Correct Answer – D
Different electolytes have different coagulation values.The coagulation behaviour of various elecrolytes was studied in detail by Hardy and Schulze. They found that
`(1)` the ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol.
`(2)` coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the ions causing coagulation.
Thus,for the coagulation of sols carrying negative charge (like `As_(2)S_(3)` Sol), `Fe^(3+)` ions are more effective than `Ba^(2+)` or `Na^(+)` ions .Similarly, for the coagulation of sols carrying positive charge (such as `Fe(OH)_(3)` sol), `PO_(4)^(3-)` ions are more efficient than `SO_(4)^(2-)` or `Cl^(-)` ions.
The two observations given above are collectively called Hardy-Schulze rule.
These observations show that utmost care should be taken to keep sols away from environment containing even small amount of elecrolytes.

Correct Answer – D