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I noticed that distilled water works well to clean away fingerprints on touchscreens or oil stains on a ceramic field after frying an egg.

But water and oil/fat are not supposed to mix, so why does that even work?

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As far as I understand, fingerprints are mostly made from sweat and substances dissolved in sweat, so I can see them being removed with water. Not sure about the oil stains though. Are you sure it's not just a sort of confirmation bias? –  Nicolau Saker Neto Jan 25 at 14:49
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2 Answers 2

There may be two different factors at play here.

First, it isn't necessarily the water that is doing most of the cleaning. I presume the water is used to wet a cloth and the cloth then wipes the surface. So the cloth may then physically remove surface contaminants. The role of the water may be just to improve the cloth's surface contact hence improving its ability to physically remove the contaminants.

Secondly, distilled water might do a better job than tap water as it has fewer dissolved solids which would leave other stains on the surface when the water evaporates.

However, the only way to be sure is to do a comparative experiment using different cleaning fluids. You might try tap water, distilled water and distilled water with a surfactant (e.g. a liquid soap). You might find that the best result uses the surfactant but that pure distilled water does a less good, but good enough job. Science isn't about single isolated experiments: we usually need to compare alternatives to know what is best.

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In fact distilled water can be acid when expose to atmosphere because $CO_2$ dissolve into forming carbonic acid: $$\ce{H_2O(l) + CO2(g) <=> H2CO3}$$ Although the $pH$ usually in not very low (normally it is around 5) this can help to removing the greasy stains.

I think also the diffusion of the salts present in greasy stain in water can improve the water cleaning capabilities.

Finally one important thing is that distilled water don't leave residue in the surface. Normal water or other detergents usually contain salt or other substance that can leave halos in the surface. So the final result could appear better even if is not the best substance for solubilize fats.

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