Identifying transition metal ions

To identify Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺ ions:

1. Take three test tubes and add equal amounts of unknown metal ion solution.
2. Add sodium hydroxide solution dropwise to the first test tube. Observe if a precipitate forms. Keep adding sodium hydroxide until excess is reached. Note any precipitate colour changes and if anything dissolves.
3. Repeat step 2 using ammonia solution. Note any observed precipitates or colour changes in excess.
4. Add sodium carbonate solution dropwise to the final test tube. Observe any precipitate formation and evolution of gas bubbles.
5. Compare your observations of precipitates, colour changes and gas formation to the known reactions of Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺ ions summarised in the table below.

*goes brown standing in air as [Fe(H₂O)₄(OH)₂] is oxidised to [Fe(H₂O)₃(OH)₃].

Identifying group 2 ions

Group 2 ions can be identified through precipitation reactions with sodium hydroxide and sulfuric acid solutions. Additionally, group 2 ions can be distinguished by their characteristic flame test colours.

Precipitation reactions

To confirm the presence of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions:

1. Take a clean test tube and add sodium hydroxide solution dropwise to a sample containing an unknown group 2 ion.
2. Observe if a white precipitate forms initially. Continue adding sodium hydroxide until an excess amount has been added.
3. Note if the white precipitate remains or disappears with excess addition.
4. Repeat steps 1-3 using sulfuric acid.
5. Compare your observations to the known reactions of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ summarised in the table below.

Flame tests

To confirm the presence of Ca²⁺, Sr²⁺, and Ba²⁺ ions:

1. Dip a clean nichrome wire loop into a sample of the unknown ion solution. Record the flame colour observed when placing the loop into a Bunsen burner.
2. Compare observations to the known reactions and flame test colours below to identify the group 2 ion.

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.