Grade by Grade
E. Earth and Space Systems · Heat in the Environment

Changes of State and the Particle Theory

E2.3 E2.1 A1.2 A2.1About 50 minutes
Learning goals
  • I can use the particle theory to explain what happens to particles during melting, freezing, evaporating, boiling and condensing.
  • I can explain why the temperature stays the same while a pure substance changes state.
  • I can describe how volume changes during changes of state.
  • I can read and interpret a heating curve.

Words to know

change of state
A change from one state (solid, liquid or gas) to another.
melting point
The temperature at which a solid turns into a liquid. For pure water: 0 °C.
boiling point
The temperature at which a liquid boils, turning into a gas all through the liquid. For pure water at sea level: 100 °C.
heating curve
A graph that shows how the temperature of a substance changes over time as it is heated steadily.
evaporation
A liquid changing into a gas at its surface. It can happen at any temperature.
condensation
A gas changing into a liquid as it cools.

Remember from Grade 5

You learned the names of the changes of state: melting, freezing, evaporation (and boiling), condensation, sublimation and deposition. You learned that melting, evaporating and sublimating absorb thermal energy, and freezing, condensing and deposition release it.

Now you will dig deeper: what happens to the temperature and the volume during a change of state, and why.

A strange thermometer reading

Put a thermometer in a glass of ice and water and leave it on the counter. The room is warm, so heat keeps flowing into the glass. But the thermometer stays at about 0 °C for a long time, until almost all the ice is gone. Where is that energy going?

A glass full of ice cubes and water on a counter, with drops of water on the outside of the glass.
Heat flows from the warm room into the glass, but it goes into melting the ice, so the ice water stays at 0 °C until all the ice is gone. AI-generated image

Energy goes into breaking apart, not speeding up

During a change of state, the energy flowing in is used to overcome the attraction between particles, not to make them move faster.

  • Melting (0 °C for water): the particles of ice are held in place by attraction. Added energy breaks them out of their fixed places so they can slide past each other. Until all the ice has melted, the temperature stays at 0 °C.
  • Boiling (100 °C for water at sea level): added energy lets particles escape the attraction of the liquid completely and become a gas. The temperature stays at 100 °C while the water boils. (Turning up the stove makes it boil faster, not hotter.)
  • Going the other way: when water vapour condenses or water freezes, the particles come back together and release that energy to the surroundings. The temperature again stays steady during the change.

Only after a change of state is finished does the temperature start to change again.

0100°Cmeltingboilingtime
A heating curve for water (not to scale). Ice warms up to 0 °C, then the line is flat while it melts. The water warms to 100 °C, then the line is flat while it boils. Only then does the steam (water vapour) get hotter.

Volume during changes of state

  • Solid → liquid: for most substances, the liquid takes up a little more space. (Water is the exception: ice → water shrinks by about 9%.)
  • Liquid → gas: a huge expansion. Water vapour at 100 °C takes up more than a thousand times the volume of the liquid water it came from, because the particles are now far apart.
  • Gas → liquid → solid: the volume decreases again (except that water expands when it freezes).

This is why a lid rattles on a pot of boiling water: water vapour takes up much more space and pushes the lid up. The tiny cloud you see above the pot is not the water vapour itself. Water vapour is invisible. The cloud is tiny droplets of liquid water that formed when the vapour cooled in the air.

Evaporation cools you down

Evaporation happens at the surface of a liquid, at any temperature. The fastest-moving particles escape, so the particles left behind have a lower average speed. That means the liquid gets cooler.

This is how sweating works. Sweat evaporates from your skin and takes thermal energy with it. Dogs pant for a similar reason: water evaporates from their tongue and mouth.

the FASTEST particlesescape from thesurfacethe slower onesstay behind, so theaverage speed drops:the liquid cools
Evaporation is like the fastest runners leaving a race. The particles left behind are slower on average, so the liquid (or the sweat on your skin) gets cooler.
Worked example

Let's read a heating curve

Look at the heating curve. (a) What is happening on the first flat part? (b) Why is it flat? (c) What is happening on the sloped part between 0 °C and 100 °C?

  1. (a) The first flat part is at 0 °C. The ice is melting.
  2. (b) The energy going in is being used to break particles out of their fixed places, not to make them move faster, so the temperature does not rise.
  3. (c) Between 0 °C and 100 °C all the water is liquid. The energy now makes the particles move faster, so the temperature rises.

(a) melting, (b) energy goes into overcoming attraction, (c) liquid water warming up.

