Testing for carbonates (CO3^2-)
4 marks · NormalA teacher places a small amount of white powder in a test tube and adds dilute nitric acid. Bubbles appear. To confirm the identity of the gas, another experiment is performed with limewater.
Write the formula for the ion being tested.
State the name and formula of the gas formed.
Describe the chemical test for this gas.
State the positive result for this test.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) CO3^{2-}
(b) Carbon dioxide, CO2
(c) Bubble the gas through limewater (calcium hydroxide solution)
(d) Limewater turns milky/cloudy
Must show: ion formula, gas identification, limewater test
Also accept: CO2 or carbon dioxide for the gas; limewater or Ca(OH)2(aq) for the test
Common mistakes: incorrect ion formula; hydrogen or oxygen as the gas
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct formula for carbonate ion: CO3^{2-}.
B1 (b) · 1 mark · Correctly states carbon dioxide and its formula (CO2).
B1 (c) · 1 mark · Describes limewater (calcium hydroxide) test.
B1 (d) · 1 mark · Positive result: limewater turns milky/cloudy.
Worked solution
a) The ion being tested is carbonate: CO3^{2-}.
b) The gas formed is carbon dioxide (CO2).
c) Bubble the gas through limewater.
d) The limewater turns milky to show CO2.