Among the following, the false statement is:

We begin by recalling a basic fact from the study of colloids: every colloidal particle present in a sol carries an electric charge, and this charge is the reason why identical particles repel each other and the sol remains stable. The sign of this charge (whether it is positive, $$+$$, or negative, $$-$$) depends upon the nature of the dispersed phase and the dispersion medium.

Natural rubber latex is a familiar lyophobic colloid in which tiny particles of rubber (the dispersed phase) are distributed uniformly in water or a weak aqueous medium (the dispersion medium). Experimental measurement of the electrophoretic mobility of these rubber particles shows that they move toward the positive electrode (the anode) when an external electric field is applied. Because like charges repel and unlike charges attract, a particle that drifts toward the anode must be negatively charged. Hence, rubber particles in latex carry a negative charge, symbolically written as $$\text{rubber}^{-}$$.

Option A states, in words, “Latex is a colloidal solution of rubber particles which are positively charged.” We have just reasoned that the charge is actually negative, so Option A is not correct, i.e., it is false.

Now let us briefly examine the remaining statements to confirm that they are true.

Option B: Tyndall effect is the scattering of light by colloidal particles. A true solution has particles of molecular size (roughly $$10^{-10}\,\text{m}$$) which do not scatter light appreciably, whereas a colloidal particle (size $$10^{-7}\,\text{m}$$ to $$10^{-9}\,\text{m}$$) scatters light and produces a visible beam. Therefore this effect indeed allows us to distinguish a colloidal solution from a true solution. Thus, Option B is true.

Option C: Clouds consist of colloidally dispersed water droplets that generally carry an electric charge, usually negative. If sand particles with a large surface area are first given a positive charge and then sprayed into the cloud from an aeroplane, electrostatic attraction occurs between the oppositely charged species, leading to coagulation of the water droplets. Coagulation forms larger droplets which then fall as rain. This technique of “artificial rain” is a practical application and is based precisely on charge neutralisation, so Option C is true.

Option D: Lyophilic sols are intrinsically more stable than lyophobic sols because solvent molecules form an adsorption layer around each colloidal particle, providing an additional safeguard against coagulation. Nevertheless, if a sufficiently high concentration of a strong electrolyte is added, the ions of the electrolyte can screen or neutralise the charge on the sol particles and also disturb the solvation layer. Once this protective shield is lost, particles can aggregate and the sol coagulates. Therefore, the statement that “Lyophilic sols can be coagulated by adding an electrolyte” is indeed correct, i.e., Option D is true.

Because Options B, C, and D are all true, but Option A contradicts the established negative charge on rubber latex particles, Option A is the only false statement.

Hence, the correct answer is Option A.