Unit Five: Physical States of Matter

Lesson 2: Vapor Pressure

• Comptencies
• Starter Activities
• Main Activites

Competencies

• Explain vapour pressure
• Determine vapour pressure of a liquid
• State the factors that affect vapour pressure of a liquid
• Give examples of the effects of vapour pressure from daily life

Materials

Diagrammatic representation of vaporization

Note for the teacher

The lesson of vapour pressure can be contextualized because students have experiences in their day-to-day activities. The most important point is understanding what happens at particle level during vaporization and why do different liquids have different vapour pressures? Students should also understand the concepts such as volatility and viscosity since they encounter them in their lives frequently. To depict the concept of vapour pressure it is advisable to conduct the experiment on the student textbook page 193.

Starter activity

The lesson can be started by inviting students to discuss the process of cooking and boiling in the kitchen at home.

Let the students discuss round their desks the following points;

• When food is cooked in a closed container, why does the inner part of the cover gets wet after some time?
• Sometimes the cover of the cooking utensil flips of by an internal effect. Explain these phenomena in chemical terms.                                            
• Use figures to explain your ideas.

Give them some time and ask them to explain their understandings.

Although the concept seems simple, students may come up with different views regarding the phenomena. Some of them may say ‘the liquid is splashed up to the cover”, the others may think simply the surroundings of a liquid is usually wet like river sides.  Some the students may guess that liquid vaporizes and condenses on the cover.

Regarding the removal of the cover of the cooking utensil, they may also think that the liquid it self pushes the cover. Some of them may unable to imagine that vapour pressure has such energy. They should also be able to draw figure like the following one.

Evaluation

What makes the cover of the cooking utensil wet during cooking?

What pushes the cover off the cooking utensil?

Main Activity

Now you may define vapour pressure as the partial pressure developed above a liquid. Let the students investigate the factors that affect vapour pressure of a liquid by doing the following activities

Organize the students in to groups and give them to the following activities to discuss and come up with kinetic theory of matter explanations.

Activity 1

If you keep equal amount of water in equal sized open beakers, the first being under temperature of 200C and second under temperature of 450C , the content of the second water reduces significantly than the first one.

Activity 2

If you keep equal amounts of benzene and water in equal sized open beakers under the same temperature, the volume of the benzene reduces significantly in the same time.

1. Use the concept of intermolecular forces of attraction to explain the difference.
2. How do you classify the two types of liquids?

The group representatives should present their observations to the class. The class may ask questions and discussion should be extended among students.

You should assist the students in their discussion to associate the concepts to the kinetic theory of matter and the intermolecular forces of attraction.

Students should come to the understandings like:

• According to the kinetic theory of matter, the particles possess kinetic energy and potential energy. The kinetic energy depends on the temperature of the particles. As temperature increases the kinetic energy of the particles increases, as a result they become more energetic to overcome the attraction force of the neighbouring molecules. Likewise, the particles of the water at temperature of 450C obtain more kinetic energy than they were at 200C. Therefore more molecules escape from the former resulting in more reduction of the volume in a given period of time.
• When benzene and water are compared, water possesses strong intermolecular attraction force between its molecules which is known as hydrogen bonding. But benzene has very weak intermolecular forces of attraction, i.e. the London force. At the same temperature, more energy is required to break the forces between the water molecules than benzene molecules. So, more benzene vaporizes than water in a specific period of time.
• Substances which vaporize at lower temperature like benzene are called volatile substance. Volatile substances are substances having weak intermolecular forces. They usually have higher vapour pressure than the non-volatile substances at a particular condition of temperature and pressure.

Evaluation

Students can be asked question like:

1. What are the factors that affect vapour pressure of a liquid?
2. What are volatile liquids?

Concluding activities

Let the students discuss and classify the following substances as volatile and non-volatile. The substances are: ammonia, water, benzene, ethanol, perfume, sulphuric acid, and edible oil.

Check whether the students can perform the classification as