A smell you can't see — answer key

1. SQ1 · Practice

Model answer. It spread out from the pad: the nearer strips turned first, and the waits got longer. Strip 1, nearest the pad, turned first and strip 6 last, so the ammonia spread out from the pad, where its concentration was highest. The waits grew, so it did not move along at a steady speed. The amount of ammonia stayed the same: the lid kept it inside, and diffusion only moves particles; it doesn't make or destroy them. The same particles spread through more space, so the concentration near the pad fell.

A full answer includes

• I chose “It spread out from the pad: the nearer strips turned first, and the waits got longer”: strip 1 turned first, strip 6 last, and each wait was longer than the one before.
• I stated that the amount of ammonia stayed the same: the lid kept it in, and spreading moves the particles without making or destroying any.

Also accept

• Spread out from the pad, nearest strips first, with longer and longer waits; the amount stayed the same, because the closed tank kept all the ammonia in.
• The statement about spreading out from the pad; no change in the amount: the ammonia particles spread through more space, but none are made or destroyed.

Watch for

• “A gas fills its container at once.” — Not a long tank of still air. The strips turned one after another, so the ammonia took time to reach each one.
• “There is less ammonia once it has spread out.” — There is the same amount, spread through more space. Its concentration near the pad falls, but none leaves the closed tank.

2. SQ2 · Practice

Options as numbered on the pupil sheet: 1 — Near the pad there are many ammonia particles; further along the tank there are few.; 2 — So the ammonia has a net movement away from the pad, until its concentration is the same everywhere.; 3 — Each ammonia particle moves in a straight line until it collides with another particle.; 4 — So more ammonia particles cross any point from the pad side than from the far side.; 5 — Every collision sends it off in a new, random direction..

Model answer. Order: one particle moves in a straight line until it collides; each collision sends it in a new, random direction; there are many ammonia particles near the pad and few further along; so more cross any point from the pad side; so there is a net movement away from the pad. Each particle moves in a straight line until it collides, and every collision sends it off in a new, random direction, so some ammonia particles do head back towards the pad. But there are many more ammonia particles near the pad than further along. At any point in the tank, more of them happen to cross from the pad side than from the far side. That difference is a net movement away from the pad, and it lasts until the concentration is the same everywhere.

A full answer includes

• Order: each particle moves in a straight line until it collides with another particle; every collision sends it off in a new, random direction; there are many ammonia particles near the pad and few further along; so more of them cross any point from the pad side than from the far side; so there is a net movement away from the pad until the concentration is the same everywhere.
• I said that some ammonia particles do move back towards the pad, because their directions are random; the net movement comes from the difference in numbers on the two sides.

Also accept

• Straight line until a collision, new random direction, many near the pad and few far away, more cross from the pad side, net movement away from the pad.
• Starting with “many near the pad, few further along” and then the two steps about how one particle moves is fine too, as long as “more cross from the pad side” and the net movement come last.

Watch for

• “Every ammonia particle moves away from the pad.” — Each one moves at random, so some head back towards the pad. The gas spreads away from the pad only because more particles cross from the crowded side.

3. SQ3 · Practice

Model answer. The wrong statement is the one about particles making their way to the emptier end because they need more room. No particle aims anywhere or needs room. Each ammonia particle moves at random and changes direction at every collision, so some are moving back towards the pad. The ammonia spreads towards the far end because there are more ammonia particles near the pad: more of them cross the middle of the tank from that side than from the other. The result is a net movement from higher to lower concentration.

A full answer includes

• I chose “The ammonia particles make their way to the emptier end of the tank because they need more room.”
• I explained that no particle aims anywhere or needs anything: each one moves at random, in every direction, and changes direction at every collision.
• I gave the correct reason: there are more ammonia particles near the pad, so more of them cross from that side than back, which gives a net movement towards the far end, where the concentration is lower.

Also accept

• The “need more room” statement: particles don't choose a direction; more cross from the crowded pad side simply because there are more of them there.
• The statement about making for the emptier end is wrong: their motion is random, and the net movement towards the far end comes from the higher concentration near the pad.

Watch for

• “The statement that some particles move back towards the pad is wrong, because the ammonia only moves away from the pad.” — Single particles move in every direction, including back towards the pad. Only the net movement is away from the pad.
• “The statement about more particles crossing from the pad side is wrong, because with random motion the same number must cross each way.” — Random motion means no direction is favoured for any one particle. But there are more particles on the pad side, so more of them cross from there.

4. SQ4 · Practice

Model answer. (a) From X to Y the peak gets lower and wider: the concentration at the centre falls, and near both ends it rises from almost zero. (b) The ammonia particles move at random and change direction at every collision, so as many head left as right. On each side there are more particles near the centre than further out, so more cross outwards than come back. This net movement outwards lowers the peak and widens it, and it is the same on both sides, so the curve stays symmetrical. (c) In the end the graph is a flat line: the concentration is the same all along the tank. It is the spread-out average, below the peak of Y, above Y's lowest points and not zero. The particles are still moving and colliding, but as many cross each way, so the concentration no longer changes.

