NaCl is an electrolyte. it decomposes into Na+ and Cl-. These attack the charged colloidal particles and thus discharge them. Thecolloidal particles then coagulate.
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When NaCl is added to ferric oxide sol, it dissociates to give Na+ and Cl- ions. Particles of ferric oxide sol are positively charged. Thus, they get coagulated in the presence of negatively charged Cl- ion