Page 20 - Understanding NCERT Science 09
P. 20
Q uestions b. 573 K • • • dish and keep it inside a cupboard or
on a shelf in your class.
Record the room temperature.
1. Convert the following temperature
Record the time or days taken for the
to celsius scale:
evaporation process in the above cases.
a. 300 K
Repeat the above three steps of activity
2. What is the physical state of
water at:
observations.
o
o
a. 250 C b. 100 C ? on a rainy day and record your
3. For any substance, why does the • What do you infer about the effect of
temperature remain constant temperature, surface area and wind
during the change of state? velocity (speed) on evaporation?
4. Suggest a method to liquefy
You must have observed that the rate of
atmospheric gases.
evaporation increases with–
• an increase of surface area:
1.5 Evaporation We know that evaporation is a surface
Do we always need to heat or change pressure phenomenon. If the surface area is
for changing the state of matter? Can you increased, the rate of evaporation
quote some examples from everyday life where increases. For example, while putting
change of state from liquid to vapour takes clothes for drying up we spread them out.
place without the liquid reaching the boiling • an increase of temperature:
point? Water, when left uncovered, slowly With the increase of temperature, more
changes into vapour. Wet clothes dry up. What number of particles get enough kinetic
happens to water in the above two examples? energy to go into the vapour state.
We know that particles of matter are • a decrease in humidity:
always moving and are never at rest. At a given Humidity is the amount of water vapour
temperature in any gas, liquid or solid, there present in air. The air around us cannot
are particles with different amounts of kinetic
hold more than a definite amount of
energy. In the case of liquids, a small fraction
of particles at the surface, having higher water vapour at a given temperature. If
kinetic energy, is able to break away from the the amount of water in air is already high,
forces of attraction of other particles and gets the rate of evaporation decreases.
converted into vapour. This phenomenon of • an increase in wind speed:
change of liquid into vapours at any It is a common observation that clothes
temperature below its boiling point is called dry faster on a windy day. With the
evaporation. increase in wind speed, the particles of
water vapour move away with the wind,
1.5.1 FACTORS AFFECTING decreasing the amount of water vapour
EVAPORATION in the surrounding.
Let us understand this with an activity. 1.5.2 HOW DOES EVAPORATION CAUSE
COOLING?
Activity _____________1.14
In an open vessel, the liquid keeps on
• Take 5 mL of water in a test tube and evaporating. The particles of liquid absorb
keep it near a window or under a fan. energy from the surrounding to regain the
• Take 5 mL of water in an open china energy lost during evaporation. This
dish and keep it near a window or absorption of energy from the surroundings
under a fan. make the surroundings cold.
• Take 5 mL of water in an open china
MATTER IN OUR SURROUNDINGS 9
Rationalised 2023-24
