What it shows
Soap helps to lift dirt and oily grime off the skin, and rubbing for longer removes more of it. A harmless coloured powder stands in for germs so that you can see how much is removed in a given time.
The science
Soap molecules have one end that attracts water and one that attracts oil. They surround oily dirt and let rinsing water carry it away. Rubbing physically loosens the dirt. Time matters because soap needs time to reach all corners such as between fingers and under nails.
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.
- Washable coloured powder such as turmeric, cocoa or herbal gulal labelled skin-safe, mixed with a little oil
- Bar or liquid soap
- A tap with clean water or a mug and bucket
- A stopwatch
- A white towel or kitchen paper
- A camera or phone
Safety
For this project: Use skin-safe powder only (not rangoli or craft colours) and keep it away from eyes, mouth and cuts; anyone with skin allergies or sensitive skin should only observe. Wash off any leftover colour. An adult supervises. This shows cleanliness only and does not test for real germs.
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.
Step 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.
Mix powder with a few drops of oil and rub a thin coat on both palms, then dry for a minute.
Take a photo of the palms as the starting condition.
Wet the hands, apply soap and rub for exactly 5 seconds, then rinse for 5 seconds.
Photograph the palms against a white sheet and note how much colour is left.
Re-coat the hands and repeat with 10 seconds of rubbing, then 20 seconds.
Give each photo a score from 0 (clean) to 5 (very coloured) with your friends not knowing which time was used.
Make a table of times and scores and draw a bar graph.
Note which parts of the hand keep colour longest, such as thumbs, fingertips and between fingers.
What you should see
Less coloured powder remains with longer washing, and fingertips, thumbs and between the fingers keep colour longest, which tells you where to rub.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01How does soap remove oily dirt?
Its molecules surround oil droplets so that water can wash them away.
02Why is 20 seconds suggested for washing?
It gives soap and rubbing enough time to loosen dirt everywhere.
03What is the powder in this test?
A safe stand-in for dirt and germs, not an actual germ test.
04Why score the photos blind?
So expectations do not influence the scores.
Take it further
Compare washing with water only, with soap, and with an alcohol sanitiser on the same coating.
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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