Four solutions were added to separate test-tubes of aqueous sodium carbonate and the observations were recorded.
Which of the following observations is incorrect?
A)add: barium chloride | observation: white ppt formed
B)add: copper(II) sulfate | observation: blue ppt formed
C)add: dilute hydrochloric acid | observation: effervescence produced
D)add: sodium hydroxide | observation: white ppt formed
Answer: D
- D is incorrect as no ppt can be formed. Recall: all sodium salts are soluble.
- A is correct → white ppt is barium carbonate.
- C is correct → carbon dioxide is formed.
Common question: why is B correct? Shouldn't copper(II) carbonate be green?
Explanation: In aqueous solutions, carbonate ions can react with water to produce hydroxide ions; they are weak alkalis.
CO32– + H2O → HCO3– + OH– So essentially, copper(II) ions will react with the hydroxide ions to form copper(II) hydroxide which is a blue ppt.
Further reading: why is copper(II) hydroxide formed instead of copper(II) carbonate since both are insoluble?
Ans: copper(II) hydroxide (Ksp ~ 10–20) is more insoluble than copper(II) carbonate (Ksp ~ 10–10) so it is preferentially formed. The smaller the Ksp value, the more insoluble a salt is.
