What it shows
An effervescent tablet fizzes in water as its citric acid and bicarbonate react. The same tablet crushed to powder finishes in a fraction of the time, because the reaction happens only at the solid surface and powder has far more surface. Timing whole, halved, quartered and powdered tablets gives a clean rate curve.
The science
The reaction between citric acid and sodium bicarbonate can only take place where water reaches the solid. A whole tablet exposes only its outside; cutting it in half exposes new faces, and crushing it into powder exposes an enormous area. More exposed surface means more particles collide with water per second, so gas is released faster and the fizz ends sooner. Rate therefore increases with surface area even though the amount of reactant is the same.
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.
- Twelve effervescent tablets (vitamin C or antacid type) or ENO sachets
- Four identical glasses
- Water at room temperature, measured 100 ml per glass
- A stopwatch
- A spoon and a clean sheet of paper for crushing
- A balance if available
- Notebook
Safety
For this project: Do not swallow the tablets or the fizzing water during the test. Crush tablets inside folded paper so powder does not fly into eyes.
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.
Pour exactly 100 ml of water into each of the four glasses and let them stand so they are all the same temperature.
Prepare one whole tablet, one cut in half, one cut in quarters and one crushed to powder in folded paper.
Drop the whole tablet into the first glass and start the stopwatch; stop when the last bubble rises.
Repeat with the halves, the quarters and the powder in their own glasses, timing each.
Run the full set three times with fresh tablets and water, and average the times.
Calculate a rough rate for each as one divided by the time taken.
Plot rate against the number of pieces, treating the powder as a very large number.
Describe in your report how the curve would change if warm water were used, as a prediction for the viva.
What you should see
The whole tablet takes the longest, typically a minute or more, halves and quarters are faster, and the powder finishes in a few seconds, giving a rate that climbs steeply with surface area.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the powder react fastest?
It has the most surface in contact with water, so more collisions happen each second.
2Does crushing change the amount of gas made?
No, the same mass of tablet makes the same gas; only the speed changes.
3Why average three runs?
To reduce the effect of timing errors and small differences between tablets.
4Give an everyday example of surface area and rate.
Small wood chips catch fire faster than a log, and powdered sugar dissolves faster than cubes.
Take it further
Repeat the powder test in cold, room-temperature and warm water to separate the effect of temperature from surface area.
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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