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Why don't particles of a colloid settle but those of a suspension do? - Is it because they are really small and so remain in constant motion which overcomes the force of gravity and prevents them from settling? - Is it because they are charged and so they repel each other and stay suspended in the solution as they try to go away from each other?

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Particles in colloid absorb either positive or negative ion preferentially from solution depending on there surface. This develops a potential on colloid surface called Zeta potential. Due to this potential colloid particles repel each other and can't coagulate.

Particle size also play a major role. With decreasing size surface to volume and hence surface to weigh ratio increases. So, same volume of colloid particles have higher surface area than suspension. So, colloid surface have enough charge to keep them apart whereas suspension don't have.

Conclusion Both the point you've mentioned is responsible for prevention of colloid coagulation.

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