This is a very interesting example. I'll give a short answer here, and leave a full explanation to someone more familiar with the syntax of this particular verb.
Verbs often take preposition phrases as a Complement. In the active voice version of the Original Poster's sentence, we see the verb add take a Direct Object and a prepositional phrase functioning as Locative Complement:
- Someone added water [to the mixture].
In the sentence above we see water as a Direct Object, and the phrase to the mixture functioning as the Locative Complement.
Usually when we see verb plus preposition phrases like this, the preposition phrase cannot become the Subject of a passive construction. So for example if we passivise Someone slept in the bed, we get:
Here the Complement of the Preposition in has become the Subject of the passivised sentence. In contrast, the preposition phrase in the bed itself cannot become the Subject:
- *In the bed was slept. (ungrammatical)
This is the pattern that we generally see with such sentences. We cannot move the prepositional phrase into the Subject position. We can only move the noun phrase from within the preposition phrase. So the first examples in the following pairs of sentences are grammatical, and the second ones are not:
- The notes were referred to
- *To the notes were referred.
- The protesters were spoken to.
- *To the protesters were spoken
- The goblets were drunk out of.
- *Out of the goblets were drunk.
- The problem was looked into.
- *Into the problem was looked.
The active version of the Original Poster's example is:
- Someone added water to the mixture.
We can of course make water the Subject of a passive version of the sentence:
- Water was added to the mixture.
But we should expect to be able to make the noun phrase the mixture the Subject of a passive version of the sentence as well:
- *The mixture was added water to. (ungrammatical)
But this gives us a poor result. Very strangely, we do seem to be able to move the whole preposition phrase to the Subject position here. Many grammarians say that this is not possible. However, there is no doubt that the only viable solution to the Original Poster's malformed sentence, without moving mixture out of the Complement phrase, is to move the entire prepositional phrase into Subject position:
- To this mixture was added water.
Now, exactly why this should be possible, I have no idea. However, very strangely indeed, it actually is. Maybe someone else can explain why ...
Rathony has suggested that maybe there is some phrase movement here. He is correct, I believe. I think my version of the sentence is actually one where the prepositional phrase has been fronted and the Subject post-posed. In other words my version of the sentence is a convoluted version of:
- To the mixture water was added.
Here the Subject of the sentence is actually, of course, water. So, this made me think that perhaps this post should be deleted. However, there still remains the puzzling question, as detailed above, of why the following isn't grammatical:
- *The mixture was added water to.
So I've turned it into a wiki instead.