Problem 106 - Chemistry An Atoms First Approach

Question

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

Step-by-Step Solution

Answer

A colloidal suspension is stabilized by Brownian motion and electrostatic repulsion. Brownian motion prevents particles from settling due to gravity, while electrostatic repulsion prevents aggregation. Adding heat increases the system's temperature and kinetic energy, which can overpower the stabilizing forces, leading to destabilization and particle settling. Similarly, adding an electrolyte introduces ions that can reduce the surface charges on particles, weakening electrostatic repulsion and causing particles to aggregate and settle out.

Step 1: Define colloidal suspension

A colloidal suspension is a heterogeneous mixture in which small particles (typically in the size range of 1 nm - 1 µm) are dispersed throughout a continuous medium, such as a liquid or gas. The particles are too small to be seen with the naked eye and usually do not settle out of the mixture because they are kept in suspension by various forces, such as Brownian motion and electrostatic repulsion.

Step 2: Describe forces that stabilize a colloidal suspension

There are two primary forces which stabilize a colloidal suspension: 1. Brownian motion: This is the random motion of particles within the suspension, caused by collisions with molecules in the surrounding medium. Brownian motion keeps the colloidal particles in constant motion, preventing them from settling due to gravity. 2. Electrostatic repulsion: Colloidal particles often have an electric charge on their surface, either due to adsorption of charged species or dissociation of surface groups. These charges create a repulsive force between particles, preventing them from coming too close to each other and aggregating. These two forces work together to maintain the stability of the colloidal suspension and prevent the particles from settling or clumping together.

Step 3: Explain the effect of adding heat on colloidal suspension

Adding heat to a colloidal suspension increases the temperature of the system, which results in an increase in the kinetic energy of the particles and the surrounding medium. This increased kinetic energy causes the particles to move faster, thus increasing Brownian motion. However, as the temperature increases, so does the thermal energy, which can overpower the stabilizing forces. When Brownian motion can no longer counteract the particles' tendency to aggregate, the colloidal suspension may destabilize, allowing particles to settle out or clump together.

Step 4: Explain the effect of adding an electrolyte on colloidal suspension

Adding an electrolyte to a colloidal suspension introduces ions into the system. These ions can adsorb onto the surface of the colloidal particles, reducing or even neutralizing the surface charges on the particles. As a result, the electrostatic repulsion between particles is weakened or eliminated. Without sufficient repulsive forces, the particles can come closer together and aggregate, eventually leading to destabilization and settling out of the suspension. In conclusion, a colloidal suspension is stabilized by a combination of Brownian motion and electrostatic repulsion. Adding heat or an electrolyte can destabilize the suspension by overpowering the stabilizing forces, ultimately causing the suspended particles to settle out.