What it shows
Hard water leaves chalky scale of calcium carbonate that dissolves in acids and gives off carbon dioxide. You compare vinegar, lemon juice and citric acid solution on chalk as a model of scale and measure the mass lost.
The science
Scale is mostly calcium carbonate. An acid reacts with it to form a soluble calcium salt, water and carbon dioxide, which is why bubbles appear. A stronger acid, or one with more acid per spoon, dissolves more per minute. Citric acid is stronger than the acetic acid in vinegar.
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.
- Pieces of blackboard chalk of equal weight
- Vinegar, lemon juice and citric acid solution
- Plain water
- Four small glasses
- A kitchen scale (to 0.1 g if possible)
- A stopwatch
- A spoon and tissue
Safety
For this project: Use only mild kitchen acids. An adult must supervise, and the used liquids should be washed away with plenty of water. Keep the liquids away from the eyes, and wear gloves if the skin is sensitive.
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
7 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 four pieces of chalk of about the same size and note the weights.
Pour 50 ml of each liquid into its own glass and label the glasses.
Drop one chalk piece into each glass and start the stopwatch.
Count the bubbles rising in ten seconds at the start for each glass.
After 30 minutes lift each chalk out, dry it on tissue and weigh it again.
Calculate the mass lost and the percentage lost.
Rank the liquids and write a short note on which would work best for a kettle.
What you should see
The citric acid solution removes the most chalk, vinegar and lemon juice follow and plain water changes almost nothing. The bubbles are carbon dioxide.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What gas bubbles from chalk in acid?
Carbon dioxide.
2What is scale made of?
Calcium carbonate left by hard water.
3Why is plain water included?
As a control.
4Why not use strong acids at home?
They are dangerous and damage metal.
Take it further
Test the effect of warming the acid to 50 C (adult only) on the rate.
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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