Prescriptive Activities: Facet Cluster 2.2: PF#64-65
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Prescriptive #64-65:
60 The student does not understand the processes and energy changes involved in phase
changes.
64 The student believes molecules break apart and reform during state changes.
65 The student believes bubbles from a pure liquid boiling are composed of a substance that is
different than the pure liquid.
Background:
In this activity you will analyze water as it boils
Materials:
 Student handout
 Hot plate (or Bunsen burner with ring stand, ring clamps and wire gauze)
 100 mL beaker
 Cobalt chloride strips (also called water moisture test strips)
 Watch glass (ideal chilled but not necessary)
 Tongs
Activity:
Add about 20 mL of water to the beaker (this does not need to be exact; you just want the beaker
to be about 1/5 – ¼ full). Place the beaker on the hot plate and turn the hot plate to high. Use
caution when using the hot plate; do not burn yourself or melt the cord. Using the tongs, dip the
cobalt chloride paper into the water. Record your observations.
Once the water is close to boiling, place the watch glass over the top of the beaker so the beaker is
completely covered. You should notice some drops of liquid starting to form on the bottom side
of the watch glass. What do you think this liquid is? How did it get there?
Using the tongs, carefully remove the watch glass, flip it over and place on the table. Dip the
cobalt chloride paper into the liquid that collected on the watch glass. Record your observations.
Adapted from Emily Borda
Prescriptive Activities: Facet Cluster 2.2: PF#64-65
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Finally, use the tongs to hold a piece of the cobalt chloride paper in the beaker but above the
water. Record your observations.
Questions:
1. What causes the cobalt chloride paper to change colors?
2. Since the water in the beaker and the liquid on the watch glass had the same effect on the
cobalt chloride paper, what does this tell you about the liquid on the watch glass?
3. What happened to the cobalt chloride paper when you held it just above the boiling water?
What does this tell you about the gas in the beaker of boiling water?
Information:
The image1 on the left shows three water molecules
[labeled (1), (2) and (3)]. The solid lines (labeled b) represent
the bonds that hold the hydrogen and oxygen atoms together
within the molecule. The dotted line (labeled a) represents the
force holding two water molecules near each other. This force is
called an intermolecular force – meaning a force between
different molecules, just like interstate highways are roads that
go between different states.
4. If the intermolecular force, a, was broken, what would
remain? What would you call this substance(s)?
1
Image source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html
Adapted from Emily Borda
Prescriptive Activities: Facet Cluster 2.2: PF#64-65
Teacher Page
5. If we focus on one water molecule [in this case molecule (3)] what
would be left if the bond (b) was broken? What would you call this
substance(s)?
6. When water boils, which do you think breaks - the bonds (b), intermolecular forces (a),
both or neither? Support your answer using the evidence from the investigation using
cobalt chloride paper.
7. If you generalize your findings from this activity, what can you say about changes that
happen to liquids? When a liquid boils or turns to a gas, what type of gas does it form?
How do the atoms in the gas relate to the atoms of the liquid?
Teacher Notes
The cobalt chloride should be blue before putting it in the water. If you do this activity on a
particularly humid day or climate, it may have turned pink from the moisture in the air. If this
happened, have the students briefly place the paper on the hot plate to dry it off before each of the
tests.
Students may think the cobalt chloride turns color when it encounters a liquid. If students put this
for Question 1 you may want to show them that it is water that makes the paper turn colors not
any liquid. To test this, use the cobalt chloride paper to test other clear liquids like ethanol,
hexane, isopropanol, etc. You can do this as a demo. Note: since these liquids are volatile, water
from the air may condense on the paper as it cools due to the evaporation of the liquid and may
cause the paper to change color. In order to avoid confusion, it is best if you test these liquids and
then quickly throw away the test paper.
Adapted from Emily Borda