SSS TWO PHYSICS
FIRST TERM
DEFINITION OF TERMS
 Boiling
point of a substance is defined as the
temperature at which its saturated vapour
pressure becomes equal to the external
atmospheric pressure
 Relative humidity is the ratio of the mass of
water vapour in a given volume of air to the
mass of water vapour required to saturate the
same volume of air at the same temperature. It
is expressed in percent. It is measured with a
hygrometer. It is used by weathermen for
making weather forecast.
 Evaporation is the escape of water molecules
into the atmosphere from a water contained
body. It causes cooling.
Factors affecting evaporation
 Temperature
 Surface
 Wind
 Humidity/dryness of air
 Pressure
Nature of liquid/viscosity/density
Factors affecting boiling point of a liquid
 Nature of the liquid
 External atmospheric pressure
 Presence of impurities
 Degree of its molecular cohesive force
N/B
 The extra heat giving to boiling water is
not to increase the temperature but to break
the intermolecular forces in the molecules of
water.
Differences between boiling and evaporation
Evaporation
 This is the change from
liquid to vapour at
temperature below normal
boiling point.
 Takes place slowly at the
liquid surface.
 Takes place at all
temperatures.
Boiling
 This is the change from
liquid to vapour at the
boiling point.
 Occurs throughout the
entire volume of the liquid.
 Takes place at a particular
temperature.
Differences between boiling and evaporation
Evaporation
Boiling
• Temperature needs • Temperature
not be steady
remains steady
during evaporation.
during boiling.
 Wind assists
 Wind has no effect
evaporation.
on boiling.
The similarity between
evaporation and boiling is
that both involve liquid
phenomenon.
Assignments
The mass of water vapour in a given
volume of air at 20 degree Celsius is
0.05g. The air requires 0.15g of water
vapour to saturate it at the same
temperature. Calculate the relative
humidity of the air.
 Describe an experiment to show that
evaporation causes cooling.

 Define
heat capacity and state its unit.
 A quantity of pepper soup of mass 800g poured into
a plastic container with a tight fitting lid has a
temperature of 30 degree Celsius. The container is then
placed in a microwave oven, rated 1200W and operated
for 3minutes.
 Calculate the final temperature attained by the soup.
(Assuming no heat losses).
 Explain why containers with tight-fitting lids are not
suitable for use in microwave cooking.
When the soup is brought out and allowed to cool, a
dent is observed on the container. Explain.
( Take specific heat capacity of the soup = 4000J/kgK)