When do we say "I stirred salt into water to make salty water" or "I stirred salt into water to make salt water"?

Question

I often hear people say "salt water" rather than "salty water".

According to my study, "salt water" is water that has salt.

Say, I have a sore throat and I want to gargle with "salt water" or "salty water".

According to the dictionary, they say "Gargle with salt water if your throat is sore.".

There are 2 examples

Example 1: I add some salt into some water but I don't stir the water. And I am very sure that I can't gargle with that water because the salt has not dissolved in the water yet. Some part of that water is salty but others are not.

Example 2: I add some salt into some water and I stir the water. I am sure I can use this water because the salt has dissolved into the water.

So the water in example 1 and 2 is called "salt water" or "salty water".

When do we say "I stirred salt into water to make salty water" or "I stirred salt into water to make salt water"?

Also, why do most people say "I want to drink sugary milk" but not "I want to drink sugar milk"?

Answer 1

This is explained with a handful of points:

1. Words that are typically nouns can be also used as adjectives to modify other nouns. Sticking to food examples, we can have "apple pie," "chicken soup," etc.
2. Many nouns also have an adjective form, like salty. When they don't, we can even force them to by making up constructions like "apple-y." —Oops, that one is actually in the dictionary. "Vanilla-ish," then. Just because these forms exist doesn't prevent the original noun form being used in the way described in #1. Beefy exists, but we can still have "beef stew," "beef steak," etc.
3. I don't want to give the impression that there is a hard and fast rule about these two usages, but we tend to use the first construction, using a noun as a modifier, to say something about what something is made of or identified by, and the adjective form to say something about how something seems or its qualities. If I say "it's a very chickeny soup," I mean that it has a flavor that reminds one of chicken, or perhaps a lot of chicken in it. But if I decided to make chicken-flavored ice cream, I would probably call it "chicken ice cream," since the chicken part is part of its identity.
4. This distinction allows for a lot of confusion. You make the good point that "salt water" is in fact "salty." And chicken ice cream is surely... chickeny. Don't get confused by that point; one is an identifier, the other a descriptor.
5. But let's talk about "salt water" vs. "sugar milk." The other thing at work here is that salt water is a phrase. Phrases are combinations of words that are used in that order so often that they take on their own identity and can act as if they're one big word. This means that things we talk and think about often are more likely to become phrases. "Chicken ice cream" is not a phrase. "Ice cream" is (it's not just "icy cream"!). "Chocolate milk" is popular; sugar in milk is not, so "sugar milk" is not a recognized phrase. You could say it; there's nothing wrong with using "sugar" instead of "sugary"; there are "sugar cookies" and "sugar plums." It's just that your hearer might not understand what the phrase means, because "sugar milk" isn't discussed often. (Thankfully.)

So don't get confused: it has nothing to do with how much the salt has dissolved in the water (I can put ice in water and call it "ice water" even though it hasn't dissolved, or dissolve sugar in water and call it "sugar water"). It has to do with 1) whether the noun + noun combination already exists as a phrase and is often used for that thing, and 2) whether I want to simply describe or to identify a thing.