Pupil sheet · Diffusion
Lighter, faster — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. identify / state
In the ring tube, ammonia (Mr 17) spreads from one end and hydrogen chloride (Mr 36.5) from the other, at the same temperature. Complete each answer.
The gas with the lower Mr
Its rate of diffusion, compared with the other gas
The white ring forms nearer the end of which gas?
Nudge (only if you are stuck): Compare the two Mr values first. Then ask: which gas covered more of the tube before they met?
2. explain
Put the steps in order to explain why the white ring forms nearer the hydrogen chloride end of the tube.
Write the number of each step in order.
- The two gases meet nearer the hydrogen chloride end.
- Ammonia particles are lighter (relative molecular mass 17, against 36.5), so they move faster on average.
- There they react and make a white ring of ammonium chloride.
- Both gases are at the same temperature, so their particles have the same average kinetic energy.
- Both gases zigzag through the air, but in the same time the ammonia spreads further along the tube.
- First: ______
- Second: ______
- Third: ______
- Fourth: ______
- Fifth: ______
If both gases have the same average kinetic energy, why do the lighter particles move faster?
Nudge (only if you are stuck): Start with what is the same for both gases. End with the ring.
3. predict / explain
Each row of the table is a ring-tube experiment at room temperature. One gas spreads from the left end and one from the right end, both put in at the same moment, and where they meet they form a white solid. Predict where the ring forms in each tube.
| Tube | Left end | Right end |
|---|---|---|
| 1 | hydrogen iodide, Mr 128 | ammonia, Mr 17 |
| 2 | methylamine, Mr 31 | hydrogen bromide, Mr 81 |
| 3 | gas P, Mr 44 | gas Q, Mr 44 |
| 4 | hydrogen chloride, Mr 36.5 | trimethylamine, Mr 59 |
Write the number of the matching option.
- Nearer the left end
- In the middle
- Nearer the right end
- Tube 1: ______
- Tube 2: ______
- Tube 3: ______
- Tube 4: ______
Explain your prediction for tube 4.
Nudge (only if you are stuck): In each tube, which gas is faster? The faster gas covers more of the tube before they meet.
4. evaluate / explain
A pupil explains the ring tube like this: “Hydrogen chloride molecules are heavier, so air molecules can't knock them off course as easily, and they get further along the tube.” (a) Use the position of the ring to evaluate the pupil's idea. (b) Give a correct explanation, using particles.
(a) What the ring position shows about the pupil's idea
(b) A correct explanation, using particles
Nudge (only if you are stuck): If hydrogen chloride really got further, which end would the ring be nearer?
5. identify / explain
A gas jar of pale yellow-green chlorine (Mr 71) stands under a gas jar of air, mouth to mouth, with a glass plate between them. (Chlorine is toxic, so this is a teacher demonstration.) The plate is pulled out, and the jars are left sealed together and still. Which statements are correct?
Tick every correct statement.
- Chlorine molecules are heavier than the nitrogen and oxygen molecules in air, so at the same temperature they move more slowly on average.
- Chlorine is denser than air, so none of it can spread up into the top jar.
- Once both jars are evenly coloured, the chlorine molecules stop moving.
- Each chlorine molecule changes direction every time it collides with another molecule.
- The chlorine can spread upwards only if there is a draught between the jars.
- Bromine (Mr 160) in the bottom jar would take longer than chlorine to spread evenly through both jars.
Choose one statement you rejected and explain what is wrong with it.
Nudge (only if you are stuck): Which statements give the gas a pull in one direction, or need moving air?
6. predict / explain
Ethylamine is a gas with Mr 45. Like ammonia, it reacts with hydrogen chloride (Mr 36.5) to make a white solid. A teacher puts cotton wool soaked in ethylamine solution into the left end of a long tube, and cotton wool soaked in concentrated hydrochloric acid into the right end, at the same moment. (a) Predict where the white ring forms. (b) Explain your prediction, using particles. (c) The teacher repeats the experiment with the plugs swapped: hydrogen chloride at the left end, ethylamine at the right. Predict where the ring forms now.
(a) Where the white ring forms
(b) Why, using particles
(c) Where the ring forms with the plugs swapped
Nudge (only if you are stuck): Compare the two Mr values. Which gas is the lighter one this time?