What it shows
Clean rain is slightly acidic, around pH 5.6, because carbon dioxide dissolves in it. Sulphur and nitrogen oxides from vehicles and industry make it more acidic. Collecting rain in clean containers at three sites and testing with pH paper shows whether local pollution is lowering the pH.
The science
Carbon dioxide in air dissolves in rain to form weak carbonic acid, which gives natural rain a pH near 5.6. Burning fuels releases sulphur dioxide and nitrogen oxides, which form sulphuric and nitric acid in clouds and bring the pH down to 4.5 or lower, harming leaves, fish and stone buildings. Dust from dry soil can push the pH the other way, towards neutral, which is common in north Indian cities. Comparing sites and the first rain of the season with later rain shows these effects.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three clean glass jars or bottles, rinsed with distilled water
- Three wide funnels
- Universal indicator pH paper, range 1 to 14, or a pH meter borrowed from school
- Distilled water as a control
- Labels and a notebook
Safety
For this project: Do not drink the collected water. Rinse with distilled water, not tap water, which has its own pH.
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.
Set a funnel and jar at three sites at least 1 metre above the ground: near a main road, a residential area and an open field.
Put them out just before the rain and bring them in within an hour of it stopping to limit dust and evaporation.
Test the distilled water with pH paper first and record its colour as the control.
Dip a fresh strip in each sample, compare with the chart and record the pH.
Record the date, the site and whether it was the first rain after a dry spell.
Repeat for at least five rain events through the season.
Tabulate and plot pH against date for each site.
Comment on which site is most acidic and whether the first rains differ from later ones.
What you should see
A table of 15 or more readings. Rain near the main road often reads 5 or below, the field reads near 5.5 to 6, and the first shower of the season is usually the most polluted.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is clean rain slightly acidic?
Carbon dioxide dissolves in it to form carbonic acid.
2Which gases cause acid rain?
Sulphur dioxide and nitrogen oxides from burning fuels.
3What damage does acid rain do?
It harms leaves and aquatic life and dissolves marble and limestone on buildings.
4Why rinse jars with distilled water?
Tap water contains dissolved salts that would change the pH.
Take it further
Leave a marble chip in a sample of the most acidic rain for a month and look for changes in its surface.
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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