Exp 6.1 C - to detect the presence of carbonate ions and hydrogencarbonate ions in aqueous solution

Question 1

Apparatus required (7)

Answer 1

boiling tube, test tube, one holed rubber stopper to fit boiling tube, delivery tubes, dropper, spatula, wash bottle

Question 2

chemicals required (6)

Answer 2

any soluble carbonate salt (e.g. Na2CO3, K2CO3) and any soluble hydrogencarbonate salt (e.g. NaHCo3, KHCO3), dilute HCl, limewater, magnesium sulfate solution, deionised water

Question 3

List the 8 steps required for this experiment

Answer 3

1. Place a small amount of the carbonate salt in a boiling tube to a depth of about 1cm.
2. Insert a one-holed rubber stopper into the boiling tube.
3. Remove the one holed rubber stopper, add 1/4 test tube of dilute HCl to the carbonate salt in the boiling tube and quickly replace the rubber stopper.
4. Observations: A brisk effervescence (fizzing) is observed as the acid comes in contact with the carbonate.
5. Bubble the gas through limewater. The gas given off turns the limewater milky.
6. Repeat the experiment using the hydrogencarbonate salt instead of the carbonate salt.
7. To distinguish between carbonate and hydrogencarbonate ions, add a small amount of the carbonate salt to a test tube 1/4 full of deionised water. Add 1/4 test tube of magnesium sulfate solution to the carbonate solution. A white precipitate is formed. No precipitate is formed in the hydrogencarbonate solution.
8. Repeat the above using hydrogencarbonate salt.

Question 4

Why is fizzing observed?
Why does the limewater turn milky?
Give the formulas for fizzing in both the carbonate and hydrogen-carbonate.

Answer 4

The fizzing is caused by the carbonate or hydrogencarbonate ions reacting with the acid to give off CO2.

carbonate: CO3^2- + 2H+ –> CO2 + H2O
Hydrogen carbonate: HCO3^- + H+ –> CO2 + H2O

The CO2 reacts with the limewater to produce calcium carbonate (chalk/limestone).

Question 5

What is the conclusion of the main experiment?

Answer 5

Both carbonate and hydrogencarbonate ions react with dilute HCl to give off CO2 gas.

Question 6

What are the 3 conclusions to distinguish between the two ions?

Answer 6

1. Carbonate and hydrogen-carbonate ions are distinguished from each other using magnesium sulfate solution.
2. Magnesium sulfate solution added to a solution containing carbonate ions gives a white precipitate. Magnesium sulfate solution added to a solution containing hydrogen-carbonate ions gives a clear solution.
3. Magnesium carbonate is insoluble in water but magnesium hydrogen-carbonate is soluble.

Question 7

Give the chemical equations for the formation of a precipitate and no precipitate with the magnesium sulfate solution

Answer 7

Mg2+ + CO3^2- –> MgCO3 ↓

Mg2+ + 2HCO3- —> Mg(HCO3)2

Question 8

How can a white precipitate be formed with the hydrogencarbonate solution and magnesium sulfate solution? Give the equation for this

Answer 8

The solution must be boiled

Mg(HCO3)2 –> MgCO3↓ + CO2 + H2O