Question

Four salt solutions containing cations A, B, C and D were taken in different test tubes. Caustic soda was added to each one of them and the following observations were made:

Salts containing cations	Observation on adding caustic soda
A	White chalky ppt. soluble in excess
B	White ppt. insoluble in excess
C	Blue ppt. insoluble in excess
D	Reddish brown ppt. insoluble in excess

Identify A, B, C and D.

Answer 1

Cation	Identification
A	A is zinc ion (Zn2+) because, on adding caustic soda (NaOH), it forms a white chalky precipitate of zinc hydroxide, Zn(OH)2, which dissolves in excess NaOH.
B	B is a magnesium ion (Mg2+) because, on adding caustic soda (NaOH), it forms a white precipitate of magnesium hydroxide, Mg(OH)2, which is insoluble in excess NaOH.
C	C is a copper(II) ion (Cu2+) because, on adding caustic soda (NaOH), it forms a blue precipitate of copper(II) hydroxide, Cu(OH)2, which is insoluble in excess NaOH.
D	D is ferric ion (Fe3+) because, on adding caustic soda (NaOH), it forms a reddish-brown precipitate of ferric hydroxide, Fe(OH)3, which is insoluble in excess NaOH.