What it shows
Tea and coffee are mildly acidic, and the acidity depends on how long they brew and whether milk is added. Measuring pH with indicator paper for black tea, green tea and coffee brewed for different times shows how extraction of acids changes with time and how milk partly neutralises them.
The science
Tea leaves and coffee beans contain organic acids such as citric, malic and quinic acids, as well as tannins, that dissolve into hot water. The longer the brew, the more of these compounds are extracted, so the pH drops slightly. Green tea is less processed and typically gives a pH nearer neutral than black tea, and coffee is usually around pH 5. Milk contains proteins and phosphates that act as a buffer and bind acids, so adding milk raises the pH toward neutral. Indicator paper changes colour with the hydrogen ion concentration and is read against the printed scale.
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.
- Black tea leaves, green tea leaves and filter coffee powder
- Universal indicator paper or pH strips from the school lab or a chemist
- Hot water (adult pours) and a kettle
- Six identical cups, a tea strainer and a timer
- Milk, 50 ml
- A white tile and tweezers for the strips
- A notebook for the table
Safety
For this project: An adult must pour the hot water and the brews must cool before any strip is dipped. Do not drink brews that have had indicator strips in them.
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.
Put one level teaspoon of black tea in each of three cups and ask the adult to add 150 ml of hot water to each at the same time.
Remove the leaves from the first cup after 1 minute, the second after 3 minutes and the third after 6 minutes by pouring through a strainer.
Let all three cool to room temperature, dip a strip in each for two seconds, place it on the tile and read the pH against the scale.
Repeat the three brewing times with green tea and with coffee.
Add 20 ml of milk to a fresh 3-minute black tea and measure its pH after cooling.
Measure the pH of the plain hot water and the milk as controls.
Record all readings in a table with rows for each drink and columns for brewing time, and plot pH against time for each drink.
Repeat the whole set once more to check the readings agree within half a pH unit.
What you should see
pH falls slightly with longer brewing for each drink, black tea and coffee sit around pH 5 while green tea is closer to 6 or 7, and the milk tea reads higher than the same tea without milk.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does longer brewing lower the pH?
More of the natural acids and tannins are extracted into the water.
02Why does milk raise the pH?
Milk proteins and phosphates act as a buffer and neutralise some of the acid.
03What does pH 5 mean compared to pH 7?
pH 5 has a hundred times more hydrogen ions than pH 7, so it is mildly acidic.
04Why cool the brews before testing?
Indicator colours are calibrated at room temperature and hot liquids can damage the strips.
Take it further
Measure the pH of the same tea brewed in bottled, tap and borewell water to see whether the water itself shifts the result.
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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