What it shows
Each substance makes crystals of its own fixed shape: salt forms cubes, sugar forms slanted blocks, and alum forms pointed double pyramids. Looking at grains with a magnifier shows that crystals are not random lumps but neat, repeating shapes.
The science
In a crystal the tiny particles are packed in a repeating pattern, and the pattern decides the outer shape. Salt particles pack in a cube pattern, so even the smallest grain is a tiny cube. Sugar molecules are a different shape and pack differently, so sugar crystals look like tilted blocks. Because the pattern is always the same for a substance, crystal shape can help identify it.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A pinch each of table salt, sugar, alum powder and washing soda
- A magnifying glass or a mobile phone camera on zoom
- Black paper or a dark plate
- A toothpick for moving grains
- Pencil and paper for drawings
Safety
For this project: Keep the grains away from the mouth and eyes; washing soda is not for eating. Wash hands after the activity.
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
7 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.
Sprinkle a few grains of salt on the black paper and spread them out with the toothpick.
Look at the grains through the magnifier and turn the paper to catch the light on their flat faces.
Draw the shape of two or three grains, noting the square faces of the salt cubes.
Repeat with sugar grains and draw them; look for sloping, uneven faces.
Repeat with alum and washing soda, drawing each carefully.
Compare the drawings and write one sentence describing how each crystal differs.
Ask a family member to give you a mystery grain of either salt or sugar and identify it using only the magnifier.
What you should see
A set of drawings showing cube-shaped salt, slanted sugar blocks and pointed alum crystals, and the ability to tell salt from sugar by shape alone without tasting.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What shape are salt crystals?
Tiny cubes.
2Why do all salt grains have the same shape?
Because the particles inside always pack in the same repeating pattern.
3Can you tell salt from sugar without tasting?
Yes, by looking at the crystal shape under a magnifier.
4What happens to the shape when salt dissolves?
The crystal breaks apart into particles too small to see; the shape returns when the water evaporates.
Take it further
Dissolve a little salt in water, let a drop dry on a dark plate in the sun and look at the new cubes that form.
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.
Free ebooks for this project
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