What it shows
The coloured coating on candies such as Gems is often a mixture of several food dyes. Water climbing up filter paper carries the dyes at different speeds, so a single dot of colour spreads into separate bands and shows what the mixture contains.
The science
Chromatography separates a mixture because its parts travel at different speeds. Each dye sticks to the paper fibres a different amount and dissolves in water a different amount. A dye that dissolves well and sticks weakly rides the water front and goes far; a dye that clings to the paper lags behind. Dyes that end up at the same height on two strips are probably the same dye.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One packet of coloured sugar-coated candies
- Coffee filter paper or white blotting paper cut into 2 cm by 10 cm strips
- A glass with 1 cm of water
- A pencil and a clip or tape
- A dropper or a toothpick
- A white plate
Safety
For this project: Do not eat candies that have been handled on the plate; use a fresh one if you want to eat.
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.
Put one candy on the plate with a few drops of water on it for a minute so the colour washes off into a small puddle.
Draw a pencil line 2 cm from the bottom of a strip; pencil does not dissolve, ink would.
Dab a dot of the coloured puddle on the pencil line with the toothpick, let it dry and dab again three times to make it strong.
Clip the strip to the pencil and rest the pencil across the glass so the strip tip sits in the water but the dot stays above it.
Watch the water climb; as it passes the dot the colours start to spread upward.
Take the strip out when the water is 1 cm from the top and mark the water front with the pencil.
Repeat for every colour of candy and dry all the strips side by side.
Note which colours split into two or more bands, and which bands match across different candies.
What you should see
Green and brown coatings usually split into blue and yellow or red bands, while some colours stay as one band. The dried strips make a colourful chart showing which dyes the factory mixed.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why do the colours separate?
Each dye moves up the paper at its own speed, depending on how well it dissolves and how much it sticks to the paper.
2Why draw the start line in pencil?
Pencil does not dissolve in water, so it will not run and spoil the test.
3What does it mean if green splits into blue and yellow?
The green coating is a mixture of a blue dye and a yellow dye.
4Where is chromatography used?
In laboratories to check food colours, medicines and even inks in forged documents.
Take it further
Try the same test with salt water instead of plain water and see whether the bands move differently.
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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