I could see boiling oil being it's own word because when used as a weapon it's unlikely to actually be boiling and yet still be called "boiling oil".

As you note, we have a specific word for solid water. Additionally, in colloquial speech “water” almost exclusively refers to the liquid state. Finally we have “steam” to refer to the gaseous state.

So English arguably has three unique words for the three common states of H2O.

It's a great question, and is tough to answer intuitively without speaking a native language that actually has such a word.

I would agree that "boiling milk" and "boiling oil" are very unlikely to get separate words, unless one of them happens to be an extremely common thing that people encounter a lot and that has special practical implications.

Milk might be a special case, in that it essentially is just water with some other stuff dissolved. It is to water as salt water is to water... but more so.

My guess would be that the single word might get pressed into service like "ice" does, but I think we'd have to find languages that include this word and survey native speakers. It could vary.

Nearly everyone encounters boiling water in everyday life, but do most people ever see other liquids boiling, even once, and especially during the historical periods that shaped our current languages? If not we might be getting into something like technical language, where daily life lines up poorly and terms and jargon get formalized.

What do you mean by "why we would have"? Dictionaries aren't prescriptive, they're descriptive. If by "we" you mean English speakers, clearly you don't have a word for that. But if you mean some Slavic languages, they do. Likewise, English has "ice", while other languages simply call it "frozen water". Or take the example from the linked article, "at home", which some languages do have a separate word for. I don't think many languages have a distinct word for "at work" though, or "at the shop". That simply reflects that being at home is a more common and generally important concept, just like boiling water is more important in some sense than boiling milk.

Asking for the "reasons" behind a certain word existing is sort of like asking why the human body looks the way it does. Sure, scientists may have good theories why it was evolutionary advantageous to have five fingers and no tail, but in the end the only answer that's for certain is, "because it evolved that way". So the answer is, "we" have a word for boiling water because people found it useful to have such a word.

Arguably it depends more on the atmospheric pressure to get boiling water as close as possible to 100°C.

The general rule of thumb is that black tea (i.e. fermented tea leaves) should be brewed at 100°C, green tea (non-fermented tea leaves) should be brewed around 80°C to avoid it being bitter and white tea (young, non-fermented tea leaves) is best at around 70°C.

Cool, going back over them I'm actually surprised at the strength of the substitution test, thus far I haven't really encountered one that strongly goes against the test if a suitable synonym is picked.

There are a few things for which English simply doesn't have anything to substitute and those are harder to assess. boiling is one but so would "blood" in "blood pressure", obviously replacing it with another liquid has basically the same meaning eg water pressure, oil pressure but as far as I can tell there's literally no synonym for blood.

I those cases I try to use a stand in from another language to see of the substitution works. for for example "sangre" in Spanish so "sangre pressure" which doesn't seem to affect it's meaning much so I'd argue it's exclusion.

Conversely "Red tape" cannot be "roja tape" and a "caliente dog" is one trapped in a car not a food.

> the term is still more commonly referring to the assembly of a wiener or frankfurter wrapped in a bread of some sort

I had that disagreement in an alpine resort once. A seller was vending some sort of sausage stuffed in a bread, i was hungry so I walked up to them with money in hand and said "A hot dog please" while pointing at the only thing they were selling. The lady was mortified by my utterance, and was not willing to accept the money until I agreed with her that it is a bratwurst and not a hot dog. :D The disagreement felt a bit academical, but given that she was holding the hot dogs hostage and money does not taste that good she won the argument.