Colour change in a teacup

As the revision period looms (I mean, when is there a time when we don’t have to revise?), tea becomes my constant companion. Whether this may be in the form of offering a shoulder to cry on as maths questions refuse to make sense or, by living up to its expectations, that tea can, in fact, solve all your problems by creating the polite deception of wanting to boil the kettle for everyone when you just wish to escape the confining shackles of textbooks.

It was as I was pouring over my chemistry notes on hydrogen and hydroxide ions that tea made another unlikely addition to its already enviable personal statement; it is a real-life example of a colour change.

I am sure this has limitations. For example, I drink my tea with lemon and no milk.

However, I was still intrigued about this magic in my tea cup and here is what I found in my educational revision break.

50 shades of tea

By adding a lemon slice or juice, black tea lightens significantly (shown by figures 1 and 2), and although I have never tried this, by adding lemon to green tea, the tea loses its signature colour and becomes colourless [1].

img_6320

Fig 1. Tea before lemon has been added.

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Fig 2. Tea after lemon has been added.

Lemon juice is an example of citric acid, which has the chemical formula of C6H8O7. It is a weak tricarboxylic acid that is found mainly in citrus fruit, but also a variety of different fruit and vegetables. Lemons and limes have a particularly high citric acid content as it can make up to 8% of the dry mass of the fruit and the concentration can reach 0.30 mol/L whereas this is significantly lower in fruit such as oranges and grapefruit where the concentration amounts to 0.005 mol/L.  Citric acid itself was isolated by the chemist Carl Wilhelm Scheele in 1784 by crystallising it from lemon juice. The acid can exist in both an anhydrous form, which is formed when it is crystallised from hot water, and a monohydrate form which is created when the acid is crystallised from cold water instead.

Anyway, back to tea.

Tea leaves are rich in polyphenols-a group of chemicals that accounts for almost one third of the mass of a dried leaf and much of the tea’s colour and flavour is due to these compounds. One group of polyphenols is called thearubigins and they are the red-brown pigments which are found in black tea. This group of polyphenols can make up between 7% and 20% of the total mass of dried black tea, and more interestingly, these thearubigins are weak ionising acids, and the anions (otherwise known as negatively charged ions) they produce are highly coloured. For example, if the water used to brew the tea is alkaline, then the colour of the tea will be deeper because of the greater ionisation of the thearubigins, and because the colour of black tea is influenced by the concentration of hydrogen ions in the water [3].

This means that if lemon juice, i.e. citric acid, is added to the tea, then the hydrogen ions will suppress the ionisation of the thearubigins and this will make the tea a lighter colour [3]. Sorry – no real magic this time.

On a side note, however, you may be interested to know that theaflavins-the yellow-coloured polyphenols present in black tea-are not actually involved in the colour change that is associated with a change in acidity [3].

Conclusion

The colour change that you see in your tea cup as you sip slowly in order to extend the tea-drinking break is in fact just evidence of a change in acidity-much like a change in litmus paper that you may have seen in a Chemistry lab.

References

[1]  http://www.thenakedscientists.com/HTML/questions/question/2413/

[2] https://en.wikipedia.org/wiki/Citric_acid

[3] http://superbeefy.com/why-does-black-tea-lighten-or-change-color-when-you-add-lemon-juice-and-what-causes-the-chemical-change/

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