Answers · States and changes of state

States and changes of state — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. SK1 · Final check

Model answer. (a) Both statements are wrong. Squashing a gas pushes its particles closer together; the particles stay the same size. Melting keeps the same H₂O molecules: they leave their fixed positions and move around each other, but they don't break apart. (b) It is a gas. It can also be compressed easily (and has no fixed shape). Its particles are far apart, randomly arranged and move quickly in all directions.

A full answer includes

I said gas particles stay the same size when squashed; only the spaces between them get smaller.
I said melting keeps the same H₂O molecules; only their arrangement and motion change.
I identified the sample as a gas and gave another property, such as being easy to compress or having no fixed shape.
I described its particles as far apart, randomly arranged and moving quickly in all directions.

Also accept

(a) Wrong twice: particles keep their size, only the gaps shrink; melting keeps intact H₂O molecules. (b) Gas; easily compressed; particles far apart, random, moving quickly in all directions.

Watch for

“Melting makes new substances.” — Melting is a change of state. The same H₂O molecules are there before and after; only their arrangement and motion change.
“Particles shrink when a gas is compressed.” — The particles keep their size. Compressing a gas reduces the empty space between them.

2. SK2 · Final check

Model answer. (a) Melting point 17 °C; boiling point 118 °C: the temperature stays constant at these values. (b) Liquid. (c) Q is boiling. The energy supplied overcomes the forces of attraction between particles instead of making them move faster, so the temperature stays at 118 °C. (d) Evaporation: it happens only at the surface and below the boiling point, without bubbles.

A full answer includes

Melting point 17 °C and boiling point 118 °C, read from the constant temperatures.
At 11 minutes Q is a liquid: it is between its melting and boiling points.
From 16 to 20 minutes Q is boiling: the energy overcomes forces of attraction between particles, so they don't move faster and the temperature stays constant.
At 80 °C Q evaporates: only from the surface, below its boiling point, without bubbles.

Also accept

17 °C and 118 °C; liquid; boiling, energy overcomes forces between particles so the temperature is constant; evaporation at the surface below the boiling point.

Watch for

“The melting point is the temperature at the start of heating.” — Look for a temperature that stays the same for several readings: that is a change of state.

3. SK3 · Final check

Model answer. (a) Liquid at 25 °C, gas at 80 °C. (b) The temperature falls to 56 °C, stays flat while R condenses, falls to −95 °C, stays flat while R freezes, then falls again to −120 °C. (c) While R condenses or freezes, forces of attraction pull the particles closer together and energy is released. This replaces the energy lost to the surroundings, so the temperature stays constant until the change is complete.

A full answer includes

Liquid at 25 °C; gas at 80 °C.
Flat at 56 °C while R condenses and flat at −95 °C while it freezes; falling before, between and after.
During the change, energy is released as forces of attraction pull the particles together; it replaces the energy lost, so the temperature stays constant.

Also accept

Liquid, gas; flat at 56 °C (condensing) and −95 °C (freezing); energy released as particles are attracted together balances the energy lost.

Watch for

“On cooling, the flat parts happen at different temperatures from heating.” — For a pure substance, condensing happens at the boiling point and freezing at the melting point: the same temperatures as on heating.