A full answer includes

• (a) I said the concentration at the centre falls and near the ends it rises: the peak gets lower and wider.
• (b) I said the particles move at random, so as many head left as right; on each side more cross outwards from the crowded centre than come back, so there is a net movement outwards (lower, wider peak) that is the same on both sides (symmetrical).
• (b) I did not say the particles want, try or aim to go anywhere: no particle aims anywhere.
• (c) I predicted a flat line: the same concentration everywhere, below the peak of Y, above Y's lowest points and not zero (the spread-out average). I said the particles are still moving, but as many cross each way, so there is no net movement.

Also accept

• (a) Centre falls, ends rise; the peak gets lower and wider. (b) Random motion, as likely left as right; more particles cross outwards from the high concentration at the centre than back, equally on both sides. (c) A flat line at the average level, not zero; the particles are still moving, but there is no net movement.
• (c) “The concentration is the same everywhere” counts as a flat line, and “diffusion stops but the particles keep moving” is fine too.

Watch for

• “The ammonia spreads because the particles push each other away from the crowded centre.” — Gas particles are far apart and only affect each other when they collide. The spreading comes from random motion plus there being more particles at the centre.
• “In the end, the particles stop moving.” — The particles of a gas never stop moving. When the concentration is even, as many cross each way, so nothing more changes.

5. SQ5 · Practice

Model answer. (a) In a liquid the particles are close together, touching their neighbours. A purple particle can hardly move before it collides with a water particle and changes direction, so it gets very little distance in a given time. In a gas the particles are far apart, so each one travels much further between collisions, and a gas spreads much faster. (b) Stirring moves the whole liquid, carrying the purple particles along with the water. That is the liquid being moved, not diffusion. In still water the purple spreads only by diffusion, which takes days.

A full answer includes

• (a) I said that in a liquid the particles are close together, touching their neighbours, so a purple particle collides with water particles almost all the time.
• (a) I said each purple particle therefore moves only a tiny distance before changing direction, so it gets much less far in the same time than a gas particle, which travels much further between collisions.
• (b) I said that stirring moves the whole liquid and carries all its particles along together: that is not diffusion, so it tells her nothing about how fast diffusion is.

Also accept

• (a) Liquid particles are close together, so there are many more collisions and each particle moves only a short way before changing direction. (b) Stirring moves the liquid itself and carries the particles along; that isn't diffusion.
• (a) “Less space between the particles, so the purple particles keep bumping into water particles” counts.

Watch for

• “Diffusion is slower in a liquid because the particles in a liquid move more slowly.” — The reason to give is spacing, not speed. Liquid particles are close together, so a particle collides almost all the time and gets very little distance before changing direction.
• “Stirring speeds up diffusion.” — Stirring moves the whole liquid and carries the particles along. That spreading is not diffusion, which needs no stirring at all.

6. ST1 · Apply

Model answer. (a) The dye spreads out from the hole, where its concentration is highest, into the jelly around it, where there is little or none. It spreads equally in all directions, and more and more slowly: the circle took 1 hour to reach 1 cm across but 4 hours to reach 2 cm. (b) The dye particles move at random and keep colliding with water particles, so no direction is favoured. In every direction, more dye particles cross outwards from the crowded centre than back, so the circle grows evenly all round. The same amount of dye is spread through more and more jelly, so its concentration falls and the colour gets paler. (c) The jelly can't flow, so nothing carries the dye along. Any spreading must come from the dye particles' own random motion: diffusion.

A full answer includes

• (a) I said the dye spreads out from the hole, from a higher to a lower concentration, equally in all directions, and that it spreads more slowly as the circle grows: twice as wide took four times as long.
• (b) I said the dye particles move at random and collide with water particles, so no direction is more likely than another; in every direction more cross outwards from the crowded centre than come back, so the circle grows evenly all round.
• (b) I said the circle gets paler because the same amount of dye is spread through more jelly: its concentration falls.
• (c) I said that if the jelly could flow, moving liquid could carry the dye along; because it can't, the spreading must be diffusion alone.

Also accept

• (a) Outwards from the hole in all directions, slowing down as it goes. (b) Random motion, every direction equally likely; net movement from high to low concentration in every direction; the same amount in more jelly, so paler. (c) No flowing liquid to carry the dye, so the spreading is diffusion alone.
• (b) “More dye particles move out of the crowded middle than come back in” counts as the net movement.

Watch for

• “The dye spreads out because it wants to fill the dish.” — No particle aims anywhere. Dye particles move at random; more cross outwards than back because there are more of them near the hole.
• “The circle gets paler because some of the dye gets used up.” — None is used up. The same amount of dye is spread through more jelly, so its concentration is lower.