CHAPTER 9

THERMAL PROPERTIES OR MATTER

Internal energy is made up of kinetic energy and potential energy.

Kinetic energy
-It is due to the vibration of the strong interatomic or intermolecular bonds.
-The higher the temperature, the more vigorous the vibrations of the particles.
-In liquid and gases, kinetic energy is due to the movement of the particles instead of vibrations.

Potential energy
-It is due to the compression and rarefaction of the intermolecular bonds as the particles vibrate.
-The force and the distance between the particles determines the amount of potential energy stored in the bonds.

MELTING AND SOLIDIFICATION

Melting

-A process when a solid is heated and changes to liquid
-Occurs at a definite or constant temperature

Substance                        Melting point/°C
Ethyl alcohol                    -117
Ice                                    0
Wax                                  57
Tin                                   232
Gold                                 1067

Solidification

-The reverse process of changing a liquid to a solid is called freezing or solidification
-A pure substance will freeze at a temperature equal to its melting point

BOILING AND CONDENSATION

Boiling

-A process when a pure liquid changes into vapour when heating

Substance                       Boiling point/°C
Hydrogen                        -253
Oxygen                           -183
Ether                               35
Alcohol                           79
Water                              100
Mercury                          357

Condensation

-It is the reverse of boiling.
-When the vapour of the same substance is cooled, it changes into liquid at the same constant temperature as in boiling.
-Heat is given out

EVAPORATION

Evaporation
-The change of state of a liquid into its vapour at any temperature

Evaporation vs Boiling

Boiling                                             Evaporation
Occurs at a fixed                            Occurs at any
temperature                                    temperature

Quick process                                Slow process

Takes place within           Takes place only on the
the liquid                             surface of the liquid

Bubbles are formed            No bubbles are formed
in the liquid                                      in the liquid

Temperature remains      Temperature may change
constant during boiling

Heat supplied by an            Heat supplied from the
energy source                                  surroundings

Everyday effects and uses of evaporation

-Cooling effect on the skin when you apply cologne or when perspiration evaporates

-Wet clothes or puddles of water on pavements drying up in the open air

-Sponging someone with a high fever using water so that as the water evaporates, heat is removed from the patient’s body

Factors affecting the rate of evaporation

Temperature

The higher the temperature, the higher the rate of evaporation. A warmer liquid means more molecules in the surface are energetic to escape.

Humidity of the surrounding air

The rate of evaporation decreases when humidity increases.

Surface area of the liquid

The bigger the exposed surface area of the liquid, the higher the rate of evaporation. A larger surface area means more molecules can escape from the surface.

Movement of air above the surface of the liquid

If you are wet with sweat and you stand underneath a ceiling fan that is set to the maximum speed, you will find that the evaporation of your perspiration is very rapid. You cool down faster in moving air than in still air. The same applies to blowing your wet hair dry using a hair dryer. This is because the moving air removes the molecules of the liquid as soon as they escape from the surface. Therefore, the rate of evaporation increases.

Pressure

Decreasing atmospheric pressure increases the rate of evaporation.

The nature of liquid

The lower the boiling point of a liquid, the higher the rate of evaporation.

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