A sealed sample tube of gas has been handed to you. The tube is fitted with a bung and a short delivery tube, and you are told only that the gas is one of h...
Question 1 Report
A sealed sample tube of gas has been handed to you. The tube is fitted with a bung and a short delivery tube, and you are told only that the gas is one of hydrogen, oxygen, carbon dioxide or chlorine. Four quick bench tests will settle which it is. Carry them out in the order given, because two of the tests destroy the sample.
First hold a piece of damp blue litmus paper at the open end of the delivery tube for a few seconds, then withdraw the litmus paper and look at it. Second, bubble a little of the gas through limewater in a boiling tube and watch the limewater. Third, collect a little of the gas in a small dry test tube and hold a lighted splint at the mouth of that test tube. Fourth, collect a further sample and lower a glowing splint into it.
Table 11.1
| test | observations |
|---|---|
| damp blue litmus paper held in the gas | |
| gas bubbled through limewater | |
| lighted splint at the mouth of the test tube | |
| glowing splint lowered into the gas |
(a) Enter your observations for all four tests in Table 11.1. [6]
(b) Name the gas in the sealed tube, and give the one test result that identifies it beyond doubt. [3]
(c) Explain why the litmus test is carried out before the splint tests rather than after them. [3]
(d) For each of the three gases the sample turned out not to be, give the single result that would have identified it. [3]
(e) Suggest why a glowing splint, rather than a lighted one, is used to test for oxygen. [2]
(f) Chlorine is one of the four possibilities. State the precaution needed while testing for it and give the reason. [2]
Answer Details
This is a gas-identification practical. Four bench tests are done in a fixed order because the splint tests burn or consume the sample, so the non-destructive litmus test must come first. Reading the four results together points to a single gas; the observations below are the consistent set for the sample actually supplied.
(a) [6] Record what each test shows:
| test | observations |
|---|---|
| damp blue litmus paper held in the gas | no change (the paper stays blue) |
| gas bubbled through limewater | limewater stays clear (no milkiness) |
| lighted splint at the mouth of the test tube | a squeaky "pop" |
| glowing splint lowered into the gas | the splint goes out (does not relight) |
Marks are awarded as a consistent set [6]: damp blue litmus no change (it would turn red then bleach white only for chlorine); limewater stays clear (it would turn milky only for carbon dioxide); the lighted splint gives a squeaky pop (hydrogen) rather than being extinguished; the glowing splint is not relit (only oxygen would relight it).
(b) [3] The gas is hydrogen [1]. The result that identifies it beyond doubt is the squeaky pop when a lighted splint is held at the mouth of the tube [1], because hydrogen is the only one of the four gases that burns explosively with that sound; none of oxygen, carbon dioxide or chlorine gives a pop [1].
(c) [3] The litmus test is done before the splint tests for any three of these reasons (max 3): the splint tests burn or consume the sample [1]; chlorine would bleach the litmus, and that is the only test that reveals chlorine [1]; doing litmus first keeps that evidence available before the sample is used up [1]; a sample that has been burnt cannot be retested [1].
(d) [3] The single identifying result for each gas the sample was not: hydrogen would give a squeaky pop with a lighted splint [1]; oxygen would relight a glowing splint [1]; carbon dioxide would turn limewater milky [1].
(e) [2] A lighted splint would keep burning in any gas that supports combustion, so it could not tell those gases apart [1]; only oxygen will relight a splint that is merely glowing, so using a glowing splint gives an unambiguous positive test for oxygen [1].
(f) [2] Precaution when testing for chlorine: work in a fume cupboard, or keep the tube stoppered and do not inhale the gas [1]; the reason is that chlorine is toxic and harmful to breathe in [1].
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