Objectives

At the end of the lesson, students should be able to:

• define melting and freezing.

• define boiling and condensation.

• interpret the heating and cooling curve.

Internal energy The internal energy of a body is the combination of the

total kinetic energy (due to molecular motion) and the

potential energy (due to intermolecular forces) of the

molecules in the body.

An increase in temperature leads to an increase in the

kinetic component of the internal energy.

Melting and Freezing

Melting and Freezing Melting is a process in which a substance changes its

state from solid to liquid.

The temperature at which this happens is called the

melting point of the substance.

Solid Liquid

Melting and Freezing Freezing is a process in which a substance changes its

state from liquid to solid.

The temperature at which this happens is called the

freezing point of the substance.

Liquid Solid

Melting point = Freezing point

Boiling and Condensation

Boiling and Condensation Boiling is a process in which a substance changes its

state from liquid state to gaseous state.

The temperature at which this happens is called the

boiling point of the substance.

Liquid Gas

Boiling and Condensation Condensation is a process in which a substance changes

its state from gaseous state to liquid state.

The temperature at which this happens is called the

condensation point of the substance.

Gas Liquid

Boiling point = Condensation point

Take Note! During the change of state

(melting/freezing/boiling/condensation), the

temperature of the substance remains constant.

For example, when pure ice is melting at 0oC, it will

remain at 0oC until the ice has melted completely. The

temperature of pure water will increase beyond 0oC,

when more thermal energy is added to the water.

This is a temperature-time graph. In this example, ice is being heated from -10oC. As you can see, the temperature of ice rises to 0oC.

When the temperature of ice reaches 0oC, the ice melts. During the change in state, there is no change in temperature even though thermal energy is still being transferred to the water.