Boiling and Evaporation explained using Kinetic Theory. Factors that affect Evaporation.

Subject :

Basic Science

TERM 

SECOND TERM

Week

Week 8

CLASS 

JSS 2 / Basic 8

Topic :

Boiling and Evaporation explained using Kinetic Theory.

Factors that affect Evaporation.  

Instructional Materials :

• Gas cylinder
• Bunsen Burner
• Kettle for boiling water
• Pictures of objects in solid, liquid and gaseous state of matter

Reference Materials

• Scheme of Work
• Online Information
• Textbooks
• Workbooks
• 9 Year Basic Education Curriculum

Previous Knowledge :

The pupils have been taught

  Explanation of some phenomena using kinetic theory. Changes in the states of matter

in their previous lesson

Behavioural Objectives :  At the end of the lesson, the learners will be able to

• Explain the meaning of boiling
• Explain the meaning of evaporation
• State the factors that affect Evaporation.

Entry Behaviour 

Learners are familiar with  Meaning Of Energy (Kinetic and Potential Energy)

Content :

EVAPORATION AND BOILING

Evaporation vs. Boiling

Evaporation occurs on the surface of liquid and it is a vaporization of liquid. It is a state of transition from liquid to gaseous state. The process occurs slowly and cannot be seen as well. It occurs when there is exposure of water to air and water molecules change into vapor and these vapors rise up and form clouds.

Boiling occurs on the entire mass of liquid and it is the vaporization of liquid. It occurs rapidly. It happens when the vapor pressure of the liquid is equal to the pressure exerted by the environmental pressure on liquid. It is a state of phase transition. The boiling occurs in three different stages: nucleate boiling, transition boiling and film boiling. There are no such stages for evaporation.

Boiling occurs when the temperature of the liquid is greater than the boiling point of the substance. Evaporation can occur at any temperature. It occurs as long as the substance  remains  liquid at a particular temperature. Evaporation occurs when there is an increased energy present and occurs rapidly. It occurs from the bottom of the container when allowed to boil. The bubbles form at the bottom of the container and then rise on top of the container. In boiling, bubbles do not form at the bottom and rise to the surface. Evaporation occurs at room temperature and therefore, occurs at a slower rate when compared to boiling. In boiling, there is formation of bubbles as it is a complex physical process and these bubbles are formed on a heated liquid. There is cavitation and acoustic effects seen in boiling. There is no such bubbles formed in evaporation and there is no cavitation and acoustic effect present in evaporation.

The microscopic difference between evaporation and boiling is as follows:

1. Evaporation occurs on the surface of the liquid whereas boiling occurs at the entire length of liquid.

2. Boiling occurs rapidly whereas evaporation occurs slowly.

3. Evaporation occurs at any temperature whereas boiling occurs at a specific temperature.

4. The motion of particles is fast in boiling whereas in evaporation few particles move slowly and few at a faster rate.

5. There is formation of bubbles in boiling, but bubbles are not seen in evaporation.

Boiling and Evaporation

Kinetic Theory explains boiling as follows. In a liquid such as water, the particles are in continuous motion but the speed of movement is not as fast as in gas. When heat is applied to water in a beaker or container, the particles of water gain energy. This additional energy makes the particles move faster than before. As more and move particles gain higher energy they move rapidly in all directions. The mass of water begins to move physically at a particular temperature. At this temperature, the particles acquire greater kinetic energy and begin to escape into vapour. At this stage, boiling is said to be taking place. Boiling is therefore a state in which all particles of the liquid have acquired energy and are moving rapidly in all directions, especially in the direction of the open mouth of container.

Using Molecular Structure to Explain

Kinetic Theory

Kinetic theory can be used to explain certain phenomena in nature to show the differences in them. Such phenomena are evaporation, boiling and vapour pressure.

Evaporation

Evaporation is a process whereby molecules of a liquid with higher kinetic energy escape through the liquid surface into the space above the liquid. Such molecules are said to vaporize.

In any given liquid sample, some particles possess more kinetic energy than others. So, when such energetic particles come near the liquid surface, they break away from the attractive forces of other nearby liquid particles and escape into the space above. When this happens, we say the liquid is evaporating.

