What it shows
A torch beam is invisible in clear salt water but shows up as a bright path in dilute milk or starch water. The particles of a colloid are big enough to scatter light, and those of a true solution are not.
The science
In a true solution such as salt in water, the particles are ions or molecules smaller than 1 nm and do not scatter visible light. Colloids such as milk and starch water contain particles of 1 to 1000 nm which scatter light, and the beam becomes visible: the Tyndall effect. Suspensions such as chalk or mud in water have larger particles that scatter light too, but they settle on standing and can be filtered, which colloids cannot.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A bright torch or phone light
- Five clear glasses
- Salt, sugar, milk, starch (cornflour), chalk powder or mud
- Water
- A cardboard sheet with a small hole
- A dark room
- A notebook
Safety
For this project: Do not shine the torch into anyone’s eyes. Do not taste the mixtures. Wipe up spills.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make five liquids: a spoon of salt in a glass of water, a spoon of sugar in water, a few drops of milk in water, half a spoon of cornflour boiled in water and cooled, and a pinch of chalk or mud stirred in water.
Darken the room and cover the torch front with the card with a small hole, to make a narrow beam.
Shine the beam through the side of each glass and look from the side, not from the front. Note whether the beam is visible inside the liquid.
Write in a table which glasses show a visible beam and which show none.
Leave all five glasses for 30 minutes and note which one has settled to the bottom. This is the suspension.
Pour the chalk water and the milk water through filter paper, and see which liquid becomes clear.
Shine the torch in a smoky or foggy setting such as a steamy bathroom door or sunbeam through dusty air, and note that the same effect makes sunbeams visible.
Classify each of the five liquids as solution, colloid or suspension in a table with the reason.
What you should see
The salt and sugar solutions show no beam, the milk and starch show a clear beam, the chalk water shows a beam but settles and is filtered out.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the Tyndall effect?
The scattering of light by particles in a colloid, making the beam visible.
2Why does a salt solution not scatter light?
Its particles are too small.
3How do you tell a suspension from a colloid?
A suspension settles and can be filtered; a colloid does not.
4Where do we see this in daily life?
Sunbeams in a dusty room and headlight beams in fog.
Take it further
Add a few drops of lemon juice to the milk water and see what happens to the beam as the proteins clump.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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