Hydrates are solid compounds which contain water. This water is included with the chemical formula of the compound because there is a set ratio of water to compound.
Alright so let's talk about hydrates and what they are, hydrate is actually a compound with a specific number of water molecules bound to its atom but the atom is inside. Just kind of like these water molecules are trapped within a salt. A salt don't forget it's a ionic compound. So what does it actually kind of look like if we picture it and draw it up here ionic compounds we know are cations and anions kind of attracted to each other and so when they're actually being formed what happens is these water molecules get trapped within there and get really attracted to these positive and negative charges within it and they get trapped within the salt and the number of water molecules that get trapped in comparison to the actual molecule the ratio of atoms is dependent on the salt itself. So if you're actually to look at a hydrate to see what it looks like it, hydrates are crystals just like ionic structures are. However they do have like a blue color to them, they might have like an [ear dicing] color there sometimes. But they have a characteristic blue color that makes it pretty much seen when you look at the crystal itself. So this is an example of what a hydrate mint look like if you find one.
Alright if you were to name hydrates because they're ratio of water molecules to salts, you want to be able to and they're not bound to each other like these guys aren't bounded to anything. But they're just trapped within a crystal, so we would have like recognized that, so by doing that if we had one molecule for example one water molecule for every one salt we would've just represented this way and saying that we have this little dot tells us that the ratio for everyone water molecule you have one salt. So if you we were to name this we would've names this compound ammonium oxalate mono hydrate. So we'd the salt just like we always would ammonium oxalate and then we'd say alright we have one water molecule attached to it, so mono like we would for a covalent compounds we use the same prefixes.
And this little dot tells us it's trapped within there however it's not bonded to anything in there. If you had two for every salt, for example calcium chloride you would say the same thing calcium chloride instead of mono you'd say di, dihydrate and you would represent that by this little dot here which does not mean multiply, this little dot tells us we just have two for every one of these we within a structure so two hydrates. So on and so forth so we get to 3 sodium acetates trihydrate represent that with a dot and a 3 so on and so forth and use the same prefixes as you would do if you were naming a covalent compound. And those are hydrates in a nut shell.