What it shows
Fizzy drinks and juices contain phosphoric, citric and carbonic acids. Counting the drops of a fixed baking soda solution needed to neutralise a measured volume of each drink, watched with a natural indicator, ranks them by acid content and links the result to tooth enamel damage.
The science
Sodium bicarbonate is a weak base that reacts with the acids in the drink, releasing carbon dioxide and forming the sodium salt of each acid. The more acid a drink contains, the more baking soda solution is needed before the acid is used up. The indicator shows the moment the solution stops being acidic: red cabbage shifts from pink to purple blue, and phenolphthalein turns faintly pink. Because the same baking soda solution is used for every sample, the drop count is a fair comparison of total acid, which is what attacks tooth enamel.
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.
- Small amounts of a cola, a lemon soda, an orange juice, a packaged mango drink and plain water
- Baking soda solution: 5 g dissolved in 100 ml water
- Red cabbage indicator or a few drops of phenolphthalein from the school lab
- A dropper or 1 ml syringe
- A 10 ml syringe or measuring cylinder
- Five white cups or small beakers
- Notebook
Safety
For this project: Do not drink any sample after indicator is added. Phenolphthalein, if used, must be handled by the teacher.
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 each drink into a cup and stir it for a minute to let the fizz escape, since dissolved carbon dioxide would use up baking soda unevenly.
Measure 10 ml of the first drink into a clean cup and add ten drops of indicator; note the colour.
Add baking soda solution one drop at a time, swirling after each, and watch for fizzing and for the colour to change.
Stop at the first drop where the colour holds at the neutral or basic shade for thirty seconds, and record the number of drops.
Repeat for each drink and for plain water as a control, rinsing the cup between samples.
Do every drink twice and take the average.
Rank the drinks in a bar chart from most to least acidic by drop count.
Soak a piece of eggshell in the most acidic drink and in water for two days as a visual link to enamel, and record what you see.
What you should see
Water needs no drops, the mango drink and orange juice a moderate number, and the cola and lemon soda the most, typically several times more than the juice; the eggshell in cola softens and loses its shine while the one in water stays unchanged.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which acid is found in colas?
Phosphoric acid, along with carbonic acid from the dissolved gas.
2Why stir out the fizz before testing?
Dissolved carbon dioxide forms carbonic acid that escapes unevenly and would spoil the comparison.
3What does the indicator show?
The point at which all the acid has been neutralised by the baking soda.
4Why does eggshell soften in cola?
Its calcium carbonate reacts with the acid, in the same way that acid attacks tooth enamel.
Take it further
Measure the pH of each drink with pH paper and discuss why pH and total acid do not rank the drinks in exactly the same order.
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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