A student is given a white solid that may be a carbonate. Plan an experiment, using dilute hydrochloric acid, to show that the solid is a carbonate by colle...
Question 1 Report
A student is given a white solid that may be a carbonate. Plan an experiment, using dilute hydrochloric acid, to show that the solid is a carbonate by collecting and testing the gas that is given off.
(a) Describe the method the student would use, naming the apparatus used to add the acid and to collect and test the gas. [4]
(b) Name the gas produced and state the result of the test that confirms it. [2]
(c) State one safety precaution the student should take during this experiment. [1]
Answer Details
This tests planning a gas-collection experiment and the confirmatory test for a carbonate. Carbonates react with acids to give off carbon dioxide, which turns limewater milky.
(a) A clear method scores up to four marks: [4]
- Place a portion of the white solid in a test-tube. [1]
- Add dilute hydrochloric acid from a dropping pipette (or measuring cylinder). [1]
- Fit a bung carrying a delivery tube that leads into a second test-tube containing limewater. [1]
- Bubble the gas given off through the limewater and watch for a change. [1]
(b) The gas produced is carbon dioxide. [1] The positive result is that the limewater turns milky (cloudy white), because carbon dioxide forms insoluble calcium carbonate with the calcium hydroxide in limewater. [1] The reaction of the solid with acid is \[\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{H}_2\text{O} + \text{CO}_2\]
(c) One safety precaution, for example wear eye protection / safety goggles, or add the acid carefully to avoid splashing. [1]
Exam tip: the milky limewater test confirms carbon dioxide, and effervescence with acid is the general sign of a carbonate.
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