LSS_Phase Changes Lesson

Phase Changes

When a substance changes from one state of matter to another it is called a phase change.   One important feature of the phase change is that at a certain temperature the changes tend toward physical equilibrium. This means that the rate of exchange between states of matter is equal.  

For example, consider a liquid evaporating in a sealed container. Molecules with enough kinetic energy escape to the gaseous phase. When they hit the liquid surface, they lose some of their KE and return to the liquid phase. This is condensation and is shown below in picture (a). Recall that the rate of condensation depends upon the surface area, the IMA of the molecules, and the temperature. It also depends upon the concentration of molecules in the vapor state. At first the rate of evaporation is greater than the rate of condensation, shown in (b). As the concentration of vapor molecules increases, so does the rate of condensation. This happens until equilibrium is reached, shown in (c).   Note that this does not mean that there are equal numbers of gaseous and liquid molecules. It means that the rates of condensation and evaporation are equal. Once this equilibrium is reached, the concentration of vapor molecules remains constant.

Evaporation of Water in a Closed Container demonstrating the following: Molecules in a vapor state Molecules undergoing vaporization Molecules undergoing condensation

An equilibrium can also be reached between the solid/liquid phase. This occurs at the melting/freezing point.

Vapor Pressure

You might wonder why we are talking about vapor pressure in the module on liquids and solids. Remember that evaporation occurs naturally and therefore vapor is present. A vapor is a gas that should normally be a liquid or solid under those conditions. As liquid evaporates, the vapor exerts a pressure on the surroundings. This pressure increases until equilibrium is reached and then remains constant.

Factors That Affect Vapor Pressure

Let's discuss the factors that do not affect vapor pressure (VP) and see if you can determine the factors that do affect vapor pressure based on what you have already learned!

  • Although surface area does affect the rate of evaporation, it does not affect the equilibrium VP. This is because surface area affects the rate of condensation equally.
  • Volume has no effect on VP because evaporation and condensation only occur at the surface
  • The size of the container does not affect VP. The reason requires you to think through a situation where the size of a container increases.

Let's examine this step by step:

  1. If the volume of space increases, initially the VP decreases (because there will be the same number of molecules occupying a larger space).  
  2. Fewer molecules will therefore strike the liquid surface, decreasing the rate of condensation.  
  3. The rate of evaporation remains the same (because nothing has changed that would affect this).  
  4. Evaporation continues until the concentration of vapor molecules increase to reestablish equilibrium.  
  5. This causes the VP to increase back to its original amount.

In solids, some particles at the surface can have enough kinetic energy to move straight into the gas phase. This is called sublimation. This process is rare.

Vapor pressure is explained further in this Khan Academy Video.

Boiling

We have already discussed the relationship between boiling point and IMA, but let's revisit boiling as it relates to vapor pressure. Visualize a pot of boiling water on the stove. Think about what is actually inside those bubbles. At first, you might think that it is air, but it is not. It is the vapor of the liquid that is boiling, in this case, water. This vapor bubble exerts a vapor pressure. But unlike in evaporation, this is not occurring at the surface. The bubble often forms at the bottom of the pot, near the heat surface. Since vapor is less dense than the liquid surrounding it, it will rise. When it gets to the surface, it potentially could escape into the atmosphere. However, the atmosphere exerts pressure as well. It pushes down on the liquid and the vapor bubble trying to escape. The only way the vapor bubble can escape is if its VP is slightly larger than the atmospheric pressure. The point where vapor pressure equals atmospheric pressure is the real definition of boiling point.  

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

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