This document discusses melting and boiling. It defines melting as the change from solid to liquid that occurs at a specific melting point temperature for each substance. Ice melts at 0°C. Boiling is defined as the change from liquid to gas, which occurs at the boiling point temperature for each substance. Boiling involves particles gaining enough energy to break free of liquid form. Evaporation only occurs at the surface of a liquid, while boiling can occur below and above the surface as bubbles form and rise. Melting and boiling points are specific to each pure substance and involve heat being used to weaken or break molecular bonds without changing temperature.

1. MELTING AND BOILING

2. MELTING
 When ice is heated, it melts.
 Ice melts at 0oC.
 The heat applied is used to break the forces of
attraction between molecules.
 As the ice melts, temperature remains constant.
 The heat applied to melt a substance is called Latent
Heat.
 Latent heat means ‘hidden’ heat.
 Since there is no change in temperature, the heat
applied is hidden.

3.  Melting - change from solid to liquid
 Melting point - SPECIFIC temperature when
melting occurs.
 Each pure substance has a SPECIFIC melting
point.
 Examples:
 M.P. of Water = 0°C
 M.P. of Nitrogen = -209.9 °C
 M.P. of Silver = 961.93 °C
 M.P. of Carbon = 3500.0 °C

4. Melting Point
 Particles of a solid vibrate so fast that they break free
from their fixed positions.
Solid Liquid
Increasing Thermal Energy
Melting point

5. Vaporization
 Vaporization – change from liquid to gas
 Vaporization happens when particles in a liquid gain
enough energy to form a gas.
GasLiquid
Increasing Thermal Energy
Boiling point

6. Two Kinds of Vaporization
 Evaporation – vaporization that takes place only on
the surface of the liquid
 Boiling – when a liquid changes to a gas BELOW its
surface as well as above.

7. Boiling Point
 Boiling Point – temperature at which a liquid boils
 Each pure substance has a SPECIFIC boiling point.
 Examples:
 B.P. of Water = 100°C
 B.P. of Nitrogen = -195.79 °C
 B.P. of Silver = 2162 °C
 B.P. of Carbon = 4027 °C

8. EVAPORATION BOILING
-takes place at all temperatures - takes place at a constant temperature
(boiling point)
-takes place at a slow rate - takes place at a faster rate
-particles move slowly - particles move rapidly
-no bubbles seen - bubbles are formed deep in the liquid
- - bubbles expand, rise, burst and release
large amounts of vapour
-takes place on the surface of the liquid - takes place throughout the liquid
-depends on temperature, surface area and - depends on the rate of heat supply
state of air above the surface
- Brings about cooling - no cooling

9. Boiling Point and Melting Point
-150
-100
-50
0
50
100
150
200
Temperature
time
WATER – H20
Melting point
Boiling point
solid
liquid
gas
 Melting and boiling occur at constant temperatures.
 The heat supplied is hidden ‘latent’.
 Bonds between molecules are weakened in melting and broken boiling.
 More energy is required for boiling than melting.