Vapor Pressure


Rob Rouse

Talent Unlimited High School, Manhattan


Summer Research Program for Science Teachers

August 2007



Unit: The Physical Behavior of Matter


Topics: Vapor pressure of different substances and how they relate to intermolecular forces.


Aim: What is vapor pressure and how is it related to intermolecular forces?


Do Now: Complete 1- 3. Write in complete sentences.

1) How is boiling point related to the strength of an intermolecular force?




2) Rank the following substances in order of increasing melting point.


                                    H2(g)               CaCl2(aq)                    H2O(l)




3) What will happen if a glass of water is left uncovered at room temperature, for several weeks?






  1. Water
  2. Isopropyl alcohol
  3. Hexanes
  4. Worksheet


Procedure of Lesson:

  1. Students work individually to answer the do now questions on the handout.
  2. As a class, we answer the do now questions and stop to discuss the third one in detail.
  3. In the front of the room, on the chalk board (a portable chalk board can be used) has been divided into three vertical rows. The students are informed that the three lanes are for a vapor pressure race.
  4. The instructor calls for three volunteers. The Volunteers are chosen. Each volunteer is responsible for a specific spray bottle. The three spray bottles each contain a different substance. When told to, the three students simultaneously spray two squirts of the liquid onto the chalk board in different lanes.     
  5. Students are asked to record their observations of what happens on their handout. Three time keepers record the time it takes for the different liquids to evaporate. 
  6. Students will find that the liquids all evaporate at different rates. The hexanes begin to evaporate immediately, the Isopropyl alcohol takes slightly longer, and the water takes the longest to completely evaporate. This will spur a discussion where the students will discuss why they think the liquids were and why the liquids evaporate at different rates.
  7. Students are instructed how vapor pressure relates to intermolecular forces (something they should already have been exposed to). The way this can be done is to use pictures to explain the interactions occurring in 1) hydrogen bonds 2) dispersion forces  3) molecule ion interactions.

  8. Students work in groups to answer the following questions.

a. Arrange the substances in order of evaporation, from fastest to slowest evaporation.



b. Using your answer to question #1 above, arrange the substances in order of strongest to weakest intermolecular forces.



c. Using your answer to #3 above, arrange the substances in order of increasing boiling point.



d. Arrange the substances in order of decreasing melting point.



          e. What is the relationship between intermolecular force strength and vapor pressure?




   9.   Students are instructed how to read a vapor pressure chart.  



 Summary: Students must answer the following questions.

   1.  Identify the type of intermolecular force in each substance (you can abbreviate):


            HF(g) _____               NaF(aq) _____            F2(g) _____                 H2(l)_____


   2.  Arrange the following substance in order of decreasing boiling point: 1= lowest 4=highest


            H2O(l) _____              Xe(g) _____                Ne(g) _____                CaCl2(aq) _____


   3.  Draw the molecule ion attraction between H2O and Na+




 Homework: Complete the worksheet.



National Standards:


Content Standard A:

As a result of activities in grades 9-12, all students should develop

          1) Understandings about scientific inquiry

Content Standard B:

As a result of their activities in grades 9-12, all students should develop an understanding of

1) Structure of atoms

2) Structure and properties of matter

3) Interactions of energy and matter