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# Density and Buoyancy: Mixing Hot and Cold Water

Media Type:
Video

Running Time: 3m 20s
Size: 10.0 MB

Source: ZOOM

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We've all seen substances change as a result of changes in temperature. Water heated on the stove eventually changes from a liquid to a gas and seemingly disappears as it disperses into the air. But even at lesser extremes, temperature affects the density of water. In this video segment adapted from ZOOM, a cast member demonstrates how temperature affects the way water molecules react with one another.

Background Essay

Density is a measure of a substance's mass relative to the amount of space it takes up. Dense substances, like iron and other metals, are massive -- they weigh a lot but take up very little space. Gases, in contrast, take up a great deal of space and weigh very little.

Density is a measurable characteristic of all substances and is generally recorded in grams per cubic centimeter. Iron, for example, has a density of 7.9 g/cm3. By comparison, the density of oxygen -- at sea level and a temperature of 20 degrees Celsius (C) -- is 0.0013 g/cm3.

For many substances, including most solids, density varies only slightly with changes in temperature. The density of gases, and to a lesser extent that of liquids, changes as ambient temperatures rise and fall. The molecules of these substances generally become more active and more spread out -- less dense -- as temperatures rise. Conversely, they become more tightly packed as temperatures decrease.

Water follows these general rules, but with two exceptions that are important to life on Earth. First, water is less dense as a solid than as a liquid. This is because of the way water molecules arrange themselves when they form ice crystals. The crystalline structure of ice holds molecules at greater distances from one another than when they are in a liquid form. This phenomenon explains why ice is less dense and floats on water. If this weren't the case, lakes, ponds, and oceans, even in temperate parts of the world, would likely freeze permanently.

Second, water is most dense at about 4 degrees C. As it warms up from 0 degrees and begins to near the 4-degree mark, it becomes increasingly dense. Then, like most other substances, it becomes less and less dense as temperatures continue to rise. In spring and fall, temperature changes cause the densest water at the surface of a lake or pond to fall to the bottom, where it replaces warmer or cooler water. This causes nutrients that have settled at the bottom and oxygen concentrated near the surface to become more evenly distributed throughout a lake or pond, thus making them more available to the aquatic organisms that need them.

Discussion Questions

• Why did the warm water float on top of the cold water?
• What would happen if you reversed the bottles, placing the one with warm water on top?
• What do you think would happen after an hour? After four hours? How would the temperatures compare then?
• Would you have been satisfied with the cast member's explanation if he had not done the second "control" experiment? Are there more "control" experiments you would have liked him to try?

• Transcript

KENNY: Check this out. Do you see how the red water floats to the top of the clear water? Why do you think that is? There's nothing in the water but food coloring. But the red water and the clear water are different in another way. Do you know how they're different?

Well, since you can't feel the water, I'm going to tell you. The red water is warm and the clear water is cold. And warm water floats on top of cold water. Brandi M. of Mansfield, Texas, sent us this phenom. Here's a way you can test it out for yourself.

Okay, first, you want to take a bottle and fill it with cold water. Then, you want to take another bottle and fill it with very warm water and add some food coloring. Now you want to take an index card and put it on top of the bottle with cold water. Okay.

Then, slowly flip over the cold water bottle while holding the index card in place and put it on top of the warm water bottle. Now, carefully... carefully remove the index card so that the spouts are aligned. And...watch in amazement.

See, it's starting to go up. The reason that warm water floats and cold water sinks is because that warm water is less dense than cold water. Density is something that makes things float so warm water can actually float on cold water.

To test it out, I'm going to try this with cold clear water and cold red water instead of warm red water. So, they're both cold. See? The colors mixed less.

So now I know that the red water floated to the top of the clear water because it was warmer and not because it was colored red. Try this at home with two bottles of warm water. Do you think they'll mix the same way? Test it out.

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