Identifying carbonate and hydrogencarbonate ions

To confirm the presence of carbonate (CO₃^2-) or hydrogencarbonate (HCO₃^-) ions:

1. Add dilute nitric acid (HNO₃) dropwise to a sample containing an unknown ion.
2. Observe carefully for effervescence caused by bubbles of carbon dioxide (CO₂) gas being evolved: CO₃^2-_(aq) + 2H⁺_(aq) ➔ CO_2(g) + H₂O_(l)

HCO₃^-_(aq) + H⁺_(aq) ➔ CO_2(g) + H₂O_(l)

Confirm the presence of CO₂ by passing bubbles of the gas through limewater (calcium hydroxide solution), in a connecting test tube. If CO₂ is present, the limewater will turn cloudy due to the formation of a white calcium carbonate precipitate, confirming the original presence of carbonate ions in the sample:

CO_2(g) + Ca(OH)_2(aq) ➔ CaCO_3(s) + H₂O_(l)

Identifying sulfate ions

To confirm the presence of sulfate (SO₄^2-) ions:

1. Add dilute nitric acid (HNO₃) dropwise to a sample containing an unknown ion.
2. Add a few drops of a solution containing aqueous Ba²⁺ ions, such as barium chloride (BaCl₂) or barium nitrate (Ba(NO₃)₂), to the sample.
3. Observe carefully for the formation of a white precipitate of barium sulfate (BaSO₄): Ba²⁺_(aq) + SO₄^2-_(aq) ➔ BaSO_4(s)

Identifying halide ions

To distinguish between chloride, bromide, and iodide ions:

1. Add dilute nitric acid (HNO₃) dropwise to a sample containing an unknown ion.

2. Add silver nitrate solution (AgNO₃) dropwise.

3. Observe the colour of any precipitate formed and compare to the known precipitate colours of silver halides:

4. Take separate portions of the silver halide precipitate and add to test tubes containing dilute ammonia solution (NH₃) and concentrated ammonia solution (NH₃).

5. Observe solubility of precipitate in each ammonia solution. Compare observations to the known solubilities of silver halides in dilute and concentrated NH₃ summarised below:

6. The solubility tests in ammonia solutions can definitively confirm the identity of the halide ion originally present by distinguishing between the silver halides.

Identifying ammonium ions

To confirm the presence of ammonium (NH₄⁺) ions:

1. Add sodium hydroxide solution dropwise to a sample containing ammonium ions. Gently warm the mixture. The hydroxide ions react with the ammonium ions to produce ammonia gas: OH^-_(aq) + NH₄⁺_(aq) ➔ NH_3(g) + H₂O_(l)

2. Moisten a piece of red litmus paper and hold over the test tube. The paper will turn blue if ammonia gas is present, thereby confirming that ammonium ions were originally present in the sample.