Water changes phases when heated or cooled. When water is heated, the water molecules gain kinetic energy and escape from the liquid phase as bubbles and steam. If the water is boiled for over 10 minutes, the volume will decrease due to evaporation. When cooled, the reverse occurs as water vapor molecules lose energy and condense back into liquid water droplets.

1. ACTIVITY 5
WHAT CHANGES TAKE PLACE WHEN WATER IS HEATED OR
COOLED?

2. OBJECTIVES
• 1. describe what happens to water when it is
heated;
•2. describe what happens to water when it is
cooled;
•3. represent through drawings/illustrations what
happens to the particles of
•water when it is heated and then cooled; and
•4. explain the processes taking place at the sub-

3. QUESTIONS AND ANSWERS
•Q1. Describe what you observe in the water
inside the beaker and above the level of water.
•There are bubbles formed at the bottom of
the beaker and bubbles in the boiling water.
There is also steam observed above the liquid.
•Q2. What do you think is inside the bubbles that
form when the water boils? Where did they come
from?

4. •The bubbles are water in the gaseous state. (It is
very common for students to say that the bubbles
formed are air. At the start of the heating
process, however, the tiny bubbles that form are
due to the air dissolved in the water. This is not
boiling.)
•Q3. If you keep the water boiling for more than
10 minutes, what do you think will happen to the
amount of water in the beaker? Why?
•The volume of water in the beaker will decrease if
water will be kept boiling for more than 10 minutes
because there will be rapid evaporation of water

5. •Q4. Where did the water go?
•As the water is heated and the temperature of the
water rises, the molecules gain more kinetic energy
and they move faster. More molecules have the
energy to overcome the forces of attraction of
the neighboring molecules. These molecules escape to
the gaseous phase.
•Q5. Can you explain by illustration how the water
changes from liquid to gas? What is happening to the

6. PART B. COOLING WATER
•Q6. Describe what you observe in the water inside
the beaker and at the bottom of the watch glass.
•Q6. As the water began to cool, droplets formed
under the watch glass that covered the beaker
containing hot water. There are also drops of water
formed on the inside wall of the beaker. Some of
these drops of water were observed falling to the
water inside the beaker.
•Q7. Where does the water at the bottom of the
watch glass come from?

7. • The water droplets at the bottom of the watch glass are the molecules of
water that escape from the liquid and go into the gaseous phase.
These water vapor rise and some molecules touch the glass.
• Q8. Can you explain by illustration how the water changes from gas to
liquid?