What is the primary test to verify whether a solution is in colloidal form or not?

Surface tension of lyophilic sols is

The protecting power of lyophilic colloidal sol is expressed in terms of

Assertion : The micelle formed by sodium stearate in water has $-COO^{-}$ groups at the surface.
Reason : Surface tension of water is reduced by the addition of stearate.

Equal volumes of two $AgI$ sols are mixed together. The first sol is obtained by adding $AgNO_3$ to a slight excess of $KI$,and the second sol is obtained by adding $KI$ to a slight excess of $AgNO_3$. What happens when they are mixed?

$A$ micelle formed during the cleansing action by soap is