What it shows
Chalk and marble are both calcium carbonate, which acid rain slowly eats away on old buildings such as the Taj Mahal. Pieces in vinegar fizz and lose mass, while pieces in plain water stay unchanged, so the loss can be measured and compared.
The science
Calcium carbonate reacts with acids to form a soluble calcium salt, water and carbon dioxide. The gas appears as bubbles and the solid slowly dissolves away. Vinegar is a much stronger acid than real acid rain, so the model runs in days rather than centuries, but the chemistry is the same. Rainwater becomes acidic when it dissolves sulphur dioxide and nitrogen oxides from burning fuel. Softer chalk dissolves faster than hard marble because it is more porous and has more surface for the acid to reach.
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.
- Four pieces of blackboard chalk and four small marble chips or pieces of a broken marble tile
- Vinegar, 1 cup
- Plain water, 1 cup
- Four small glasses
- A kitchen balance that reads grams, or a ruler
- Labels
Safety
For this project: Vinegar stings eyes and cuts; wash hands after handling. Marble edges can be sharp; an adult should break any tile pieces.
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.
Weigh or measure each chalk piece and marble chip and write the numbers on the labels.
Put one chalk and one marble chip in a glass of vinegar and another pair in a glass of water.
Watch the vinegar glass; bubbles appear on both pieces, far more on the chalk.
Leave the glasses for a day, then lift the pieces out, pat dry and weigh them again.
Return them to the liquid for two more days, weighing once a day.
Also put a third chalk piece in vinegar diluted with four parts water as a closer model of weak acid rain.
Make a line graph of mass against day for every piece.
Compare the surfaces with a magnifier and note pitting on the vinegar pieces.
What you should see
The chalk in vinegar loses most of its mass and crumbles within a day, the marble in vinegar loses a little and becomes pitted, the dilute vinegar pieces change slowly, and the water pieces stay the same.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What are chalk and marble made of?
Calcium carbonate.
2What were the bubbles?
Carbon dioxide released when the acid reacted with the carbonate.
3What makes rain acidic?
Gases such as sulphur dioxide and nitrogen oxides from burning fuels dissolve in it.
4Why did the chalk dissolve faster than the marble?
Chalk is soft and porous, giving the acid more surface to attack.
Take it further
Coat one marble chip with candle wax before soaking and see whether a coating can protect stone.
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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