## Exploring our world: Why does ice float?

### You can help teach youth about density, mass and volume by exploring why ice floats.

What is density? Density is a term used to describe the relationship between the mass of matter and its volume or space it takes up. We can calculate density by taking the mass (weight) of an object and dividing it by its volume (D = m/v).

Most matter increases density when it changes state from a liquid to a solid. Ask children, “Are hot things more or less dense than cold things?” (With younger children, you might say, “Which is heavier?”, even though that is not technically correct.) Even from a young age, people understand that cold things are typically denser than warm things. Hot air rises. When you go swimming in a lake, it is usually colder in deeper water. Then why does ice float? Why does the density of water decreases when it is a solid? You can help youth explore density.

Give a youth a golf ball and a ping pong ball and ask, “Which weighs more, or feels heavier?” They should be able to feel that the golf ball is heavier. Next ask, “Is one bigger? Or are they about the same size?” The youth may say the ping pong ball is bigger, but they are roughly the same size. Because their volume is roughly the same, we can compare their weight when talking about density and conclude the density of the golf ball is greater than the ping pong ball. Remember, density is the relationship between mass and volume.

Another way to explore density with youth is to take a group of youth and explain each of them represents a molecule. Ask them to stand in a group with their arms held out and spread out so that just their fingers touch another youth’s fingers. Explain this represents a liquid as you gently push your way between the youth. We can easily move through or stir liquids. Next, have the youth move farther apart so they are not touching; this is a gas. Finally, ask the youth to pack tightly together, and then attempt to move between them; this is a solid. Tap a table or other solid and explain it is not possible to move through a solid because its density increased. Throughout this explanation, the youth remained the same – so their mass was constant, but their volume or the amount of space they took up as a group changed. As a gas they took up the most space, and as a solid they took up the least. Solids generally have a smaller volume but greater density than gases or liquids.

But what about water? How does water decrease in density as a solid? The answer can be found in the unique shape and polarity of water molecules. A water molecule is made up of two hydrogen and one oxygen atoms. The oxygen atom is in the center with the hydrogen atoms bonded to the oxygen almost like ears. Polarity refers to slightly negative charge on the oxygen atom and the slightly positive charge on the hydrogen atoms, which attract other molecules so that the slight positive charges sticks to the slight negative; linking water molecules together forming a structure with many empty spaces. In liquid water, the links are forming and breaking over and over as the molecules move so that the empty spaces don’t stay empty. In ice, solid water, the molecules are barely moving, the links become more permanent and the spaces remain until the ice melts. If you look closely at an ice cube, you can see many tiny air bubbles trapped within the spaces.