A student added dilute sulfuric acid, a portion at a time, to green copper(II) carbonate in a beaker in order to prepare copper(II) sulfate. She recorded he...
Question 1 Report
A student added dilute sulfuric acid, a portion at a time, to green copper(II) carbonate in a beaker in order to prepare copper(II) sulfate. She recorded her observations in a results table. Table 2.1 shows the table with some observations missing.
Table 2.1
| Stage of the experiment | Observation |
|---|---|
| dilute sulfuric acid added to the green solid | |
| gas bubbled through limewater | |
| green solid added until no more reacts |
(a) Complete the table by writing the missing observation at each stage. [3]
(b) Name the gas that is produced and give the result of the test on the gas. [2]
(c) Explain why the student adds the green solid until it is in excess. [1]
Answer Details
This question tests the observations when a metal carbonate reacts with an acid to make a salt, and the test for the gas produced.
(a) The completed table is:
| Stage of the experiment | Observation |
|---|---|
| dilute sulfuric acid added to the green solid | effervescence (bubbles); the green solid dissolves to give a blue solution |
| gas bubbled through limewater | the limewater turns milky / cloudy |
| green solid added until no more reacts | fizzing stops and some green solid stays undissolved |
Acid on the carbonate gives effervescence and a blue solution [1]; the gas turns limewater milky [1]; in excess, the fizzing stops and solid remains [1]. [3]
(b) The gas is carbon dioxide [1]; it turns limewater milky [1]. This is the standard test for \(CO_2\), where \(CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O\), the white calcium carbonate causing the cloudiness. [2]
(c) The green solid is added in excess to make sure all of the acid has reacted [1], so no acid is left to contaminate the salt. The leftover solid is later removed by filtering.
Exam tip: "blue solution" identifies a copper(II) salt, and "limewater turns milky" identifies carbon dioxide, so a carbonate is present.
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