Expansion, Contraction, and Ice

~ how heat changes the size of things ~

Alberta Grade 6 Science
Organizing IdeaMatter — Understandings of the physical world are deepened by investigating matter and energy.
Guiding QuestionHow can the particles of matter be influenced by heating or cooling?
Learning OutcomeStudents investigate how particles of matter behave when heated or cooled and analyze effects on solids, liquids, and gases.
This lesson covers: Expansion as the typical response of matter to heating; contraction as the typical response to cooling; water’s unusual property of having greater volume in solid form than in liquid form; why the surface of a large body of water freezes first and the insulating sheet of ice that forms protects aquatic life.
1

Heat makes things bigger

In the thermometer lesson, the liquid expanded when warmed and shrank when cooled. Most materials behave the same way: solids, liquids, and gases all expand when heated and contract when cooled.

The cause is particle motion. Hotter particles move faster and push each other slightly further apart, so the material takes up more space overall.

↓ click anywhere in the chamber to warm the bar ↓
the rule
Heat → expand. Cool → contract. Gases expand the most, solids the least, but the direction is the same for nearly every material.
2

But water is the famous exception

For most substances, the solid form is denser than the liquid form, so a chunk of solid metal dropped into a pool of melted metal will sink to the bottom.

Water does the opposite: when it freezes, it expands instead of contracting. That makes solid water (ice) less dense than liquid water, which is why ice floats.

why does this happen?

When water freezes, its molecules lock into a hexagonal crystal pattern with empty space between them. The same number of molecules takes up more room — so the ice is bigger and lighter than the water it came from.

other substances that do this
A handful of other substances also expand when they freeze, including silicon, bismuth, and cast iron. Water is the most familiar example, and the only one whose unusual behaviour is essential for life on Earth.
3

How ice saves the fish

Because ice floats, lakes and ponds freeze from the top down rather than the bottom up. The ice layer acts as an insulating cover that slows heat loss from the water below. Even on very cold winter days, the water beneath the ice stays liquid, typically near 4°C — the temperature at which water is densest. Fish, plants, and other aquatic life survive the winter under the ice.

freezing warm
air temperature: 20°C
what if ice sank?
If ice were heavier than water (as most solids are heavier than their liquids), lakes would freeze from the bottom up every winter. New ice would sink and pile up until the entire lake became a solid block. Aquatic life would not survive the winter. Water's unusual density behaviour is one of the reasons life on Earth is possible.
← previous: thermometers ↑ back to all activities → next: the engineering challenge