In coagulation, once zeta potential forces are reduced below the ______________, the particles in suspension will start to coalesce?

• A. Molecular forces
• B. Ionic forces
• C. Covalent forces
• D. Van der Waals forces

Answer: (D)

In a suspension, stability comes from electrostatic repulsion between particles created by the electric double layer; zeta potential measures how strongly those particles repel each other. When the magnitude of that zeta potential drops below a certain level, the electrostatic barrier weakens enough that attractive forces between particles can take over. The main attractive force at play here is van der Waals forces, which are short-range attractions present between nearby particles. Once repulsion is reduced, these van der Waals attractions pull particles together, causing coalescence and aggregation.

Other options aren’t the driving mechanism in this context: molecular forces are too general to pinpoint the inter-particle attraction responsible for coagulation; ionic forces describe the electrostatic part of the interaction rather than the attractive pull that dominates after destabilization; covalent forces are strong chemical bonds that don’t normally govern physical coagulation in suspensions.