A suspension with a zeta potential of -35 mV is considered stable or unstable?

• A. Stable
• B. Unstable
• C. Both possible
• D. Not enough data

Answer: (A)

The main idea is that colloidal stability is driven by electrostatic repulsion between particles, quantified by zeta potential. The stronger the magnitude of the zeta potential, the more repulsion keeps particles apart and prevents aggregation. A common rule of thumb is that an absolute zeta potential around 30 mV or higher indicates a stable suspension. Here the zeta potential is -35 mV, and the magnitude is 35 mV, which exceeds the threshold. The negative sign just indicates the charge type; stability depends on how strongly charged the particles are, not the sign. So this suspension is considered stable under typical conditions. Remember that very high ionic strength or other stabilizers can modify stability, but with -35 mV, stability is expected.