(a) Do colloids made only of gases exist? Why or why not? (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?

Colloids are a type of mixture that consists of two or more substances uniformly distributed throughout a powdery or granular substance or a liquid, but in which the individual particles do not dissolve, and instead are merely suspended (unlike solutions where the solute is dissolved). The size of particles in a colloid generally ranges between 1 nm and 1000 nm. The substance that is dispersed in a colloid is called the dispersed phase, and the medium in which it is dispersed is called the dispersion medium.

Colloids made only of gases do not exist because, in a gas, the particles are already in a state of continuous, random motion with large spaces between them. As a result, gas particles freely mix with each other, rather than forming a colloid in which particles remain suspended without dissolving. In a colloid made up of gas particles only, there would be no distinction between the dispersed phase and the dispersion medium because the particles would simply mix together instead of forming a suspension.

To determine whether a given colored preparation is a solution or a colloid, one can perform the following experiments: 1. Tyndall effect: Shine a beam of light through the colored preparation and observe if the light path is visible or dispersed. In a colloid, the light path will be visible due to the scattering of light by the suspended particles (this is called the Tyndall effect). In a solution, the light path will not be visible, as the particles are dissolved and do not scatter light. 2. Filtration: Filter the colored preparation through a suitable filter paper. If the colored substance passes through the filter paper, it is a solution, as the dissolved particles are too small to be trapped in the filter. If the colored substance is retained by the filter paper, then it is a colloid, as the particles are larger and can be trapped in the filter. 3. Sedimentation and centrifugation: Allow the colored preparation to stand undisturbed for a suitable period of time. If the particles settle at the bottom, then it is a colloid. For faster results, centrifuge the sample. If particles are separated by centrifugation, it is a colloid. However, if the particles remain uniformly distributed and do not settle or separate, then it is a solution.