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Heat and Cold: How Water Changes Shape · Heating and cooling cause water to change between solid, liquid, and gas — and those changes can be reversed · NGSS 2-PS1-4

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Heat and Cold: How Water Changes Shape

with Atlas

Atlas the friendly guide stands in a bright kitchen lab, holding a tray with three clear cups — one full of chunky ice, one of liquid water, and one gently steaming — and points at each cup with a cheerful smile.

Observe what happens to ice when heat is added to it
Observe what happens to liquid water when heat is removed from it
Identify melting, freezing, and evaporation as changes that can be caused by adding or removing heat
Predict whether a change of state caused by heating or cooling can be reversed

Key terms

Melting: When heat turns a solid into a liquid.
Freezing: When taking away heat turns a liquid into a solid.
Evaporation: When liquid water turns into invisible gas.
Reverse: To change something back the way it was.

Adding Heat

When you add heat to water, it changes. Heat makes ice melt into liquid water, and you can watch the edges drip. If you keep adding heat, warm liquid water turns into invisible gas and floats into the air. So adding heat moves water from solid toward liquid, and from liquid toward gas. Heat gives the tiny water particles more energy so they can spread apart and move more freely.

Taking Heat Away

Taking heat away does the opposite. When you put liquid water in a freezer, the cold takes heat away, so the water slows down and freezes into solid ice. When invisible water gas cools on a cold glass, it turns back into tiny liquid drops. Melting and freezing are opposite changes, and we can go back and forth by adding or removing heat. Nothing is lost, the water just changes form.

Worked examples

Order three cups by heat added.

Solid ice is the coldest, so it has the most heat removed.
Liquid water is in the middle, with some heat added.
Evaporating warm water is the hottest, with the most heat added.

Answer: Ice, then liquid water, then evaporating warm water.

Hi, I am Atlas! Look at my three cups. The first cup holds solid ice — it is hard and keeps its shape. The second holds liquid water that can pour and splash. The third holds warm water that is slowly turning into invisible gas in the air. When we ADD heat to ice, it melts into liquid water. You can watch the edges of an ice cube get wet and drip — that is the change happening right in front of you! When we REMOVE heat — like putting water in a freezer — the liquid slows down and freezes back into solid ice. Melting and freezing are opposite changes, and we can reverse them by adding or removing heat. Evaporation is what happens when liquid water turns into gas and floats away. A puddle left on the sidewalk on a sunny day slowly disappears — the water did not get destroyed, it changed into invisible gas in the air. Evaporation can happen even on a cool day; it just takes longer. This change can be reversed too: when that gas cools down, it can become liquid again (think of water drops on a cold glass). If you feel stuck, look at the three cups in the picture and ask yourself: is heat being added or removed?

Activity

Put these three cups in order from the most heat removed to the most heat added

Practice

What happens to an ice cube when you add heat to it?

How can you turn liquid water back into solid ice?

Common mistakes to avoid

A puddle that dries up is gone forever.The water turned into invisible gas and floated into the air.
Melting and freezing cannot be reversed.You can switch back and forth by adding or removing heat.

Check your understanding

What do you observe when you add heat to an ice cube?

You put liquid water in a freezer overnight. What change do you observe in the morning?

A puddle on the sidewalk slowly disappears on a cool, cloudy day. Where did the water go?

Recap

Adding heat melts ice into water and turns water into gas, while taking heat away freezes water back into ice. These changes are opposite and can be reversed by adding or removing heat.

Reflect

When have you seen water change because of heat?