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That Sinking Feeling (Water Temperature and Density)

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  • Why does the water always seem colder at the bottom of a lake? Is it because the sun only shines on the top or is there another reason? Why are icebergs so dangerous for ships at sea and what's unique about the way water changes from liquid to solid? If these questions have puzzled you in the past, or if you're bored with winter and just want to do some "cool" experiments, I've got a challenge for you!

    Here's what you'll need to play along:

    • 3 tall (12 oz.) clear glasses half full of room temperature water
    • A glass of ice water
    • A glass of hot tap water
    • A plastic soda bottle -- with cap -- filled with water
    • A freezer
    • Red and blue food coloring
    • Some ice cubes

    Drop a few ice cubes into a glass of room temperature water. What do the ice cubes do? If you said they float, congratulations. You've just proven that ice is less dense than liquid water! Ice is "cool" stuff because most substances actually get denser when they change from a liquid to a solid, but not water.

    As water freezes into ice, the volume actually increases. To check this out, take an empty plastic soda bottle and fill it completely with water. Seal it up tight with the cap and stick it in the freezer for a few hours.

    Okay, so if water gets less dense when it freezes, then what happens when different-temperature waters meet?

    Here's the challenge:

    How do different-temperature waters react when they flow together? Do they sink, float, or mix together?

    What you do:

    Begin by predicting what will happen when ice-cold water meets room-temperature water. Then predict what will happen when hot tap water meets room temperature water. Make sure you explain why you think this might happen. After you've made your predictions, put a few drops of blue food coloring into the ice-cold water and slowly pour it into one glass of room temperature water. What does it do? Was your prediction right? Now mix a few drops of red food coloring into the hot tap water and slowly pour it into another glass of room temperature water. Did it do the same as the cold? Based on your experiments, what do you think happens to the density of water as it changes temperature?

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    Notes to Teachers

    Curriculum Focus: Science/Simple Physics/Water temperature and density

    Learning Outcomes:

    • Based on the observation that ice floats on (and is less dense than) water, make a reasoned prediction about how waters of different temperatures will react when they flow together.

    • Experiment by pouring blue-colored ice water and red-colored hot tap water into separatecontainers of room-temperature water, to find out whether waters of different temperatures sink, float, or mix together.

    • Understand that when water gets hot, it gets less dense; and that when water gets cold, it gets more dense EXCEPT when it freezes and turns to ice, in which case it becomes less dense!

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    Web Links

    For more information on density, try these Web sites.

    Thinking Fountain (Density Experiment)
    Part of a larger index of science subjects, this site is a sequence of lesson plans and demonstrations that explains density with pictures, definitions, and predicted questions and answers. Links are provided.

    Professor Bubbles Home Page
    A complete unit on bubbles, which includes the history of soap, the scientific principles of bubbles, bubble tools, and solutions, is available. Photos and text are included.

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