Question

What stabilizes a colloidal suspension? Explain why adding heat or adding an electrolyte can cause the suspended particles to settle out.

Short answer

A colloidal suspension is stabilized by the balance of attractive Van der Waals forces and repulsive electrostatic forces between the dispersed particles. Adding heat increases the Brownian motion of particles, which can overcome the repulsive forces and cause aggregation and settling. Adding an electrolyte introduces ions that can shield or neutralize the charges of the particles, reducing repulsive forces and promoting aggregation and settling.

Step-by-step solution

1. Definition of a colloidal suspension

A colloidal suspension is a mixture in which one substance, typically in the form of small particles or droplets, is dispersed throughout another substance. The particles in a colloidal suspension are generally larger than individual molecules but smaller than the particles found in a conventional suspension. The dispersed particles are called the dispersed phase, while the substance in which they are dispersed is called the continuous phase.

2. Forces that stabilize colloidal suspensions

A colloidal suspension is stabilized by a balance of attractive and repulsive forces between the dispersed particles. The attractive forces are primarily Van der Waals forces, which arise from the slight imbalance of positive and negative charges within the particles, causing them to be weakly attracted to each other. The repulsive forces are often due to electrostatic repulsion, where the particles develop electric charges that cause them to repel each other. This electrostatic repulsion can be due to the adsorption of charged species on the particle surfaces or the dissociation of surface groups, creating charged particles. The balance of attractive and repulsive forces prevents the particles from aggregating and settling out of the suspension.

3. Effect of adding heat on colloidal suspension stability

Adding heat to a colloidal suspension can disrupt the balance of forces that stabilize the suspension. As the temperature increases, the Brownian motion of the particles also increases. Brownian motion is the random movement of particles caused by the constant collisions with the surrounding molecules. This increased motion can overcome the repulsive electrostatic forces between the particles, causing them to aggregate and eventually settle out of the suspension.

4. Effect of adding an electrolyte on colloidal suspension stability

Adding an electrolyte to a colloidal suspension can also destabilize the suspension. Electrolytes are substances that dissociate into ions when dissolved in a solvent, such as water. When an electrolyte is added to a colloidal suspension, the ions can interact with the charged particles of the dispersed phase, shielding or neutralizing their charges. This reduces the repulsive electrostatic forces between the particles, allowing the attractive Van der Waals forces to dominate and cause aggregation. As the particles aggregate, they become larger and can eventually settle out of the suspension. In conclusion, a colloidal suspension is stabilized by a balance of attractive and repulsive forces between the dispersed particles. Adding heat or an electrolyte can disrupt this balance, causing the particles to aggregate and eventually settle out of the suspension.