Grade by Grade
C. Matter and Energy · Properties of and Changes in Matter

Changes of State and Thermal Energy

C2.6 C2.3 A2.1 A1.2About 45 minutes
Learning goals
  • I can name the six changes of state: melting, freezing, evaporation, condensation, sublimation and deposition.
  • I can explain that matter absorbs thermal energy when it melts, evaporates or sublimates.
  • I can explain that matter releases thermal energy when it freezes, condenses or goes through deposition.
  • I can use the particle model to explain a change of state.
  • I can read and change a simple code that uses temperature to decide the state of water.

Words to know

thermal energy
The energy of the moving particles in matter. We feel it as heat. The faster the particles move, the more thermal energy.
absorb
To take in. Matter absorbs thermal energy when it warms up.
release
To give off. Matter releases thermal energy when it cools down.
change of state
When matter changes from one state to another, like ice melting into water.
sublimation
A change straight from a solid to a gas, without becoming a liquid first.
deposition
A change straight from a gas to a solid, without becoming a liquid first. Frost forms this way.
melting point
The temperature at which a solid melts. For ice, it is 0 °C.
boiling point
The temperature at which a liquid boils. For water, it is 100 °C at sea level.

Remember from Grade 2

In Grade 2 you learned that ice melts into water when it warms, water freezes into ice when it cools, water evaporates into water vapour, and water vapour condenses into drops. Now you will find out why these changes happen, using thermal energy and particles, and meet two new changes of state.

Why does an ice cube melt in your hand?

Hold an ice cube (in a plastic bag) in your hand. Your hand starts to feel cold and the ice starts to melt. Something is moving from your hand into the ice. That something is thermal energy.

Thermal energy and particles

Remember: all matter is made of tiny particles that are always moving. Thermal energy is the energy of those moving particles. We feel it as heat.

  • When matter absorbs (takes in) thermal energy, its particles move faster and spread farther apart.
  • When matter releases (gives off) thermal energy, its particles move slower and come closer together.

Thermal energy always moves from a warmer thing to a cooler thing. Your warm hand gives thermal energy to the cold ice. That is why your hand feels cold!

Icicles hang from a snowy roof edge on a sunny winter day, with drops of water falling from their tips.
The icicles absorb thermal energy from the sunshine. Their particles speed up and spread apart, so the solid ice melts into dripping liquid water. AI-generated image

Changes that ABSORB thermal energy

  • Melting: solid → liquid. The particles jiggle so much that they break out of their places and start sliding. Ice cream melting on a summer day.
  • Evaporation: liquid → gas. Particles at the surface move fast enough to escape into the air. A puddle drying up. Boiling is very fast evaporation that happens all through the liquid, with bubbles of gas. Water boils at 100 °C.
  • Sublimation: solid → gas, skipping the liquid. Dry ice (frozen carbon dioxide) turns straight into gas. In winter, some snow slowly disappears this way on cold, dry, sunny days.

Changes that RELEASE thermal energy

  • Freezing: liquid → solid. The particles slow down and lock into place. A pond freezing in December. Water freezes at 0 °C.
  • Condensation: gas → liquid. Particles of water vapour slow down and come together as drops. Drops forming on a cold glass of lemonade, or fog on the bathroom mirror.
  • Deposition: gas → solid, skipping the liquid. Frost forming on a window or on grass on a cold morning.
absorbs thermal energy →SOLIDLIQUIDGASmeltingevaporatingfreezingcondensing← releases thermal energy
Changes of state. Going right (melting, evaporating), matter absorbs thermal energy. Going left (freezing, condensing), matter releases thermal energy. Sublimation and deposition skip the liquid; see the table.
Change of stateFrom → toThermal energyExample
meltingsolid → liquidabsorbeda snowman melting in March
freezingliquid → solidreleasedwater in an ice cube tray in the freezer
evaporation (and boiling)liquid → gasabsorbedwet laundry drying on a line
condensationgas → liquidreleaseddrops on the outside of a cold glass
sublimationsolid → gasabsorbeda chunk of dry ice shrinking as it turns into carbon dioxide gas
depositiongas → solidreleasedfrost on a car window
The six changes of state.
Worked example

Let's explain a change of state with particles

Explain what happens when an ice cube melts on a kitchen counter.

  1. Where does the thermal energy come from? The counter and the air are warmer than the ice, so thermal energy moves into the ice.
  2. What happens to the particles? The ice absorbs thermal energy. Its particles jiggle faster and faster.
  3. At 0 °C the particles break out of their fixed places and start to slide past each other.
  4. The solid ice has become liquid water.

The ice absorbs thermal energy from the warmer counter and air. Its particles move faster until they can slide past each other, so the solid melts into a liquid.

Worked example

Let's explain why sweating cools you

After a soccer game you are sweaty. When a breeze blows, you feel cooler. Why?

  1. Sweat is liquid water on your skin.
  2. For the sweat to evaporate (liquid → gas), it must absorb thermal energy.
  3. It takes that thermal energy from your skin.
  4. Your skin loses thermal energy, so you feel cooler. The breeze helps the sweat evaporate faster.

