Question

Purple of cassius is a/an:
(A) Colloidal sol of gold.
(B) Colloidal sol of platinum
(C) Colloidal sol of silver.
(D) Oxyacid of gold.

Answer 1

Hint: The colloidal sol of gold, silver and platinum are formed by the reaction with tin(II) chloride also known as the stannous chloride. Purple of cassius is a purple coloured pigment.

Complete step by step answer:

• We know, purple of cassius is a purple coloured substance formed by the reaction of salts with tin(II) chloride. The salts used are the colloidal solutions.
• If we talk about the tin(II) chloride, it acts as a reducing agent.

SnCl$_2$ (in water) $\rightarrow$ Sn$^{2+}$ + 2Cl$^{-1}$

• When tin(II) chloride reacts with silver salt, it forms a shiny surface, and leads to the formation of mirrors.
• Now, if we talk about the reaction of platinum salt with tin(II) chloride, it is used as indicator and don’t represent much effective reaction
• Next is the reaction of gold salt with the tin(II) chloride. Gold is dissolved in the aqua medium and it further reduces to give a purple precipitate.
  Therefore, purple of cassius is a colloidal sol of gold.
  So, the correct option is A.

Additional information:
Purple of cassius is used as an indicator in the chemical test performed in the laboratory, and also imparts red colour to the glass.

Note: Don’t be confused between the different salts. Just remember the reactions, and the product, and their colour formed by the various salts while reacting with the tin(II) chloride.