What are these crystals from our kettle?
April 15, 2021 12:34 PM   Subscribe

We have an electric kettle that we use to boil water that we later transfer to a thermos bottle. The other day, the water that came out of the bottle looked milky. Under a microscope, the "milk" looked like this (x 125). What is this?

The water here is "hard" so we expect this to be limescale (and there's a thin and patchy limescale deposit at the bottom of the kettle), but it's the first time that I see enough calcium (CaCo3 or Ca(HCO3)2) to make regular tap water look milky, and what about the crystal needles? There are some images of needle-like aragonite but it does not seem common. Sorry for the potato quality of the image, it was taken with a cellphone through the eyepiece of my 50-year old microscope!
posted by elgilito to Food & Drink (3 answers total) 1 user marked this as a favorite
I'd say any sort of electric kettle or hot water dispenser needs periodic flushing with CLR or their equivalent. I see all sorts stuff in the water pots.
posted by kschang at 1:45 PM on April 15

Exactly what it is depends on the precise mix of minerals in your water (which varies greatly depending on the source), but basically it's crystals formed by those minerals. If you keep boiling water in the same kettle, never quite emptying it, and never flushing it, the concentration of minerals increases and eventually becomes visible as your has done.
posted by beagle at 2:45 PM on April 15

I'm guessing this question is largely for academic interest, so hopefully, a real chemist will chime in with some detailed info, but as a budget chemist (aka physicist) I will have a crack. Two thoughts:
a) aragonite is one of the phases of CaCo3, as is calcite. As you point out, aragonite has a needle-like habit and calcite is more blocky, which is not so much what you are seeing. Calcite is nominally the more stable phase at atmospheric pressure. Apparently though, aragonite is common in seawater, and the presence of metal ions like zinc can enhance the precipitation of aragonite.
b) Other common minerals in hard water deposits are calcium sulfate and magnesium hydroxide, and calcium sulfate can have a more needle-like crystal habit. There could also be other minerals, and some crystals have wildly varying crystal habits depending on the conditions.
For the rest of your question, you'll need that real chemist, although the wikipedia article on hard water is actually quite informative.
posted by neatsocks at 4:16 PM on April 15 [3 favorites]

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