It might be nice to also have an example sentence saying, "This powder is ...".

I don't think all powder is soluble in water.

You’re absolutely right. So, should we change this? Another solution would be to just delete it.

Is this sentence grammatically incorrect or does it sound unnatural? I don't think so. Because of this I believe there is no reason whatsoever to delete or change it.

Tatoeba is not Wikipedia. "All mammals are cats" is as good a sentence as "All cats are mammals". There are no requirements for sentences to be true, and there shouldn't be such requirements.

Whether or not it's grammatically correct is really of a secondary nature. When it comes to content like this, sentences should also not state falsehoods. It's part of maintaining a certain quality. The word "powder" is used incorrectly because not every powder is soluable in water, so all such a sentence can serve is to confuse people. You shouldn't create sentences just to make a point. ☺

> The word "powder" is used incorrectly because not every powder is soluable in water, so all such a sentence can serve is to confuse people.

Whether it was used correctly or incorrectly doesn't depend on whether every powder is soluble in water or not. It depends on the intended meaning.

If the author of the sentence didn't mean to state that all powder is soluble of water and wanted to say that only some particular powder is, then they did use the word incorrectly.

If they meant to say that all powder is soluble, they used the word correctly, and the sentence is fine in every other respect.

Moreover, even if the author did use the word incorrectly, there are already translations with the same meaning, and this sentence matches them perfectly.

I agree with deniko on this. If this sentence is not true, it should have some tag like 'false' or 'not true' but it's not that bad itself. This could be used in a book where the main hero thinks that 'every powder is soluable in water'. Besides, it's just an example, we could replace the word 'powder' with some other matter. I don't think you should delete it.