What it shows
Dry citric acid and baking soda do nothing together, but the moment they get wet, on the tongue or in water, they react and release carbon dioxide bubbles. The project shows that many reactions need water to start, and lets you find the mix that fizzes most.
The science
Citric acid and sodium bicarbonate are both solids, and solids cannot react well because their particles cannot reach each other. When water touches the powder, both dissolve and their ions meet: the acid gives hydrogen ions to the bicarbonate, which breaks up into water and carbon dioxide gas. The gas makes the fizz on the tongue. Equal reacting amounts give the most gas; extra of either one is simply left over.
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.
- Food-grade citric acid (nimbu phool), 2 teaspoons
- Baking soda, 2 teaspoons
- Icing sugar or powdered sugar, 4 teaspoons
- A dry bowl, dry spoon and small dry paper packets
- A glass of water
- A balance or measuring spoons
Safety
For this project: Use only food-grade citric acid from a grocery store. Taste only a pinch at a time; too much is sour and upsets the stomach. Keep the mixture completely dry until it is used.
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.
Make sure every bowl and spoon is completely dry, otherwise the mix fizzes early and is wasted.
Mix one teaspoon of citric acid, one of baking soda and two of sugar; this is mix A.
Make mix B with two of citric acid, one of baking soda and two of sugar, and mix C with one of citric acid, two of baking soda and two of sugar.
Drop half a teaspoon of mix A into the glass of water and watch and listen to the fizz.
Rinse the glass and repeat with mixes B and C, noting which fizzes longest and strongest.
Put a pinch of each on your tongue and record the fizz and taste; sour means leftover acid, soapy means leftover soda.
Pack the best mix into dry paper packets and label them with the recipe.
Explain on a chart why the dry powder does nothing until it is wet.
What you should see
All three mixes fizz in water; the mix with roughly equal citric acid and baking soda fizzes strongly with a pleasant taste, while the others leave a sour or soapy aftertaste from the leftover ingredient.
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 dry powder not fizz?
The solids cannot react until water dissolves them and lets their particles meet.
2What gas makes the fizz?
Carbon dioxide.
3Why did mix B taste sour?
It had extra citric acid left over after all the baking soda was used up.
4Why must the spoons be dry?
Any moisture starts the reaction early and the powder loses its fizz.
Take it further
Weigh a glass of water before and after dropping in a spoon of mix, and work out how much gas escaped.
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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