You are going to find out how much of the air is used up when iron rusts. The experiment works by trapping a fixed volume of air over water and letting the ...
Question 1 Report
You are going to find out how much of the air is used up when iron rusts.
The experiment works by trapping a fixed volume of air over water and letting the damp iron wool inside slowly use up part of that air as it rusts, so push the damp iron wool firmly into the closed end of the test tube where it will stay put when the tube is turned upside down. Stand the tube upside down in a beaker of water and clamp it, then record the level of the water inside the tube at the start against the scale, because it is the rise in that level over the following days that measures the air used up. Leave the apparatus undisturbed and read the water level at the same time each day, recording every reading in your table so that the change can be worked out.
Take a small pad of iron wool, dip it in water and shake off the drops so that it stays damp. Push the damp iron wool firmly into the closed end of a boiling tube. Turn the tube upside down and clamp it with its open mouth under the surface of the water in a beaker, so that a column of air is trapped above the water inside the tube. Fix a millimetre ruler beside the tube. Measure the trapped air column at once, then leave the apparatus and measure the height to which the water rises inside the tube at the same time each day for 6 days.
Table 8.1
| day | height of the water inside the tube / mm |
|---|---|
| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 |
(a) Measure and record the length of the air column trapped in the boiling tube at the start. [2]
(b) Record the height of the water inside the tube on each of the six days in Table 8.1. [5]
(c) When the water level stops rising, record the final length of the trapped air column. [1]
(d) Use your readings to calculate the percentage of the trapped air that has been used up. Show your working. [3]
(e) Name the gas that has been used up. [1]
(f) State what has happened to the iron wool. [1]
(g) Describe how you would set up a second boiling tube to act as a control for this experiment. [3]
(h) Suggest why the apparatus must be left at the same temperature for the whole 6 days. [2]
(i) Give one reason why the water may rise less than expected. [1]
Answer Details
This measures the fraction of air used up when iron rusts. Rusting is the slow reaction of iron with oxygen and water, \( 4\,\mathrm{Fe} + 3\,\mathrm{O_2} + 2x\,\mathrm{H_2O} \rightarrow 2\,\mathrm{Fe_2O_3}\!\cdot\! x\mathrm{H_2O} \). As the damp iron wool takes oxygen from the trapped air, the pressure falls and water rises up the tube; the rise measures the oxygen removed.
(a) Record the starting length of the trapped air column, read from the ruler to the nearest millimetre, for example 120 mm [1], with the unit written [1].
(b) Enter a water height for every one of the six days [1], with the values increasing from day to day [1], the last two or three values equal [1], every value to the nearest millimetre [1], read at eye level against the ruler [1]. A worked set:
| Day | Height of water / mm |
|---|---|
| 1 | 8 |
| 2 | 15 |
| 3 | 20 |
| 4 | 23 |
| 5 | 24 |
| 6 | 24 |
(c) When the level stops rising, the final air column \( = \) starting length \( - \) final rise, for example \( 120 - 24 = 96\,\mathrm{mm} \) [1].
(d) Percentage of air used \( = \dfrac{\text{rise in water level}}{\text{starting air column}} \times 100 \) [1]; substituting the figures \( = \dfrac{24}{120} \times 100 \) [1] \( = \) about 20 % [1]. Accept 15 % to 25 %, consequential on your own readings; the true value is about one-fifth because oxygen is roughly 21 % of air.
(e) The gas used up is oxygen [1].
(f) The iron wool has rusted, turning orange-brown and crumbly [1].
(g) A control tube is set up the same way with the same volume of trapped air [1], but with dry iron wool, or left empty, so that no rusting can take place [1]; clamp it in the same beaker at the same temperature and measure it each day, expecting no rise [1]. This shows the rise in the first tube is caused by the rusting and nothing else.
(h) The temperature must be kept constant because a rise in temperature makes the trapped air expand and a fall makes it contract [1]; either change would move the water level and give a change not caused by the rusting [1].
(i) The water may rise less than expected for any one reason such as: air leaked in past the mouth of the tube, the iron wool dried out, the wool was greasy so part did not rust, or it was not left long enough [1].
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