(I'm reposting my question from Physics SE where it got closed for being off-topic, but chat tells me that it might be on topic here.)
In the office I'm working in, there's an electric kettle for boiling tea water. Every day, I boil water in it.
At one point, my boss asked me to empty the kettle after making the tea, that is, pour the unused water remaining in the kettle into the sink so that the kettle is as dry as I can easily get it. His rationale was that this will stop the kettle from getting chalk residue from the hard water.
I obey my boss, both because he owns the office and so the kettle is his, and because he's an engineer. But privately, I have doubts about this request.
I know from practice at home that it is very easy to clean chalk residue from an electric kettle: you just boil water with citric acid in it, then rinse the kettle properly, and this removes all the residue. (Vinegar also works instead of citric acid, I prefer citric acid because it doesn't have the smell and is also cheap.)
On the other hand, I'm worried that if I don't leave any water in the kettle after boiling water for tea, the heating element may still be hot and its surface could become oxidized. If that happens, that's much more diffictult to fix than the chalk. Because of this, at my home, I always make sure that there is water in the kettle after boiling, including adding cold water from the tap if I used up all the hot water. I have very hard water at my home, which causes a lot of difficulties cleaning the bathroom tiles. (I do not know how hard the water is at the office.)
So what's the best practice at home? Should I leave the kettle dry or wet after boiling water?