Evaporation, boiling and melting points and their determination. Effects of impurities and pressure on boiling and melting

• Evaporation
• Boiling points
• Melting points
• Effects of impurities and pressure on boiling and melting

Evaporation

Evaporation is defined as the process by which liquid molecules breaks away from the surface of the liquid to remain as vapor. It can also be simply defined as the process by which liquid changes to gas or vapor.

Boiling point

As heat is being added to a liquid, its temperature increases steadily until at a certain temperature when rapid evaporation is seen to occur in every parts of the liquid with the bubbles of vapor escaping to the surface.  This process is known boiling.

As a matter of distinction, evaporation occurs only at the surface of the liquid while boiling occurs throughout the entire mass of the liquid.  Also evaporation takes place at all temperature but boiling occurs at a particular temperature called boiling point.

Effects of pressure on boiling

An increase in pressure at the surface of a liquid of a liquid raises the boiling point of the liquid and conversely, a decrease in pressure lowers the boiling point of the liquid.

Effects of impurities on boiling

The presence of impurities or of dissolved substances in a liquid raises the boiling point of the liquid but lowers the freezing point

Melting points

Melting point is a temperature at which a solid substance has its bond broken such that it now flows as liquid. It is also defined as the constant temperature at which a substance changes state from solid to liquid. On the reverse, the constant temperature at which a substance changes state from liquid to solid is called freezing point

APPLICATION IN PRESSURE COOKER

The fact that increased pressure raises the boiling point is put into a useful application in the pressure cooker.  The increased pressure of the trapped gas above the liquid raises the boiling of the liquid inside the cooker.  This provides a high cooking temperature needed to conserve fuel and save time.

APPLICATION IN REFRIGERATORS

Refrigerators make use of the cooling effect of evaporation.  The volatile liquid such as liquid ammonia or Freon evaporates inside copper coil surrounding the freezing compartment, supported by electric pump which reduced the pressure.  As the volatile liquid evaporates in those coils, it absorb heat from the surround air, consequently and cooling the inside of the refrigerator and its content.

The vapor produced is pumped off into the condenser, where it is compressed by the pump and condenses back to liquid.  The latent heat given out during this condensation is quickly dissipated by an arrangement of cooling fins at the back of refrigerator.

Heat is eliminated by convection and radiation to the surroundings and by conduction into fins.  The liquid is again passed into the evaporator coil and thus the level of cooling is regulated by a thermostat connected to the switch.

CLASSWORK 4

1. Define evaporation
2. What is boiling?
3. Differentiate between boiling and evaporation

ASSIGNMENT 4

SECTION A

1. A phenomenon which is used to describe the process by which a substance changes from solid to gas is called (a) evaporation (b) freezing (c) sublimation (d) melting (e) vaporization
2. What effect will reducing the surface pressure of a liquid have on its boiling point? (a) increase its boiling point (b) reduces its boiling point (c) it does not have effect (d) increases its boiling point by 5⁰C (e) none of the above
3. Which of the following statement is true about evaporation (a) evaporation occurs at all temperature except at boiling points (b) evaporation only occurs at boiling point (c) evaporation occurs at all temperature (d) all of the above (e) none of the above
4. Impurities change the boiling points of liquid. So, salt added to water will (a) increase its boiling point (b) reduces its boiling point (c) it does not have effect (d) reduces its boiling point by 5⁰C(e) none of the above
5. Which of the following statement is true for an ice if the pressure is lowered? (a) the melting point is increased (b) the boiling point is unchanged (c) the melting point is lowered (d) the melting point is unchanged (e) none of the above

SECTION B

1. Write the effect of impurities and pressure changes on melting, freezing and boiling
2. Describe an experiment to determine the boiling point of a liquid