CHEMISTRY EXPERIMENTS ON FILETM
SOLIDS, LIQUIDS, AND GASES • 1.06–1
What Happens When Water Is Heated?
Topic
Molecular motion, kinetic theory, and convection currents in hot water
Introduction
The tiny particles that make up all solids, liquids, and gases are constantly in
motion, but when they are heated up (that is, when heat energy is added to
them) they move faster and faster and bump into each other more and more
often, and with greater force. As a result of these collisions, the molecules in a
heated substance spread farther apart and the substance expands and becomes
less and less dense. In this experiment, you will observe the effects of heating
water.
Time required
30 minutes
Materials
2 identical wide-mouthed jars (20 oz size), e.g. baby food jars
2 different food-colorings in bottles with droppers
tap water
30 oz cold water (chilled in a refrigerator)
piece of thin card (e.g. index card), slightly larger than the jar mouth
rubber kitchen gloves
Safety note
Take great care when handling containers of hot liquid.
Procedure
Part A
1. Read through this part of the procedure and try to predict what will happen
after step 5. Write down your prediction.
2. Half-fill one of the jars with the hottest water the faucet will provide. Wear
the gloves as the jar will quickly become hot, and take great care not to splash
or spill as hot water can scald.
3. Half-fill the other jar with cold water from the refrigerator.
4. Set both jars on a table and wait for the water to settle completely.
5. Take one of the food-coloring droppers in each hand and carefully – at the
same time – add a single drop of one color to one of the jars and a drop of the
other color to the other jar.
6. Observe and record what happens.
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1.06–2 • SOLIDS, LIQUIDS, AND GASES
CHEMISTRY EXPERIMENTS ON FILETM
Part B
1. Read through this part of the procedure and try to predict what will happen in
steps 7 and 8. Write down your prediction.
2. Completely fill one jar with hot water from the faucet.
3. Completely fill the other jar with cold water from the refrigerator.
4. Add a drop of a different color of food coloring to each jar.
5. Place the card over the mouth of the jar filled with hot water.
6. Wearing gloves, hold the card in place and quickly invert the jar of hot water
and carefully place it on top of the jar of cold water so the necks of the jars
match up, as shown in the illustration:
jar
hot water
index card
cold water
7. Steadying the jars with one hand, carefully pull out the card and observe what
happens. Is this what you expected?
8. Holding the two jars tight together, turn the pair upside down so that the hot
water jar is at the bottom. What happens now? Record the results of the last
two steps.
Analysis
1. In Part A, did the food coloring disperse (spread out) through the hot water
and cold water at the same rate? Did the results match your predictions? If the
dispersion rates were different, which was faster?
2. Suggest an explanation for what you observed.
3. What did you observe when you removed the separator card in step 7 of Part
B? Did this result match your prediction? Why do you think the water acted
the way that it did when the cold water was on the bottom?
4. Why do you think the water acted the way it did when the hot water was on
the bottom?
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© Diagram Visual Information Ltd.
Published by Facts On File, Inc. All electronic storage, reproduction,
or transmittal is copyright protected by the publisher.
1.06
What Happens When Water Is Heated?
1. The food coloring disperses more rapidly in the hot water than in the cold water.
2. The water molecules in the hot water are moving faster than those in the cold water, so they
push the molecules of coloring through the water at a faster rate.
3. When the cold water is on the bottom and the hot water is on top, each of the food colorings
remains in its own jar. The molecules in the hot water are moving faster, so they spread out
more, making the hot water less dense than the cold water – and because it is less dense it
floats on top of the cold water.
4. When the jars are inverted so the hot water jar is at the bottom, the colors quickly mix.
Because it is less dense, the hot water immediately floats up through the denser cold water,
mixing the colors. This upward movement of the hotter water (and corresponding downward
movement of cooler water) is called convection. Because it involves the physical movement of
molecules from one part of the substance to another, convection can only occur in liquids and
gases. The principle of convection is used in warm air room-heating systems, electric kettles
and ovens, and many other appliances. It explains why hot air balloons fly, and why it is safer
to crawl along the floor of a smoke-filled room rather than trying to walk upright. On a much
larger scale, convection currents in the Earth’s mantle create the huge forces that make whole
sections of the Earth’s crust move around.