Text Adapted from: Beyond Penguins and Polar Bears is an online magazine
Activity Adapted from: Sinking Ice Cubes? by Kelley Sampeer
Overview:
When studying polar environments it is necessary to discuss ice and the formation of ice. This
lesson addresses what makes ice float.
Key Concepts:
Density is explored in this lesson, as well as the molecular formula of water and its properties.
Time:
1, 45 minute class period
Materials:
Ice cubes
Water
Isopropyl Alcohol
Two Glasses or beakers
Gum drops
Mini-Marshmallows
Toothpicks
Content Background: See Growing Floaters and Shrinking Sinkers handout
Sponsored by:
National Science Foundation
(NSF Award 0732793)
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Aim:
Does Ice float in all liquids?
Objectives:
Students will be able to:
• examine physical properties of matter.
• define density.
• compare density of substances by floating and sinking.
• determine the density of substances.
• demonstrate how floating and sinking are independent of shape or size.
Activities:
Teacher Preparation:
1. Label the beakers “A” and “B”
2. Fill beaker “A” ¾ full with water.
3. Fill beaker “B” ¾ full with isopropyl alcohol.
Ask students to define density, and then make a list of things that float in water and things that
sink in water.
Class Demonstration:
1. Ask students, why do some things float in water?
They are less dense that water.
2. Hand out Worksheet 1, and then read questions 1 and 2 prior to the demonstration and
have student write down and discuss their answers.
3. Place one ice cube in each beaker.
4. Have a discussion of what happened during the demonstration including the different
densities of water, alcohol, and ice.
5. Have students finish assessment questions.
6. Describe the solutions.
Clear, Colorless
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7. What do you predict will happen when I place and ice cube into each glass?
The ice cube in beaker “A” will float while the ice cube in beaker “B” will sink. Answers
may vary
8. Why do you think one floats while the other sinks?
Objects that are less dense than the solution they are placed in will float while objects
that are denser than the solution they are placed in will sink.
9. What are the two types of solutions?
Water and Alcohol
10. Which liquid is denser?
Water
11. Does the size or shape of the ice cube or liquid matter?
No
Assessment:
Students will write one paragraph answering the following question: Why does ice float in water
and solid iron sink in liquid iron?
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Student Worksheet 1: Density Demonstration
Watch the demonstration and answer the following questions using the following information.
The density of water is 1000 kg/m3, the density of isopropyl alcohol is 800 kg/m3, and the
density of ice is 917 kg/m3.
1. Describe the solutions.
2. What do you predict will happen when I place and ice cub into each glass?
3. Why do you think one floats while the other sinks?
4. What are the two types of solutions?
5. Which liquid is denser?
6. Does the size or shape of the ice cube or liquid matter?
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GROWING FLOATERS AND SHRINKING SINKERS
When you grab an ice cube from the freezer and drop it into your favorite after-school drink,
what happens? Your cube sinks a bit, then bobs up to the surface and just stay there, floating like
a cork or a bath toy. What you've just experienced is one of the oddest and most important events
on Earth. Ice floats in water, and that makes water weird.
Ice is just solid water. Hold an ice cube in your hand and you quickly get a wet hand, because
your body heat changes the ice from a solid to a liquid. Heat changes other substances (like iron,
for instance) from solid to liquid, too. But there's a big difference. For most substances, the solid
form sinks in the liquid form. Imagine a vat of molten iron. Toss in a chunk of solid iron, and it
sinks like a stone. Solid iron sinks in molten iron because as solid iron forms, it shrinks.
Shrinking Iron
Everything around us is made of atoms and molecules (molecules are just collections of atoms).
The atoms that make up molten iron are moving very fast. They are bouncing off each other at
high speed. As the molten iron cools, the atoms slow down a little. The slower atoms don't
bounce off each other quite so forcefully, and so the atoms end up closer together. Eventually,
the atoms are so slow that they get locked into place, in a shape called a crystal.
You might think of crystals as beautiful, see-through shiny jewels, but when scientists talk about
crystals they're talking about something else. A crystal is just an ordered arrangement of atoms.
Molten iron, if it cools slowly, forms one large crystal. If molten iron cools quickly, it forms lots
of little crystals. In either case, the solid crystal iron formed in this way sinks in molten iron. The
atoms in the crystal solid are closer together than the atoms in the liquid.
That's true for almost every material in the Universe. As the solid forms from the liquid, the
atoms get closer together, and the solid shrinks. But something weird happens as water gets
colder and changes to ice.
Growing Ice
The best way to understand what happens when water turns to ice is to build your own water
molecule. You'll need: some gumdrops, some small marshmallows, and some toothpicks.
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Every water molecule is made of three atoms: one atom of oxygen and two atoms of hydrogen.
The gumdrops will be the oxygen atoms, and the marshmallows will be the hydrogen atoms.
First stick two toothpicks into a gumdrop. You might be tempted to put the toothpicks on
opposite sides of the gumdrop, but don't. The water molecule isn't straight, but instead is bent so
that it looks a little like this:
Stick a marshmallow on the other end of each of the toothpicks. There's your water molecule.
Imagine a whole sea of these molecules floating past one another. Because of their shape, they
can get pretty close without touching. As the temperature drops, they move slower and slower,
allowing the molecules to get even closer together. Then something strange happens. Just like
cooling iron, the cooling water starts to form crystals, but these crystals have a very special
shape.
It turns out the hydrogen atoms (the marshmallows) can get close to oxygen atoms (the
gumdrops), but can't get close to other hydrogen atoms. This causes the water molecules to line
up something like this:
In this picture, each blue dot is an oxygen atom (a gumdrop), and each red dot is a hydrogen
atom (a marshmallow). Do you notice how much space there is between water molecules? That
extra space is what makes ice float. When water cools down and the molecules get closer
together, they start to form these wide-open crystals. When all the molecules form crystals, the
water has turned to ice. The wide-open crystal structure means that the freezing water didn't
shrink; it grew!
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Pop, Potholes, and Polar Bears
If you've ever left a bottle or can of pop in a freezer, you know how powerful this growing ice
can be. It can even break glass. The ice inside pushes harder and harder on the container’s walls
until the container gives way, sometimes in an explosion. The same thing makes potholes in
roads. First, water seeps into small cracks in the road. Then the water freezes. The freezing water
grows, and the cracks get bigger. More water creeps in, freezes, and grows, starting the whole
thing all over again.
Growing water isn't all bad, however. In fact, without water's weird way of growing as it freezes,
the world would be a very different place. Imagine if ice sank to the bottom of lakes, or even the
ocean. Once there, the ice would probably never melt again. Much of the world's water would be
trapped forever far below the surface of lakes and the ocean. If lakes froze from the bottom up,
fish could never survive the winter. And if ice didn't float on seawater, polar bears, seals, and
many other creatures would need to find a new way of life - because so many creatures depend
on the floating ice of the Arctic.
The next time you cool off your favorite drink with a bit of solid water, consider what an
amazing event you've just witnessed. Ice floats, and that's weird.
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