You have been given a colourless liquid in a stoppered bottle, labelled only as liquid R. Find out whether liquid R holds a carbonate. Half fill a boiling t...
Question 1 Report
You have been given a colourless liquid in a stoppered bottle, labelled only as liquid R. Find out whether liquid R holds a carbonate.
Half fill a boiling tube with liquid R. Set up a second boiling tube holding limewater, and connect the two with a bung and delivery tube so that anything given off in the first passes into the limewater. Now pour dilute hydrochloric acid into the first boiling tube and replace the bung at once. Watch both boiling tubes for one minute.
| where you looked | observations |
|---|---|
| the boiling tube holding liquid R and the acid | |
| the boiling tube holding limewater |
(a) Enter what you saw in Table 12.1. [3]
(b) Deduce whether liquid R holds a carbonate, giving the evidence. [2]
(c) State why the bung is replaced immediately rather than after a few seconds. [1]
Answer Details
This tests the standard identification of a carbonate. A carbonate reacts with dilute acid to release carbon dioxide, \( \mathrm{CO_3^{2-} + 2H^+ \rightarrow H_2O + CO_2} \); the CO2 is then carried into limewater, which it turns milky. So effervescence in the first tube plus milky limewater in the second together prove a carbonate is present.
(a) [3] Record what you saw, one mark per correct observation:
| where you looked | observation |
|---|---|
| boiling tube with liquid R and acid | effervescence / bubbles of gas; the mixture fizzes and the liquid stays colourless [1][1] |
| boiling tube with limewater | turns milky / cloudy white [1] |
(b) [2] The deduction: yes, liquid R holds a carbonate [1], because the gas given off turned the limewater milky, which identifies carbon dioxide [1], and carbon dioxide from acid is the signature of a carbonate. (A fully consistent "no", supported by no effervescence and clear limewater, would also be accepted.)
(c) [1] The bung is replaced immediately so that the gas passes down the delivery tube into the limewater instead of escaping into the room [1]. A few seconds' delay lets the first, most concentrated burst of CO2 escape, so the limewater might not turn milky and the test would fail.
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