Why Does Ice Float?

Contributor: Lindsey Congalosi. Lesson ID: 13270

In many cold places, lakes can freeze. Because ice floats to the top of water, you can walk on these frozen lakes. How does this happen, and why is it so important in the true story "Breakthrough"?


Earth Science, Physical Science

learning style
Auditory, Kinesthetic, Visual
personality style
Beaver, Golden Retriever
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Quick Query

Lesson Plan - Get It!


The movie Breakthrough tells a true story that begins on the icy surface of Lake St. Louise in Missouri. Watch this short clip from the movie.

Breakthrough | Full Scene | 20th Century FOX from 20th Century Studios:

  • Why was the lake only frozen on the top?
  • What are some clues the boys missed that would have told them that the ice was unsafe?
  • What could you do to save yourself if this ever happens to you?

To learn the answers to these questions, we first need to examine the difference between ice and water--temperature.

  • What is temperature?
  • Sure, temperature measures how hot something is, but what does hot mean?

You may already know that everything is made up of moving molecules. This movement gives the molecules kinetic energy. The faster the molecules move, the higher the kinetic energy and the higher the temperature.

If you want to make something warmer, you just have to get the molecules to move faster. This is how a microwave works! The microwaves cause the microscopic drops of water in your food to vibrate. The vibration increases the food's kinetic energy, making it hotter.


Microwaves → Vibration → More Kinetic Energy = Hotter = Your Lunch Is Ready!

Cold molecules have less kinetic energy because they move more slowly than warm molecules. This usually causes cold molecules to become very dense and squished together, like with air.

air molecules

We say usually because some substances, like water, don't work this way. Water molecules do slow down when the water is cooled, but they also do something else. Let's examine an individual water molecule to learn more.

Water is also referred to as H2O, which means that it has 2 hydrogens (H) and 1 oxygen (O).

water molecule

Water molecules appear bent because they are polar.

polar bear

Not that kind of polar!

A polar molecule is any molecule that has a difference in electric charges. In other words, one side is more positively charged and the other is more negatively charged.

See if you can determine which side of the water molecule is negative and which side is positive. When you think you know, click on the molecules to see if you're correct.

Each water molecule has two hydrogen bonds that connect the hydrogens to the oxygen. Click on one of the hydrogen bonds in the image below.

For this next part, it will be helpful to imagine yourself as a water molecule. Your body is the oxygen molecule. Your hands are the hydrogen molecules.

  • What are the hydrogen bonds?

Your arms!

warm water molecule man

Print out the Water Molecule Labels, found under Downloadable Resources in the right-hand sidebar, or create your own. Attach the O to your shirt, and hold an H in each hand.

Move your arms around. Keep them loose and fluid. This is what water is like when it is a liquid.

Slowly stiffen your arms. You're freezing! Remember to stay in the shape of a water molecule by keeping your arms in front of you. Keep going until your arms are completely stiff, pointed straight out from your body. Now you're ice! Pretty cool, huh?

cold water molecule man

Now, get 100 of your friends together and tell them to do the same thing. If you don't have 100 people available for a science demo, you can use your imagination.

Picture all of these people around you. Everyone is moving their arms loosely and fluidly. Everyone can get pretty close together.

warm water molecule group

Now, cool everyone off. Their arms should once again stiffen until they are sticking straight out from everyone's body.

  • Is everyone close together now?

cold water molecule group

No! When you represented liquid water, the fluid arm movements allowed people to squish together. Once the hydrogen bonds froze and solidified, this could no longer happen. So, even though the molecules are moving less overall, their rigid hydrogen bonds force them to spread apart.

Watch a demonstration of this process in Why does ice float in water? - George Zaidan and Charles Morton from TED-ED:

  • Have you ever left a drink in the freezer because you wanted it to cool off quickly but then forgot about it?

If so, you probably came back to a sticky, icicle mess.

  • Do you know why?

Let's say you place a can of soda in the freezer. Although it has a few other ingredients, the main one is water. The water in the soda froze, which straightened the hydrogen bonds, causing them to push apart from each other. If the soda is left in the freezer long enough, this force will break open the soda can, causing frozen soda to line your entire freezer. Do not try this at home.

Now you understand why water expands when it freezes. This still doesn't answer our original question though.

  • Why does ice float?

When the water molecules freeze and push apart from each other, there is more empty space between them. This means that ice has a lower density than water. Density refers to how smooshed together the molecules are. That's why density is measured using the unit ‘g/cm³'. It describes how much matter (g) is in each cube of space (cm³).

Liquid water has a density of 1 g/cm³. Anything with a density greater than 1 g/cm³ will sink, and anything with a density less than 1 g/cm³ will, you guessed it, float.

Solid ice has a density of about 0.9 g/cm³, causing it to float on top of liquid water. This is why lakes freeze from the top down. And it's a good thing, too. The ice on top of the lake provides protection for the creatures and plants that live in the water. If the lake froze from the bottom up, many plants and fish would die.

Watch part of Life Noggin's Why Doesn't Winter Kill All The Fish And Plants? to learn more about this:

Don't get cold feet!

Move on to the Got It? section to test your ice knowledge before it slips your mind!

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