MELTING AND BOILING

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.

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

Melting Point Particles of a solid vibrate so fast that they break free

from their fixed positions.

Solid Liquid

Increasing Thermal Energy

Melting point

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

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.

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

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 releaselarge 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 supplystate of air above the surface

- Brings about cooling - no cooling

Boiling Point and Melting Point

-150

-100

-50

0

50

100

150

200

Te

mp

era

ture

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.