Question

Purple of cassius is a/an:
A. colloidal sol of gold
B. colloidal sol of silver
C. colloidal sol of platinum
D. oxyacid of gold

Answer 1

‌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.
$SnC{l_2}(in\,water) \to S{n^{2 + }} + 2C{l^{ - 1}}$
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 an indicator and doesn'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.
Hence, the correct option is A. colloidal sol of gold.

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