Dilute sodium hydroxide solution test for metal and ammonium ions

Question 1

When is Dilute sodium hydroxide solution used?

Answer 1

Dilute sodium hydroxide solution is used in tests for some metal ions, and is a test to identify ammonium ions in compounds.

Question 2

When do we get a white precipitate?

Answer 2

If Calcium, Alluminium or Magnesium are present

Question 3

When do we get a blue precipitate?

Answer 3

Copper, Cu2+

Question 4

When do we get a green precipitate?

Answer 4

Iron(II), Fe2+ Green

Question 5

When do we get a brown precipitate?

Answer 5

Iron(III), Fe3+ Brown

Question 6

How do you distinguish between aluminium ions and calcium ions since both form a white precipitate in the test?

Answer 6

If excess sodium hydroxide solution is added:

• the aluminium hydroxide precipitate reacts to form a colourless solution
• the calcium hydroxide precipitate is unchanged

Question 7

A green precipitate forms when dilute sodium hydroxide solution is added to a solution containing a metal ion. Identify the metal ion present.

Answer 7

Iron(II) ions, Fe2+, are present.

Question 8

How to test for ammonium ions

Answer 8

Ammonium ions, NH4+, react with hydroxide ions to form ammonia and water:

NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)

This gives a test for ammonium ions:

-add dilute sodium hydroxide solution to the sample
-warm the mixture
-Ammonia gas is given off if ammonium ions are present.

Testing for ammonia
Ammonia turns damp red litmus paper blue.

Question 9

Explain what the test for ammonia tells you about the properties of ammonia.

Answer 9

Ammonia dissolves in water to form an alkaline solution, because it turns damp red litmus paper blue.