tests for aqueous cations

Question 1

What is the effect of aqueous sodium hydroxide on aluminium (Al3+)?

Answer 1

White precipitate forms, which is soluble in excess giving a colourless solution

This indicates that aluminium ions can react with sodium hydroxide to form a soluble complex.

Question 2

What is the effect of aqueous ammonia on aluminium (Al3+)?

Answer 2

White precipitate forms, which is insoluble in excess

This shows that aluminium ions do not form a soluble complex with ammonia.

Question 3

What is the effect of aqueous sodium hydroxide on zinc (Zn2+)?

Answer 3

White precipitate forms, which is soluble in excess giving a colourless solution

Zinc ions behave similarly to aluminium in forming soluble complexes with sodium hydroxide.

Question 4

What is the effect of aqueous ammonia on zinc (Zn2+)?

Answer 4

White precipitate forms, which is soluble in excess giving a colourless solution

This indicates that zinc ions can also form soluble complexes with ammonia.

Question 5

What is the effect of aqueous sodium hydroxide on calcium (Ca2+)?

Answer 5

White precipitate forms, which is insoluble in excess

Calcium ions do not form soluble complexes with sodium hydroxide.

Question 6

What is the effect of aqueous ammonia on calcium (Ca2+)?

Answer 6

No precipitate or very slight white precipitate. If a slight precipitate forms it is insoluble in excess

This indicates calcium’s limited interaction with ammonia.

Question 7

What is the effect of aqueous sodium hydroxide on chromium (Cr3+)?

Answer 7

Green precipitate forms, which is soluble in excess

Chromium ions can form soluble complexes with sodium hydroxide.

Question 8

What is the effect of aqueous ammonia on chromium (Cr3+)?

Answer 8

Green precipitate forms, which is insoluble in excess

This shows that chromium ions do not form soluble complexes with ammonia.

Question 9

What is the effect of aqueous sodium hydroxide on copper (Cu2+)?

Answer 9

Light blue precipitate forms, which is insoluble in excess

Copper ions have a distinct behavior in precipitation reactions.

Question 10

What is the effect of aqueous ammonia on copper (Cu2+)?

Answer 10

Light blue precipitate forms, which is soluble in excess giving a dark blue solution

This indicates that copper ions form a soluble complex with ammonia.

Question 11

What is the effect of aqueous sodium hydroxide on iron(II) (Fe2+)?

Answer 11

Green precipitate forms, which is insoluble in excess. On standing for 5 minutes the surface turns brown

This indicates oxidation of Fe2+ to Fe3+.

Question 12

What is the effect of aqueous ammonia on iron(II) (Fe2+)?

Answer 12

Green precipitate forms, which is insoluble in excess. On standing for 5 minutes the surface turns brown as Fe3+ is formed

Similar behavior is observed as with sodium hydroxide.

Question 13

What is the effect of aqueous sodium hydroxide on iron(III) (Fe3+)?

Answer 13

Red-brown precipitate forms, which is insoluble in excess

Iron(III) ions produce a characteristic precipitate.

Question 14

What is the effect of aqueous ammonia on iron(III) (Fe3+)?

Answer 14

Red-brown precipitate forms, which is insoluble in excess

Iron(III) ions behave similarly with both reagents.

Question 15

What is produced when ammonium (NH4+) is warmed?

Answer 15

Ammonia gas produced on warming which turns damp red litmus blue

This indicates the basic nature of ammonia gas.