Tea, of which we drink several glasses a day on average, is in fact a small chemistry experiment every time it brews. The colour lightening when you squeeze in lemon, the white stone building up at the bottom of the kettle, the thin film on top of tea as it cools… All of them have a scientific explanation.
The molecules behind the “rabbit’s blood” colour
Fresh tea leaves are green. The dark colour of black tea comes from the fermentation (really oxidation) stage during leaf processing. At this stage the catechins in the leaf react with oxygen and two groups of coloured molecules form:
- Theaflavins: give a bright orange-red colour and a lively taste.
- Thearubigins: are the source of the darker brown-red tones and the body.
The deep red “rabbit’s blood” colour Turkish tea drinkers love depends on the balance between these two groups.
What happens when you add lemon?
Some of these coloured molecules behave like the litmus paper in a school lab: they change colour depending on the acidity of their environment. In other words, they are a pH indicator.
The citric acid in lemon juice lowers the pH of the tea. In an acidic environment the structure of these molecules changes, they absorb less light and the tea becomes noticeably lighter, turning yellowish. You can also try the reverse: add a very small amount of baking soda (a base) to a glass of tea and the colour deepens.
Home experiment: Prepare two identical glasses of tea. Add lemon to one and a tiny pinch of baking soda to the other. Photograph the colour difference and tag us.
The white stone in the kettle: limescale
That hard white layer at the bottom of the kettle is calcium carbonate. Calcium and bicarbonate ions dissolved in tap water turn solid when the water boils:
Ca²⁺ + 2 HCO₃⁻ → (heat) → CaCO₃ (solid) + CO₂ + H₂O
The harder your water (that is, the more calcium and magnesium it contains), the faster limescale builds up. Limescale is an insulator: the kettle takes longer to heat the water and uses more energy.
Practical fix: Boil water with vinegar or citric acid in the kettle. The acid dissolves the calcium carbonate and the scale comes off easily. The same chemistry is the basis of descalers sold for household appliances.
The film on cooling tea
When tea brewed with hard water cools, a thin, iridescent film may form on its surface. It is a film of calcium carbonate and organic compounds, formed when the polyphenols in tea react with calcium and oxygen in the water. It is harmless but spoils the look. Much less of it forms in tea brewed with soft or filtered water, and adding lemon also reduces it.
From the glass to industry
The limescale in your kettle also forms in factory boilers, cooling towers and heat exchangers; there it is simply measured in tonnes. The solution there is not boiling vinegar every week but conditioning the water from the start: additives used at very low doses stop calcium carbonate crystals from sticking to surfaces.
From Biyopol Kimya: You can find our scale and corrosion solutions for boilers, cooling towers and closed-loop systems on our Water Treatment Chemicals page.
Frequently asked questions
- Is hard water harmful to health? The calcium and magnesium in tap water generally do not cause health problems; hardness matters more for plumbing, appliances and taste.
- Why does tea with lemon taste lighter? While the acid changes the structure of these colour molecules, it also affects how astringency is perceived; lemon’s own aroma balances the taste as well.
- Which scales more, the teapot or the kettle? Since the boiling water is in the kettle underneath, most of the limescale builds up there.