Evaporating sweat absorbs thermal energy from your skin, which cools you down.

Just for fun

Dry ice is frozen carbon dioxide at about −78 °C. It sublimates straight into gas, and the cold gas turns water vapour in the air into a thick, spooky fog. That's how some haunted houses and concerts get their creepy clouds!

Coding a water thermometer

Scientists and engineers often write code that makes a decision from a measurement. This code reads a temperature and shows which state pure water would be in.

set temperature to -5
if temperature < 0 then
show "solid (ice)"
else if temperature < 100 then
show "liquid water"
else
show "gas (water vapour)"

With temperature set to −5 °C, the code shows "solid (ice)". This simple model ignores exactly 0 °C and 100 °C, where two states can be present together. It also ignores that some water evaporates even when it is not boiling.

−20 °C0 °C20 °C40 °C60 °C80 °C100 °C120 °C100 °C: water boils0 °C: water freezes / ice meltsgas (water vapour)liquid watersolid (ice)code:−5 °C
The thermometer code as a picture. Below 0 °C pure water is solid ice, between 0 °C and 100 °C it is liquid, and above 100 °C it is a gas. The code's −5 °C is in the ice zone.
Hands-on activity

The melting race

You need: 4 ice cubes the same size from the same tray, a metal baking pan, a plastic cutting board, a folded dish towel, a small plate, a timer, paper and pencil.

  1. Question: On which surface will an ice cube melt fastest?
  2. Predict: which ice cube will melt first? Which will melt last?
  3. Put the metal pan, the cutting board, the towel and the plate side by side on the same table, away from sunny windows.
  4. Put one ice cube on each at the same time. Start the timer.
  5. Check every 5 minutes and record what you see.
  6. Conclude: Which surface gave thermal energy to the ice fastest?
  7. Bonus (condensation): fill a glass with ice and cold water and set it on a dry towel. After 10 minutes, look at the outside of the glass. Where did the drops come from?
  8. Bonus (code): in the code above, what would show if you set the temperature to 25? To 120? Change the number and say what happens.

What to look for: the cube on the metal pan usually melts fastest, because metal passes thermal energy from the room to the ice quickly. The cube on the towel usually melts slowest, because the towel traps air and slows the flow of thermal energy. The drops on the glass come from water vapour in the air that condensed when it touched the cold glass. It is not water leaking through!

Safety: no stoves, kettles or hot water for this activity. Only grown-ups boil water. Don't suck on the test ice cubes. Wipe up melted water.

Your turn

1

What is thermal energy?

2

Name the three changes of state in which matter absorbs thermal energy.

3

Name the three changes of state in which matter releases thermal energy.

4

What happens to particles when matter absorbs thermal energy?

5

What is the change from a gas straight to a solid called? Give an example.

6

At what temperature does water freeze? At what temperature does it boil?

7

Using the code, what is shown when the temperature is set to 25?

More practice: warm-up

1

Name the change: liquid → solid.

2

Name the change: solid → liquid.

3

Name the change: gas → liquid.

4

Name the change: liquid → gas.

5

Name the change: solid → gas.

More practice: on your own

1

Water in an ice cube tray freezes in the freezer. Does the water absorb or release thermal energy? Where does it go?

2

Why does your hand feel cold when you hold an ice cube?

3

Why do drops form on the outside of a cold glass of lemonade on a hot day?

4

In the melting race, why did the ice on the towel melt slowest?

5

Using the code, what is shown at −12? At 150?

Challenge

1

A pot of water is boiling on the stove at 100 °C. The burner stays on, but the water's temperature stays at 100 °C. Where is the thermal energy going?

2

On a cold, sunny February day with no melting, a snowbank slowly gets smaller. Which change of state could explain this?

3

Change the code so it shows "too cold for a swim" when the temperature is less than 20, and "swim time" otherwise. Write it.

Check your understanding

Quiz

1.

When ice melts, it...

2.

Which change of state releases thermal energy?

3.

Frost forming on a window is an example of...

4.

When matter absorbs thermal energy, its particles...

5.

Dry ice turning straight into a gas is called...

6.

Sweating cools you because...

7.

In the code, if the temperature is set to 50, what is shown?

For parents and teachers

Big idea: Ontario Grade 5 expectation C2.6 asks children to explain how changes of state occur when matter absorbs or releases thermal energy. The examples also connect to C2.3 (changes of state around us). The code (A2.1) uses a condition to decide an outcome, and the melting race is a fair test (A1.2).

How to help: Point out changes of state as they happen: frost on the car, fog on the mirror, a popsicle melting. Ask "Is thermal energy going in or coming out?"

Common difficulties:

  • Thinking "cold" flows into your hand. It is thermal energy flowing out of the hand.
  • Thinking particles themselves grow or melt. The particles stay the same; only their movement and spacing change.
  • Thinking condensation on a glass leaks through it. The water comes from the air.
  • Mixing up sublimation (solid → gas) and deposition (gas → solid).

Safety: all boiling, stove and kettle work is for grown-ups only. Dry ice must never be touched with bare hands or sealed in a container; just discuss it.