Just for fun

Why did the ice cube refuse to get any warmer than 0 °C?

It was too busy melting to think about anything else.

# Ice-water temperature logger
last_temp = thermometer.read()
repeat 15 times:
wait 2 minutes
temp = thermometer.read()
record(temp)
if temp - last_temp < 0.5 and temp - last_temp > -0.5:
say "Steady temperature: a change of state may be happening"
else if temp > last_temp:
say "Warming up"
else:
say "Cooling down"
last_temp = temp

A plan for a program (A2.1) that reads a temperature sensor every 2 minutes for 30 minutes. If the temperature changes by less than half a degree, it reports a steady temperature, a clue that a change of state may be happening.

Hands-on activity

Track the temperature of melting ice (A1.2)

You need: a glass, ice cubes, cold tap water, a kitchen thermometer (or an aquarium or outdoor thermometer), a clock, paper and a pencil.

  1. Fill the glass with ice cubes, then add cold water to about half full. Stir gently for 30 seconds.
  2. Put the thermometer in. Wait until it settles and record the temperature (time 0).
  3. Leave the glass on the counter. Stir gently and record the temperature every 5 minutes until all the ice has melted, and then for 3 more readings after that. (This can take an hour or more, so do other things in between.)
  4. Make a line graph: time on the horizontal axis, temperature on the vertical axis.
  5. Analyse: where is your graph flat? When did it start to rise?

What to look for: the temperature stays near 0 °C while there is still ice in the glass, even though the room is warmer. Once the ice is gone, the temperature starts to rise toward room temperature.

Thinking about your results: if your reading was 1 or 2 degrees off from 0 °C, think about why. Kitchen thermometers are not always exact, and the tip may have touched an ice cube or warmer water near the top.

Safety: use only cold water and ice for this activity; do not heat or boil water. If you use a glass thermometer, handle it gently, and if it breaks, a grown-up cleans it up.

Your turn

1

During melting, what is the added energy used for?

2

Why does the temperature of boiling water stay at 100 °C even when the stove is turned up?

3

What is released when water vapour condenses into liquid water?

4

Does the volume increase a little or a lot when water boils into vapour?

5

Why does sweating cool you down?

6

What is the white cloud above a pot of boiling water made of?

7

On a heating curve, what does a flat section tell you?

More practice: warm-up

1

What is the melting point of pure ice?

2

Name the change: liquid → gas throughout the liquid, with bubbles.

3

Name the change: gas → liquid.

4

Does melting absorb or release energy?

5

Is water vapour visible or invisible?

More practice: on your own

1

A glass of ice water sits in a 22 °C room for 20 minutes and still has ice in it. What temperature is the water? Explain.

2

Why do you feel cold when you step out of a swimming pool on a windy day?

3

Explain why the lid of a pot of boiling water rattles.

4

Describe the motion and arrangement of water particles as ice melts and then the water warms up.

5

In the logger program, what does the line "if temp - last_temp < 0.5 and temp - last_temp > -0.5" check?

Challenge

1

Burns from steam at 100 °C can be worse than burns from water at 100 °C. Use what you know about condensation to explain why.

2

A heating curve for an unknown pure substance is flat at 80 °C and again at 218 °C. What are its melting and boiling points? What state is it in at 150 °C?

3

Farmers sometimes spray fruit trees with water on a frosty spring night to protect the buds. Use what you learned to suggest how this could help.

Check your understanding

Quiz

1.

While ice is melting, its temperature...

2.

On a heating curve, a flat section shows...

3.

When water vapour condenses, energy is...

4.

Which change of state has the biggest increase in volume?

5.

Why does evaporation cool a liquid?

6.

The white cloud above a boiling kettle is...

For parents and teachers

Big idea (E2.3, E2.1): during a change of state, energy goes into overcoming the attraction between particles, so the temperature stays constant until the change is complete. Volume changes greatly from liquid to gas. The ice-water experiment practises collecting and graphing data (A1.2), and the logger plan introduces conditional statements in code (A2.1).

How to help: do the ice-water log together and make the graph. Ask your child to point to the flat part and explain it.

Common difficulties:

  • Expecting the temperature to keep rising while heating. Flat parts are the key idea.
  • Calling the visible cloud "steam" or "water vapour". Water vapour is invisible; the cloud is tiny droplets.

Safety: this lesson uses only ice and cold water. Keep children away from boiling water and steam; steam can cause serious burns.