Testing for carbonates (CO3^2-)
5 marks · HardYou are given a solid sample labelled 'unknown.' It could be one of the following: magnesium sulfate, sodium carbonate, or potassium chloride.
Describe how you would carry out a chemical test to determine whether this solid contains carbonate ions. Include the reagents and apparatus you would use.
Suppose the unknown is sodium carbonate. Write the ionic equation for its reaction with hydrochloric acid.
Explain how you would confirm that the gas produced in the reaction is carbon dioxide.
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.
Show answer
(a) — · must show Add dilute strong acid (e.g., HCl) to sample in test tube/beaker, State observe for bubbles/fizzing or gas produced
(b) \ce{CO3^{2-}(s) + 2H^+(aq) -> CO2(g) + H2O(l)} · must show Correct ions and states; double check charges and balancing
(c) — · must show Bubble gas through limewater, State limewater turns cloudy/milky for positive CO2
Common mistakes: Testing without naming reagents/apparatus; Missing ions or state symbols in equation; Not describing the limewater test
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Add dilute hydrochloric acid (or named suitable acid) to the solid sample.
M2 (a) · 1 mark · Observation: bubble(s)/fizzing/gas produced (accept synonyms, must link to addition of acid).
A1 (b) · 1 mark · Correct ionic equation with states:
M3 (c) · 1 mark · State pass (bubble) the gas through limewater (named or described as calcium hydroxide solution).
A2 (c) · 1 mark · State that limewater turns cloudy/milky if carbon dioxide is present.
Worked solution
a) Add a small amount of the unknown solid to a test tube. Add a few drops of dilute hydrochloric acid to the solid. If carbonate ions are present, bubbles of gas (carbon dioxide) will be seen.
b) The ionic equation is:
c) Collect the gas produced and bubble it through limewater (calcium hydroxide solution). If the limewater turns cloudy, it confirms the gas is carbon dioxide.