Particles and their behaviour · PROCESS

Changes of state

Where does the ice go when it melts?

Draft — not yet science-reviewed.

Start here

The sealed bag

Ice, water and steam turned out to be the same particles rearranged. So here is the test. A sealed bag holds an ice cube. It is weighed, left on the bench until the ice has melted, then weighed again.

Will the reading go up, go down, or stay the same?

Your turn

Commit before the balance settles it. What will the second reading say?

Think again

Name the wrong idea first. Plenty of students write: 'the ice melted, so some of it was lost.' Now read the two balance readings again.

Sealed bag with ice on a balance, before and after.

So the ice did not go anywhere. Here is what it did instead. Heating gives the particles more energy, so they move more and break away from each other. Cooling takes energy away, so they settle back together. Not one particle is made and not one is destroyed. That is why the balance never moved.

Investigate

This is the same particle lab you used last lesson, now with a heater on it. Predict first: you heat a solid steadily. What happens at the moment it melts?

Make your prediction first — then the lab runs.

The particle counter never changes. Only the arrangement and the speed do.

The six changes of state as arrows between solid, liquid and gas.

Words to know

melting
Changing from a solid to a liquid.
freezing
Changing from a liquid to a solid.

Freezing and melting happen at the same temperature for a given substance — they are the same doorway, used in opposite directions.

evaporating
Changing from a liquid to a gas.
condensing
Changing from a gas to a liquid.
sublimation
Changing straight from a solid to a gas, with no liquid in between.

Solid carbon dioxide does this — which is why it is called dry ice.

conserved
Stays the same in total. Nothing is gained or lost.

Your turn

Every arrow on that map has a name. Say the name out loud, then tap to check it.

Worked example

Watch this one set out in full, because you do the next one. A sealed flask holds 24.0 g of ice. It is left until all the ice has melted. What is the mass of water?

Formula mass before = mass after (mass is conserved)

Insert mass before = 24.0 g

Fix no particles enter or leave a sealed flask

Answer 24.0 g

Your turn

Your turn, same four steps. A sealed tube holds 18.5 g of solid iodine. It is warmed until it has all sublimed to a purple gas. What is the mass of gas? Set it out as Formula, Insert, Fix, Answer.

Check your answer
  • States that mass is conserved
  • Inserts 18.5 g
  • Answer 18.5 g with units
  • Notes the tube is sealed

Your turn

Two words students mix up constantly. Sort these: ice in a warm room; sugar in tea; chocolate in your hand; salt in water.

Think again

Say the wrong idea out loud: 'sugar melts in tea.' Sugar in tea and ice in a warm room both look like disappearing. Are they the same change?

Your turn

One more disappearing act, and it needs the same idea. Water is boiling hard. What is inside the bubbles?

Your turn

Push it one step further. If those bubbles really were air, where would the air have come from in a sealed flask that had already been boiled once?

Check your answer
  • Recognises there is no source of air in a sealed, already-boiled flask
  • Concludes the bubbles must be the water itself, as gas
  • Says no particles were created — they only changed state

Mastery ladder

① Recall

What is the name of the change from a solid straight to a gas?

② Apply

A sealed flask holds 30 g of ice. After melting completely, what is the mass of the water?

③ Explain

Explain, in terms of particles, why the mass does not change when ice melts in a sealed bag.

Mark your answer against this list
  • Says the same particles are present throughout
  • Says no particles enter or leave
  • Says only the arrangement and movement change
  • Concludes total mass is therefore unchanged

④ Produce

A student leaves a puddle of water on the playground. The next day it has gone. They say 'the water was destroyed by the sun'. Write a better explanation.

Mark your answer against this list
  • Says the water evaporated to become a gas
  • Says the particles are still there, now in the air
  • Says nothing was destroyed
  • Mentions energy from the sun giving particles more movement

Key note

Heating gives particles energy to break away; cooling lets them settle back. Mass is always conserved and every change of state can be reversed. Melting needs heat and one substance; dissolving needs a liquid and two.

Going further

Every one of those arrows runs both ways. Freeze the water and you get ice again — the same substance, not a new one. Nothing has been made and nothing destroyed. That is exactly what makes a change of state a physical change.

Your turn

Steam meets a cold window and turns into drops of water. Name that change, then name the change that would undo it.

Check your answer
  • Names condensing
  • Names evaporating (or boiling) as the reverse
  • Says it is the same substance both ways
  • Says the mass of water is unchanged

Before this lesson

At GCSE this becomes

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