Why Does Ice Float?

Explain why ice floats on water by describing what happens to water molecules when they freeze.

The hook

Here's something you've seen a thousand times and probably never questioned. Drop an ice cube in a glass of water. It floats. But almost every other substance sinks when it freezes. The solid is denser than the liquid. Water breaks that rule. When it freezes, water expands. The ice ends up about nine percent less dense than the liquid it came from. That happens because of how water molecules grab onto each other. As they slow down and lock into place, they form a wide, open, hexagonal pattern. Lots of empty space. That empty space is why lakes freeze from the top down instead of the bottom up. It's why fish survive the winter.

Mini-lesson

Density is how much stuff is packed into a certain amount of space. Scientists write it as mass divided by volume. A brick and a sponge might be the same size, but the brick has more mass squeezed into that space, so it is denser. When you put two things together, the less dense one floats on top of the denser one. That is why oil floats on water and why a helium balloon rises in air.

For almost every substance, the solid form is denser than the liquid form. When a liquid cools, its molecules slow down and move closer together. They pack in tight, like people squeezing onto a crowded bus. Less space for the same molecules means higher density, so the solid sinks in its own liquid. Solid wax sinks in melted wax. Solid iron sinks in molten iron.

Water is the famous exception, and the reason is a weak connection called a hydrogen bond. A water molecule is one oxygen atom with two hydrogen atoms attached, shaped like a wide V. The oxygen end is slightly negative and the hydrogen ends are slightly positive, so the hydrogens on one molecule are attracted to the oxygen on its neighbors. In liquid water these bonds form and break constantly as the molecules tumble past each other.

When water freezes, the molecules slow down enough that the hydrogen bonds lock in place. Each molecule ends up bonded to four neighbors in a fixed arrangement, and that arrangement is a ring of six molecules: a hexagon. Hexagons stacked in every direction make a lattice with a lot of open space inside the rings. The same molecules now take up more room, so ice is about 9 percent less dense than liquid water. That is also why a water bottle left in the freezer can crack.

Because ice is less dense, it floats. On a lake in winter, the coldest water is at the surface, and when it freezes the ice stays on top. That ice acts like a lid: it slows the rest of the lake from losing heat, so the water underneath stays liquid, usually around 4 degrees Celsius. Fish, frogs, and insects spend the winter down there. If ice sank, lakes would freeze from the bottom up and many could turn solid, killing most of what lives in them.

Activity: Model It: Liquid Water vs. Ice

1. Look at your two drawings. Which box has fewer molecules in the same amount of space? Use the word density to explain what that tells you about ice compared to liquid water.

Finish this sentence: The ICE box has ____ molecules in the same space, so ice is ____ dense than liquid water. That means ice will ____ on water.

2. In your own words, explain what a hydrogen bond is and what it does to water molecules when they freeze.

3. Why does the hexagonal pattern in ice take up more room than the loose arrangement in liquid water? Describe what is inside each hexagon.

4. Prediction: Imagine ice were denser than water and sank instead of floated. Describe what would happen to a pond during a long, cold winter, and what that would mean for the fish and other living things in it.

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