What it shows
Limewater turns milky when carbon dioxide passes through it, because insoluble calcium carbonate forms. Breath blown out turns it milky faster than ordinary air does, which shows that exhaled air has more carbon dioxide.
The science
Slaked lime is calcium hydroxide, slightly soluble in water, giving clear limewater. Carbon dioxide reacts to form insoluble calcium carbonate, which makes the water cloudy: Ca(OH)2 + CO2 gives CaCO3 + H2O. Air we breathe out holds about 4 percent carbon dioxide against about 0.04 percent in the air around us, because cells make CO2 in respiration. With much more CO2 the milkiness first increases and later may clear again as soluble calcium hydrogen carbonate forms.
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.
- Edible slaked lime (chuna), a pinch, about 2 g
- A clean jar of 500 ml water
- Two clear glasses
- Two straws
- A small bicycle pump or a plastic syringe with a tube
- A paper coffee filter
- A funnel
- Safety glasses
Safety
For this project: Slaked lime irritates the skin and eyes. An adult must prepare the limewater, and everybody must wear safety glasses. Blow out only through the straw and never suck the liquid in. Wash hands well.
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.
An adult stirs a pinch of chuna into the water in the jar, shakes well, and leaves it for a few hours to settle.
Filter the clear liquid through a coffee filter in a funnel into a clean jar. This clear liquid is the limewater.
Pour equal amounts, about 100 ml, of the limewater into two glasses labelled A and B.
Put a straw in glass A and gently blow out through it. Count the number of breaths until the liquid becomes milky.
For glass B, use the pump or syringe to push ordinary room air through a tube into the limewater with the same number of pumps and note any change.
Compare both glasses after the same time, and photograph them against a dark sheet.
Continue to blow into glass A for a few more breaths and see if the milkiness clears later.
Dispose of the limewater by diluting it with plenty of water, and note the colour change in your report.
What you should see
The glass with the breath turns milky in about 5 to 10 breaths, while the one with air pumped through it stays almost clear for much longer.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which substance is limewater?
A solution of calcium hydroxide in water.
2Why does it turn milky?
Calcium carbonate, which does not dissolve, is formed with carbon dioxide.
3Why does breath have more carbon dioxide than air?
Our cells make it during respiration.
4Why not suck on the straw?
It may pull limewater into the mouth, which is harmful.
Take it further
Compare breath after rest and after two minutes of jogging by counting breaths needed for milkiness.
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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