The document defines melting, freezing, boiling, and condensation as the processes by which substances change state from solid to liquid, liquid to solid, liquid to gas, and gas to liquid respectively. It notes that the temperature remains constant during these state changes even as thermal energy is added or removed. An example temperature-time graph is provided to illustrate how the temperature of ice remains at 0°C as it melts into water.

2. 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.

3. 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.

4. Melting and Freezing

5. Melting and Freezing
Melting is a process in which a substance changes its
state from solid to liquid.
Solid Liquid
The temperature at which this happens is called the
melting point of the substance.

6. Melting and Freezing
Freezing is a process in which a substance changes its
state from liquid to solid.
Liquid Solid
The temperature at which this happens is called the
freezing point of the substance.
Melting point = Freezing point

7. Boiling and Condensation

8. Boiling and Condensation
Boiling is a process in which a substance changes its
state from liquid state to gaseous state.
Liquid Gas
The temperature at which this happens is called the
boiling point of the substance.

9. Boiling and Condensation
Condensation is a process in which a substance changes
its state from gaseous state to liquid state.
Gas Liquid
The temperature at which this happens is called the
condensation point of the substance.
Boiling point = Condensation point

12. 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.

13. 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.

14. 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.