What’s In The Water Cooler?

As a lunchtime diversion, I’ve been carrying out an experiment – but I’ve reached the limit of my equipment and knowledge. Any ideas welcomed!

I drink very weak black Earl Grey tea. It’s too hot to drink straight from the water heater or kettle, so after briefly dangling the tea bag in the mug, I add a dash of cold water.

For years I’ve noticed that it seems to get darker when I add cold water from the office water cooler (that’s the scientist in me).

For years I’ve done nothing with this observation (that’s because I hated practical labs – I did two computer simulations for my final year projects…).

However, today I cracked. This is one mug of tea, divided equally between two glasses, with, on the left hand side, a dash of cold tap water added, and, on the right hand side, an equivalent dash of cold water from the water cooler. Both have been left to reach room temperature.

I also had a bottle of One spring water in my office, so next up was a similar brew, though this time using a standard Careers Service PG Tips teabag – one brew divided between three glasses.

Not as easy to see, as photographed from the side, but guess which one’s which?

Far left – added tap water; middle – added bottled One spring water; far right – added water cooler water.

So, in the absence of any further equipment (or time over lunch) – why does the water cooler water make tea go darker? These are the practical issues our postgraduate scientists should be working on!


5 comments on “What’s In The Water Cooler?

  1. Maybe it is because the refractive index (color) is dependent on temperature? Do they go to the same temperature if you leave them both (don’t drink the tea!) and check the next day?

    • Good guess. If I were a proper scientist, that, of course is what I would do to check. On the other hand, I’ve already chucked the test solutions away and put the glasses in the dishwasher…

      However, even in the absence of a thermometer, I’m pretty sure we left them long enough to ensure they were all at the same room temperature.

      Mineral content is another theory – I’ve read that higher mineral content (they don’t say which minerals) can make tea “dark and murky”, but not sure where Manchester tap water comes from and how it compares with bottled/cooler water. Also, don’t know why mineral content might make the tea darker.

      Now Brian Cox had finished his Star lecture, maybe he could turn his mind to more important matters 🙂


  2. One thing is to leave long enough to reach the same temperature, and another thing is to leave them long enough to let the precipitated substances to solubilize. If you have different cups, with exactly the same water and temperature, but different colouration due to different histories of temperature, then this is the problem.

    I would say that the colouration is both dependent on the temperature and on the type of water used.
    At lower temperatures, substances have lower solubility, so you may be actually be observing a dispersion of the water and the catechins (these are of the main culprits of the colouration of the tea), and therefore it will be darker.
    Also different kinds of water will have different amounts of salts/ions. Catechins will form insoluble salts with ions such as Ca2+, thus the larger the concentration of these ions in the water, the darker it will become.

    Or maybe, it is mostly dependent on the ions, and the reason it becomes darker is because the substance formed between the catechins and the Ca is darker than the catechins.

    It would be a matter of checking these hypotheses, either experimentally or with some published literature on this field 🙂

  3. Thanks Adolfo and Charlie!

    Possible culprit – something to do with the different mineral content of water cooler water (and to a lesser extent, bottled spring water) and the way it interacts with catechins or other polyphenols in tea? I’ve read (but it was on a dodgy looking website so I’m wary of believing it) that oxidation or polymerisation of catechins or other polyphenols causes discoloration (this was in relation to dental discoloration).

    Another observation – adding water cooler water seems to generate a slight froth round the edges of the glass of tea (this also happened with the spring water, but not the tap water yesterday). Hadn’t really thought about this but could be relevant?

    Anyway, if there are any RSC members out there willing to take on the challenge, I’m sure the results can be replicated with any water cooler, as we’ve changed equipment and suppliers several times over the years, all with the same result.

    Charlie – as I can’t access my.rsc.org, could you let me know if they come up with anything?

    An interesting side effect of this experiment has been the reaction of my colleagues, including those non-scientists who were really interested and wanted an afternoon of more experiments (pity I had work to do!). However most have just smiled indulgently as my inner geek was revealed.

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