Boiling

Boiling is said to take place when a liquid freely changes into vapour when it is heated. The temperature at which this happens is called the boiling point. The boiling point of water is one hundred degree centigrade.

For a liquid to boil, it must be heated. When this happens, the rate of evaporation increases. Boiling takes place because liquid molecules acquire more kinetic energy when the liquid is heated. The molecules then collide with each other and with the walls of the vessels to build up pressure in the liquid. At this time, a saturated vapour pressure for the liquid results.

This pressure builds up and increases until a temperature is reached at which the saturated vapour pressure of the liquid is equal to the atmospheric pressure at that time. At this stage, bubbles of vapour form in the liquid and rise to the surface. The liquid is now said to be boiling.

Distinction between Evaporation and Boiling

Evaporation

• Lowers kinetic energy of molecules
• Does not need heating to place
• Can take place at It takes place at any temperature

Boiling

• Increases kinetic energy of molecules
• It needs heating before it could take place
• It takes place at finite temperature

Factors influencing the rate of evaporation 

Evaporation:

Evaporation is the process by which a liquid goes off as vapour, with or without heating or boiling. In a sample of a given substance, some particles are more energetic than the others, when these energetic particles come near a liquid surface, they break from the attractive forces of the other nearby liquid molecules and go off as vapour.

When this happens we say that the liquid has evaporated. Since evaporation results in the escape of energetic particles from the liquid body, the average kinetic energy of the liquid is lowered, and this bring about a reduction in the temperature of the liquid body

The drying of clothes, and the disappearance of a puddle of water, are instances of evaporation.

Factors Affecting Evaporation:

Evaporation of liquids is affected by the following factors namely:

1. Temperature: Evaporation of liquids occurs at all temperature but the rate at which they occur differs.

2.Pressure: The rate of evaporation increases with a decrease in the pressure of the surrounding area.

3. Humidity: If the relative humidity of an environment is very high, then the evaporation rate will be reduced and vice versa.

4. Types of liquid : The rate of evaporation is always lower in electrovalent liquids eg water than in covalent liquids eg petrol

5. Area of the evaporating surface :The greater the surface area of the liquid exposed, the greater the rate of evaporation.

6. Wind speed: How slow or fast the wind is at a particular time

7. Humidity: The amount of water vapour

in the atmosphere

8. Temperature: How hot or cold the

liquid and the atmosphere are

9. Vapour pressure of liquid: The degree of saturation of liquid vapour

10. Viscosity of liquid: How thick or thin a liquid phase is.

Similarities between boiling and Evaporation 

They both depend on nature of the liquid.

2. They both depend on the area of the surface.

3. They both depend on impurities.

 Presentation

The topic is presented step by step

Step 1:

The class teacher revises the previous topics

Step 2.

He introduces the new topic

Step 3:

The class teacher allows the pupils to give their own examples and he corrects them when the needs arise

EVALUATION 

OBJECTIVES 

1. The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by the———. A. enthalpy of vaporization B. enthalpy of change C. enthalpy of condensation D. all of the above.

2. ———occurs on the surface of the liquid whereas boiling occurs at the entire length of liquid. A. Evaporation B. Melting C. Condensation D. Boiling

3. There is formation of bubbles in———, but bubbles are not seen in evaporation.

A. boiling B. melting C. evaporation D. mixing.

4. Evaporation occurs when there is an ———energy present and occurs rapidly.

A. increased B. decreased C. normal D. perfect

5. Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves. This explains——— A. pressure B. surface area C. temperature D. flow rate

THEORY

1. Explain evaporation and boiling

2. State three differences between evaporation and boiling.

3. Mention five factors that influencing the rate of evaporation.

4. Explain the effect of surface area on the rate of evaporation.

5. Explain Concentration of other substances in the air.

6. Mention five factors that influencing the rate of evaporation.

7. Explain the effect of surface area on the rate of evaporation.

8. State three differences between evaporation and boiling.

9. Mention five factors that influencing the rate of evaporation.

10. Mention four factors affecting evaporation

Conclusion :

The class teacher wraps up or conclude the lesson by giving out short note to summarize the topic that he or she has just taught.

The class teacher also goes round to make sure that the notes are well copied or well written by the learners.

He or she does the necessary corrections when and where  the needs arise.