(b) In the 1850s, Michael Faraday prepared ruby-red
colloids of gold nanoparticles in water that are still stable
today. These brightly colored colloids look like solutions.
What experiment(s) could you do to determine whether a
given colored preparation is a solution or colloid?

Verified step by step guidance

1. Understand the difference between a solution and a colloid: Solutions are homogeneous mixtures where the solute is completely dissolved, while colloids have larger particles that are dispersed but not dissolved.
2. Consider the Tyndall effect: Colloids scatter light due to the larger size of their particles, while solutions do not. Shine a beam of light through the sample and observe if the light is scattered.
3. Perform a filtration test: Solutions will pass through a filter without leaving any residue, while colloids may leave some particles behind due to their larger size.
4. Use a centrifuge: Centrifugation can separate colloidal particles from the dispersion medium, while solutions will remain uniform as the solute is dissolved at the molecular level.
5. Observe the stability over time: Colloids are stable and do not settle out upon standing, whereas suspensions (not true solutions) may settle over time.

Key Concepts

Colloids vs. Solutions

Colloids are mixtures where tiny particles are dispersed throughout a medium, but they do not settle out over time, unlike solutions where solute particles are completely dissolved. In colloids, the particle size is typically between 1 nanometer and 1 micrometer, which gives them unique optical properties, such as scattering light, leading to their characteristic colors.

Tyndall Effect

The Tyndall Effect is the scattering of light by particles in a colloid or in very fine suspensions. This phenomenon can be used to distinguish between a colloid and a true solution; if a beam of light passes through the mixture and is visible, it indicates the presence of colloidal particles, whereas a true solution would not scatter light and would appear clear.

Stability of Colloids

Colloids can remain stable over time due to the balance of attractive and repulsive forces between the dispersed particles. Factors such as particle size, charge, and the presence of stabilizing agents (like surfactants) can affect this stability. Understanding these factors is crucial for designing experiments to test the nature of the preparation, as stability can influence the behavior of the colloid.