P5 Science: Water and Its Three States

Ice, liquid water, and water vapor all come from the same substance.

3 States of Water

Water may look very different in each state, but it is still water. When water freezes, it does not become a new substance. When it boils, it does not disappear forever. It changes its state, or form.

The three common states of water are:

State of matter	What water is called	What it looks like
Solid	Ice	Hard and keeps its shape
Liquid	Water	Flows and takes the shape of its container
Gas	Water vapor	Spreads out and is usually invisible

Solid water is Ice

Water changes from a liquid to a solid when it freezes (usually at 0°C). At this temperature, liquid water changes into ice if enough heat is removed. Ice has a fixed shape. If an ice cube is placed on a plate, it keeps the shape of the ice cube tray. However, it does not flow across the plate like liquid water does. It may slide, but it stays solid until it warms up enough to melt.

During melting, ice changes from a solid into a liquid. For example, if an ice cube is left on a plate, it takes in heat from the warmer air around it and slowly changes into a small puddle of liquid water. As ice absorbs heat from its surroundings as it melts, melting ice can make things feel colder.

Melting happens when solid ice gains heat and becomes liquid water.

Liquid water flows and takes the shape of its container

Liquid water does not have a fixed shape. Instead, it takes the shape of its container. If water is poured into a cup, it takes the shape of the cup. If it spills on the floor, it spreads out.

Liquid water has these properties:

• It flows.

• It has a fixed volume.

• It takes the shape of its container.

• It can change into ice or water vapor.

  
  

Volume is the amount of space something takes up. If a cup contains 50 ml of water, the amount of water stays the same when poured into a bowl, even though the shape changes.

Water vapour is water as a gas

When water becomes a gas, it is called water vapour. Water vapour is usually invisible. This means we cannot normally see it, even when it is in the air around us. Water vapour forms when liquid water gains enough heat. As a gas, water vapour does not have a fixed shape or a fixed volume. It spreads out to fill the space around it.

Evaporation happens from the surface

Evaporation is the change from liquid water to water vapour at the surface of the water. It can happen even when the water is not boiling. For example, a puddle can dry after the sun comes out. The puddle did not vanish by magic. Some of the liquid water changed into water vapour and mixed with the air.

Evaporation happens faster when:

• The air is warm

• There is wind

• The water is spread out over a large surface

• The air is dry

  
  

That is why wet clothes dry faster on a warm, windy day than on a cool, damp day.

Boiling also changes water into gas

Boiling is another way liquid water changes into gas. Water usually boils at 212°F (100°C). When water boils, bubbles form throughout the liquid and rise to the surface. The steam you see above a boiling pot is often made of tiny liquid water droplets that form when hot water vapour cools in the air. The actual water vapour itself is invisible.

Boiling water produces water vapor, which can cool into tiny visible droplets.

Cooling water vapour can make liquid water again

Water does not only change from liquid to gas. It can also change from gas back to liquid. This process is called condensation. Condensation happens when water vapour loses heat, cools down and form tiny drops of liquid water.

You can see condensation in many places:

• Water drops on the outside of a cold glass

• Fog on a bathroom mirror after a warm shower

• Dew on grass in the morning

• Clouds in the sky

• Mist near a cold window

  
  

A cold glass of water is one of the easiest examples. The water on the outside of the glass does not come through the glass. It comes from surrounding water vapour in the air. When the vapour touches the cold surface, it cools and becomes liquid drops. This is also how clouds form. Water vapour rises into the sky. Higher air is cooler, so the vapour condenses into tiny water droplets or ice crystals. When many droplets gather together, they form clouds. If the droplets become heavy enough, they fall as rain, snow, sleet, or hail.

Condensation forms when water vapor cools on a cold surface.

Deposition and sublimation are less common changes

Most lessons focus on melting, freezing, evaporation, boiling, and condensation. These are the main changes seen in daily life. There are two other changes that can happen too:

Sublimation happens when a solid changes directly into a gas without becoming a liquid first. Some ice and snow can slowly change into water vapor in very cold, dry air.

Deposition happens when a gas changes directly into a solid. Frost can form this way when water vapour in the air changes straight into ice on a cold surface.

These changes are useful to know, but the main three states remain the same: solid, liquid, and gas.

Quick Summary

Water can change between three states of matter: solid, liquid, and gas. Solid water is ice. Liquid water is the water we drink and use every day. Gas water is water vapour, which usually cannot be seen.

Heat causes many of these changes. When water gains heat, ice can melt and liquid water can evaporate or boil. When water loses heat, water vapour can condense and liquid water can freeze.