Densities of Warm and Cool Water

Description

Using ordinary running tap water, a dramatic consequence of the density difference between warm and cool water is demonstrated by dyeing the water and layering cool water above warm and vice versa.

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Make sure that the lips of the flasks match. (Baby food jars or other matched bottles may be used.) A large tray (cafeteria tray, for example) may be used to catch spills.

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1. Add several drops of red food color to each of two 250-mL Erlenmeyer flasks; fill them with warm tap water. (If tap water is not warm, heat some tap water on a hot plate to 40-45 °C.)
2. Add several drops of blue food coloring to each of the other two 250-mL Erlenmeyer flasks; fill them with cool tap water.
3. Predict the outcome when one flask is inverted over the other.
4. Place a paper card on top of the vessel filled with warm water. Invert the flask making sure to hold the card in place. Stack it on top of one of the cool water flasks. Remove the card. Remain prepared to catch the flasks.
5. Place a paper card on top of the flask filled with cool water. Invert the flask making sure to hold the card in place. Stack it on top of the other warm water flask. Remove the card. Wide mouth containers may also be used.
6. Note any changes.

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Name ___________________________ Class _______

Teacher __________________________

DoChem004 Densities of Warm and Cool Water

1. Predict the outcome when one flask is inverted over the other.
2. Describe what happens when the warm water is on the bottom.
   • Explain.
3. Describe what happens when the warm water is on the top.
   • Explain.
4. Devise experiments to demonstrate the effects of different food colors on the outcome.
5. Devise experiments to measure the densities of both the warm and the cool water.
6. Chemical reactions require mixing. Describe how you would combine two solutions to increase mixing without mechanical stirring. Industrial plants frequently combine large amounts under conditions where mechanical mixing is expensive.

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Purpose

To illustrate a consequence of the different densities of warm and cool water.

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• 4 250-mL Erlenmeyer flask (creamery bottle, or other suitable vessel)
• 2 8 x 8 cm paper card
• warm water
• cool water
• food colors
• large pan or tray with lip

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Students can perform this as an experiment. Assign students to pairs. Give each student two vessels. Some pairs will put the cool water on the bottom, and others on the top.

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Teacher preparation: 5 minutes

Presentation: 10 minutes

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The top vessel may fall and break.

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Dispose of solutions at the sink.

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Presentation Questions:

• Devise experiments to demonstrate the effects of different food colors on the outcome.
  • The obvious experiments are to mix different colors into the warm and cool water, and repeat the experiment. To the best of our knowledge, the selection of color has no bearing on the experimental outcome: cool water sinks, and warm water rises.
• Devise experiments to measure the densities of both the warm and the cool water.
  • Determine the mass of an empty, dry flask. Fill the flask to the very top with warm water. Determine the mass of the flask when filled. Empty the flask. Fill the flask to the very top with cool water. Measure the mass of the flask with cool water. Pour the water from the flask into a calibrated vessel designed to measure volume, and measure the volume. (Try a 500-mL graduated cylinder.)

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The usual explanation given for this effect is that warm water is less dense than cool water, so it rises or floats. Have your students suggest experiments that will confirm the different densities of warm and cool water.

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The temperature at the bottom of a large, deep lake is often near 4°C while that of the top is 0°C. The inversion of fresh water lakes during the spring season, a phenomenon very important to lake ecology, is attributed to density differences. The maximum density of water occurs at 4°C. During the winter, when frozen, the surface water is less dense than the bottom water. As the water heats during spring melting, the density of the water on the surface increases. As the water on the surface warms, it becomes more dense than the water just below it and the lake "turns over." Warming above 4 °C causes the density of the surface water to be lower than that just below it, and the temperature gradient with less dense warm water on the top and more dense cool water at the bottom is established.

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• density
• warm water
• cool water
• discrepant event
• turn over
• turns over inversion

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