What it shows
Antacids are bases or salts of weak acids that react with acid and neutralise it. Counting how many drops of vinegar one tablet can neutralise lets you compare brands fairly, per tablet and per gram.
The science
Many antacids contain magnesium hydroxide, aluminium hydroxide or sodium bicarbonate. Each reacts with an acid to form a salt and water, and bicarbonates and carbonates (such as calcium carbonate) also give carbon dioxide. For example Mg(OH)2 + 2 CH3COOH gives Mg(CH3COO)2 + 2 H2O. The indicator is blue-green in a base and turns purple then pink when the acid is more than the base, so the drop where the colour changes marks neutralisation. Vinegar is only a weak model of stomach acid, which is hydrochloric acid, so the numbers compare the tablets and do not predict the effect inside the body.
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.
- 3 to 4 different antacid tablets from a pharmacy (used only for testing, never eaten)
- White vinegar, 500 ml (one tablet needs tens of ml, and you repeat each test three times)
- Red cabbage indicator, 100 ml (soak or boil chopped cabbage in water and strain)
- A dropper or a needle-less 5 ml syringe (about 20 drops make 1 ml)
- 4 small glasses
- A teaspoon and a mortar or a spoon for crushing
- A kitchen scale that reads to 0.1 g (a jeweller’s shop will weigh for you)
- A notebook and a white paper background
Safety
For this project: An adult must supervise. Do not eat the tablets or any solution you make. Wash hands after crushing the tablets and do not touch your eyes. This is a chemistry comparison and a learning model, not medical advice: do not use the ranking to choose or take a medicine.
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.
Step 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 each tablet and write down the mass in grams, because tablets of different brands differ in size.
Crush one tablet to a fine powder with a spoon and stir it into 50 ml of water in a glass. Stir for one minute, because magnesium and aluminium compounds dissolve slowly.
Add 5 ml of red cabbage indicator. The liquid should turn blue or green, showing that it is basic. Place the glass on white paper to see colours.
Add vinegar from the dropper, stirring after every few drops, and count the drops. One tablet may need several hundred drops, so go fast at first and slow to single drops when the colour starts to change.
Stop at the first purple that does not fade back to green after 30 seconds. Write down the number of drops. A common mistake is to stop at the first flash of colour near the dropping point.
Repeat for each brand and also for a half teaspoon of baking soda for comparison. Repeat each test three times and take the average.
Divide the average drops by the tablet mass to get drops per gram, and plot a bar chart of drops per tablet and drops per gram.
Rank the antacids and write which one is the best value if you also note the price of each tablet.
What you should see
A bar chart ranks the antacids by their neutralising power. You should see that a tablet that is best per tablet may not be best per gram or per rupee, and repeat tests should agree within about 10 per cent.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What type of reaction happens between an antacid and acid?
Neutralisation: a base and an acid react to give a salt and water.
02Why do you stir for a full minute before adding vinegar?
Magnesium and aluminium hydroxides dissolve slowly, so stirring makes sure they are available to react.
03Why is vinegar not the same as stomach acid?
Vinegar is a weak organic acid, acetic acid, while stomach acid is hydrochloric acid, a strong acid.
04Why should the tablets be compared per gram as well as per tablet?
Because tablet sizes differ, and dividing by mass gives a fair comparison of the active material.
Take it further
Compare milk of magnesia, baking soda and chalk powder as antacids per gram and say which is the most effective base by mass.
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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