Answers · The particle model

The particle model — return questions key

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

1. PR1 · Return

Model answer. (a) A liquid and a gas both flow, and neither has a fixed shape: each takes the shape of its container. A liquid has a fixed volume, but a gas does not: a gas spreads out to fill its whole container. (b) Melting. In the solid chocolate, the particles are in fixed positions in a regular pattern, close together, vibrating. As it melts, they break out of their fixed positions and move about, sliding past each other, though they stay close together. (c) The key is a metal: positive ions, with electrons moving freely between them. The neon is a noble gas: single atoms. In the key the ions are close together, and strong forces hold them in fixed positions, where they only vibrate, so the key keeps its shape. In the neon the atoms are far apart and the forces between them are negligible, so nothing holds them together: they move fast in random directions and spread out to fill the tube.

A full answer includes

(a) I stated that both a liquid and a gas flow (or that neither has a fixed shape).
(a) I stated that a liquid has a fixed volume, but a gas does not.
(b) I stated melting.
(b) I wrote that the particles leave their fixed positions in a regular pattern.
(b) I wrote that they move about, sliding past each other, still close together.
(b) I did not write that the particles move far apart.
(c) I stated that the key is made of positive ions, with free electrons moving between them.
(c) I stated that the neon is made of single atoms.
(c) I wrote that in the key, strong forces hold the ions close together in fixed positions, so it keeps its shape.
(c) I wrote that in the neon, the atoms are far apart and the forces between them are negligible, so they spread out to fill the tube.

Also accept

(a) both flow (no fixed shape); only a liquid has a fixed volume; (b) melting, particles leave fixed positions and slide past each other, still close; (c) key: positive ions and free electrons, strong forces, fixed positions; neon: single atoms, far apart, negligible forces.

Watch for

“(a) A gas has a fixed volume, like a liquid.” — A gas spreads out to fill whatever container it is in, so it has no fixed volume of its own.
“(b) When chocolate melts, its particles move far apart.” — In a liquid the particles are still close together, mostly touching. They move far apart only in a gas.
“(c) The key keeps its shape because its particles do not move.” — The ions in the key vibrate about fixed positions all the time. Strong forces keep them in those positions.
“(c) Neon is made of molecules.” — Neon is a noble gas: its atoms do not join up, so the gas is single atoms.

2. PR2 · Return

Model answer. (a) Through the microscope, the specks are bright points on a dark background. Each jiggles about on its own, in a random zig-zag of jumps of different lengths, and neighbouring specks jump in different directions. (b) The air molecules move fast in random directions and hit each speck from all sides. At each moment, by chance, more hit one side than the other, so the speck is pushed; a moment later another side gets more hits, and it is pushed another way: Brownian motion. (c) Each speck is a microscopic particle, made of a huge number of atoms. It is hit by the molecules of the air, which are light, fast-moving and thousands of times smaller. (d) The molecules themselves cannot be seen, but the random jiggle of microscopic particles in a suspension is just what the kinetic particle model predicts if matter is made of tiny particles in constant random motion. (e) As a gas is cooled, its particles move more slowly, so their average kinetic energy is smaller. (f) Over the 200 °C from −123 °C to 77 °C, the average kinetic energy changes by 7.25 − 3.11 = 4.14 × 10⁻²¹ J. The 3.11 × 10⁻²¹ J left at −123 °C runs out after 3.11 ÷ 4.14 × 200 ≈ 150 °C more, at about −123 − 150 = −273 °C. This lowest possible temperature is called absolute zero: there the particles have the least kinetic energy they can have.

A full answer includes

(a) I described bright specks on a dark background.
(a) I described each speck jiggling on its own, in a random zig-zag.
(a) I described neighbouring specks moving in different directions.
(b) I wrote that air molecules hit each speck from all sides.
(b) I wrote that at each moment, by chance, one side gets more hits, so the speck is pushed one way, then another.
(c) I stated that the specks are microscopic particles, made of a huge number of atoms.
(c) I stated that they are hit by molecules of the air, which are light, fast-moving and far too small to see.
(d) I stated that the molecules cannot be seen, but the random motion of the specks is what the model predicts for matter made of tiny particles in constant random motion.
(e) I described the particles moving more slowly as the gas is cooled.
(e) I wrote that their average kinetic energy is smaller.
(f) I found the fall over the 200 °C between the two values: 7.25 − 3.11 = 4.14 (× 10⁻²¹ J).
(f) I found how far below −123 °C the energy runs out: 3.11 ÷ 4.14 × 200 ≈ 150 °C.
(f) I wrote about −273 °C.
(f) I stated its name: absolute zero.

Also accept

(a) random zig-zag, each on its own; (b) uneven hits from all sides; (c) microscopic particles hit by air molecules; (d) what the particle model predicts; (e) slower, smaller average kinetic energy; (f) about −273 °C, absolute zero.

Watch for

“(c) The specks are smoke molecules.” — A speck of smoke is a microscopic particle made of a huge number of atoms. The molecules are the much smaller particles of the air that hit it.
“(b) The specks are carried by a draught in the cell.” — A draught would carry neighbouring specks the same way together. Each speck jiggles on its own, because of uneven hits from the molecules.
“(f) The energy reaches zero at −123 °C, the colder of the two temperatures.” — At −123 °C there is still 3.11 × 10⁻²¹ J. The straight line keeps falling until it reaches zero, about 150 °C colder.