Answers · The particle model

The particle model — final check key

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

1. PK1 · Final check

Model answer. (a) Z is a solid: the particles are in a regular pattern, touching. X is a liquid: close together and mostly touching, but irregular. Y is a gas: a few particles, far apart, at random. (b) 1, solid to liquid, is melting. 2, liquid to solid, is freezing, also called solidification. 4, gas to liquid, is condensation. (c) Liquid to gas can be boiling or evaporation. Boiling happens all through the liquid, with bubbles, at the boiling point; evaporation happens only from the surface of the liquid, below its boiling point. (d) In Z, the particles vibrate about fixed positions. In Y, they move fast, in straight lines, in random directions, until they hit another particle or a wall. (e) Y, the gas, can be compressed easily. Its particles are far apart, with empty space between them, and the forces between them are negligible, so pushing them closer together is easy. In Z and X the particles are already touching. (f) The pupil has it the wrong way round. In Y, the gas, the forces between the particles are negligible: that is why nothing holds the particles together and they fly about fast and far apart. The forces are strongest in Z, the solid, where they hold each particle in a fixed position. (g) The particles do not change in size or kind when a solid melts. They are the same particles, the same size; only their arrangement, separation and motion change: they leave their fixed positions and start to slide past each other.

A full answer includes

(a) I identified Z as a solid.
(a) I identified X as a liquid.
(a) I identified Y as a gas.
(b) I identified change 1 as melting.
(b) I identified change 2 as freezing (solidification).
(b) I identified change 4 as condensation.
(c) I identified boiling and evaporation.
(c) I stated one difference: boiling happens all through the liquid, with bubbles; evaporation happens only at its surface.
(d) I wrote that in Z the particles vibrate about fixed positions.
(d) I wrote that in Y the particles move fast, in random directions.
(d) I did not write that the particles in Z do not move.
(e) I stated Y, the gas.
(e) I wrote that its particles are far apart, with empty space between them, so they can be pushed closer together.
(f) I wrote that in Y the forces between the particles are negligible, the weakest of the three states.
(f) I wrote that the forces are strongest in Z, where they hold the particles in fixed positions.
(g) I wrote that the particles stay the same size: they are the same particles.
(g) I wrote that only their arrangement and motion change: they leave their fixed positions.

Also accept

(a) solid, liquid, gas; (b) melting, freezing (solidification), condensation; (c) boiling (throughout, bubbles) and evaporation (surface only); (d) Z vibrate, Y fast and random; (e) Y, particles far apart; (f) forces negligible in Y, strongest in Z; (g) the same particles, the same size.

Watch for

“(c) Change 3 can only be boiling.” — A liquid also turns into a gas below its boiling point, from its surface: a puddle dries without boiling. That is evaporation.
“(e) Y can be compressed because its particles get squashed smaller.” — The particles stay the same size. Only the empty space between them shrinks.
“(d) In Z the particles do not move.” — In a solid the particles vibrate about fixed positions all the time.
“(f) The fastest particles are held by the strongest forces.” — The particles of a gas move fast and far apart because the forces between them are negligible: nothing holds them together. The strongest forces are in the solid, Z.

2. PK2 · Final check

Model answer. (a) Brownian motion. The random motion of microscopic particles in a suspension, like the carbon in the ink, is evidence for the kinetic particle model of matter: matter is made of tiny particles in constant random motion. (b) The direction changes from one jump to the next with no pattern, and the jumps have different lengths: the path could not have been predicted. (c) The water molecules move fast, in random directions, and hit the carbon particle from all sides. At each moment, by chance, more molecules hit one side than the other, so the particle is pushed; a moment later another side gets more hits, so it is pushed another way. (d) The carbon particle is not an atom: it is a microscopic particle, made of a huge number of carbon atoms. And it is not pushed by the water as a whole, which is still, but hit by water molecules, one after another, unevenly. A correct version: “Each microscopic carbon particle is hit by water molecules.” (e) 1 × 10⁻¹⁶ kg ÷ 3 × 10⁻²⁶ kg ≈ 3.3 × 10⁹, so the carbon particle is about three thousand million times heavier than one water molecule. One hit by something so light barely moves it, far too little to see. What can be seen is the uneven part of an enormous number of hits every second. (f) For example: another carbon particle nearby moves in a different direction at the same moment, or the particle keeps jiggling about the same place instead of drifting steadily one way. A current would carry neighbouring particles along together. (g) It jiggles less, but it does not stop. In colder water the molecules move more slowly, with a smaller average kinetic energy, so they hit the particle less hard, and its jumps are smaller. (h) As the ink is cooled, its particles lose kinetic energy. At about −273 °C, absolute zero, they have the least kinetic energy they can have: there is no more energy of motion to take away, so the ink, or anything else, cannot be colder.

A full answer includes

(a) I stated Brownian motion.
(a) I stated that it is evidence for the kinetic particle model: matter is made of tiny particles in constant random motion.
(b) I described that the direction of the jumps changes with no pattern.
(b) I described that the jumps have different lengths.
(c) I wrote that water molecules, moving fast in random directions, hit the particle from all sides.
(c) I wrote that at each moment, by chance, one side gets more hits, so it is pushed one way, then another.
(d) I wrote that the carbon particle is not an atom: it is a microscopic particle, made of a huge number of atoms.
(d) I wrote that it is pushed by hits from the water molecules, one at a time, not by the water as a whole.
(d) I gave a correct version, for example: “Each microscopic carbon particle is hit by water molecules.”
(e) I divided: 1 × 10⁻¹⁶ kg ÷ 3 × 10⁻²⁶ kg.
(e) I wrote about 3 × 10⁹ times (3.3 × 10⁹).
(e) I wrote that one molecule is so light compared with the particle that one hit barely moves it; only the uneven part of an enormous number of hits can be seen.
(f) I suggested an observation: for example, a particle nearby moves in a different direction at the same time, or the particle keeps jiggling without drifting off one way.
(g) I predicted that it jiggles less, but does not stop.
(g) I wrote that the water molecules move more slowly, with less kinetic energy on average, so their uneven hits are gentler.
(h) I wrote that at about −273 °C, absolute zero, the particles have the least kinetic energy they can have.
(h) I wrote that there is no more energy of motion to take away, so nothing can be colder.

Also accept

(a) Brownian motion, evidence for the kinetic particle model; (b) random directions, different lengths; (c) uneven random hits by water molecules; (d) a microscopic particle, hit by water molecules; (e) about 3 × 10⁹, one hit far too small to see; (f) a neighbour moving a different way; (g) less, slower molecules; (h) least kinetic energy at −273 °C, nothing left to take away.

Watch for

“(c) The carbon particle moves because it is alive.” — Carbon from ink is not alive. Specks of rock, glass and smoke jiggle in just the same way, because molecules hit them.
“(d) The carbon particle is a carbon atom.” — A particle you can see through a microscope is a microscopic particle, made of a huge number of atoms. The molecules are the water’s, and they cannot be seen.
“(e) The carbon particle is about 3 × 10¹⁰ times heavier.” — Check the powers of ten: 10⁻¹⁶ ÷ 10⁻²⁶ = 10¹⁰, and 1 ÷ 3 ≈ 0.33, so the answer is 0.33 × 10¹⁰ = 3.3 × 10⁹.
“(g) In colder water the particle stops moving.” — At 5 °C the water molecules still move fast. They hit the particle less hard, so it jiggles less, but it does not stop.