Two cups of tea - one is much lighter than the other

Make your tea change colour!

There’s something soothing about a cup of lemon tea. But there’s some fun chemistry happening in that cup too!

hot hazard iconfood safety hazard iconSafety: This activity uses a hot kettle, and you may need to juice lemons. Ask an adult to help. Use clean hands and clean equipment.

You will need

  • Kettle
  • 2 teacups
  • 2 black tea bags
  • Lemon juice
  • Bicarb soda
  • Teaspoon
  • Honey

What to do

  1. The side of a kettle, with water filled up to the 0.5 litre markPut about 500 mL of water in the kettle and boil it.
  2. two teacups with teabags in themWhile the water is boiling, put a tea bag into each teacup.
  3. Someone is pouring water into a teacup with a teabag in itWhen the kettle is done, carefully pour half of the hot water into each teacup.
  4. two cups of tea brewing and a timer counting downStart a timer for five minutes.
  5. someone is taking the teabag out of a cup of teaWhen the timer is finished, take the tea bags out of each cup.
  6. two very similar cups of teaLook at the two teacups. Are the teas the same colour?
  7. someone is spooning lemon juice into a cup of teaAdd about a tablespoon of lemon juice into one cup.
  8. someone is adding white powder into a cup of teaAdd a tiny bit of bicarb soda to the other cup – no more than a quarter of a teaspoon.
  9. two cups of tea. The left cup is much lighter than the right oneLook at the two teacups. Are the teas the same colour now?
  10. someone is adding some honey to the light teaAdd a couple of teaspoons of honey to the lemon tea and enjoy.
  11. The bicarb tea will taste bad, so pour that one down the sink!


What’s happening?

Tea might seem simple, but this experiment shows there’s a lot of chemistry hiding in this drink!

Tea is a pH indicator, which means it changes colour based on how acidic it is. Adding lemon juice makes tea more acidic and look paler. Adding bicarb soda will do the opposite, making tea more alkaline and darker in colour.

Theaflavins and thearubigins are the main chemicals that give black tea its distinctive colour. These groups of chemicals are known as tannins, and they’re part of a larger chemical family known as polyphenols, which are pH sensitive.

Tannins also have a bitter taste and are astringent, which means they make your mouth pucker up and feel dry. You might notice the bitterness and astringency in tea, particularly if you drink it black with no sugar.

So what other chemicals are in a cup of tea? There are thousands! Of these, caffeine is probably the most famous (apart from water!). Caffeine is a stimulant, which means it can block feelings of tiredness or sleepiness. A cup of tea in the morning can help you wake up, but too much tea can make it hard to go to sleep, so be careful!

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One response

  1. Kamani Kalpage Avatar
    Kamani Kalpage

    Thanks for the post. There are other simple experiments kids can do with natural indicators. I often use them in my classes, across the board from year 7 -12. These indicators include;
    1. Red cabbage extract
    2. Extracts from brightly coloured flowers (usually, purple colour works really well)
    3. Turmeric
    4. Mulberry extract (from fresh fruit)
    5. Extract from black grapes
    They can be tested with vinegar, lemon juice, baking soda and clear dishwashing detergents or laundry detergents.
    I am interested in providing simple experiments to include in Double-Helix publications that kids can conduct at home or at school.
    Thank you very much.

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