?

Reminder - Don't skip these interactives! They are an excellent way to make sure that you understand the concepts being taught. You are also likely to see similar diagrams on your assessments! Also remember that the AP Chemistry Exam changed in 2013 to focus more on concepts and less on memorization of facts. This includes the interpretation of diagrams.

Remember to work on the module practice problems as you complete each section of content.  

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