Properties of colloids
Colloids compared with suspensions: they pass unchanged through filter paper, scatter light in the Tyndall effect, and coagulate when an electrolyte is added.
The science behind this experiment
Colloidal solutions pass unchanged through filter paper, quite the opposite of suspensions. Colloids also diffract light in a particular way called the Tyndall effect, and can be coagulated by electrolytes to give precipitates which can then be filtered.
Colloids collect electric charges on the surface of their particles and the repulsion of these charges, besides solvation, is the cause of the stability of the solutions.
The Tyndall effect and flocculation or coagulation due to the addition of electrolytes are other important properties.
Equipment used
- Test tubes — PLA-007
- Test tube holder — PLA-017
- Spatula double end — PLA-024
- Spirit burner — CHE-022
- Tripod stand — HEA-014
- Wire gauze — HEA-015
- Beaker 50 ml — CHE-002
- Glass rod — PLA-004
Chemicals and reagents
- Sulphur — R:11-16 S:41
- Sodium chloride
- Ethanol — R:11 S:7-16
- Distilled water
- Milk
Safety notes
- Take care when using the burner and hot glassware.
Questions for students
- Record your observations. What can you say about the two solutions? What is the name of this phenomenon?
- What happens to the colloids on adding sodium chloride and after boiling?
- What can you say about the diluted milk solution?
What to expect
The light beam passes through the solution of sodium chloride without leaving any trace. The light ray diffracts light in its path through the colloidal solution, creating a cone effect. This phenomenon is called the Tyndall effect, i.e. diffraction of light due to the presence of particles which interact with light. Adding sodium chloride and boiling the solution causes coagulation of particles and consequent loss of colloidal properties due to the formation of larger clusters. The diluted milk also shows the Tyndall effect because it contains droplets of fat/oil dispersed as a colloidal solution. In this case micelles are constituted of fat and protein particles which present a hydrophobic part and a hydrophilic part.
Disposal
Dispose of the solutions into the liquid wastes container.
Run this experiment in your classroom
This experiment is part of ATP Volta — a self-contained mobile science lab that turns an ordinary classroom into a working STEM laboratory, with over 200 experiments in physics, chemistry, biology, robotics and engineering. Gali, the AI tutor built into ATP Connect, guides students through each step and answers their questions at the bench.
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