Are "that's it" and "that's all" interchangeable? Is there any specific case where I should use one instead of the other?
That's it is an expression that means:
1 that is the main point or difficulty: “Is she going?” “That's just it—she can't make up her mind.” 2 that is enough or the end: okay, that's it, you've cried long enough.
Ref: The OED
- used to say that something has ended Well, that's it, we've finished - we can go home now. That's it! I'm not putting up with any more of her rudeness.
- used to say that something is correct You switch the computer on at the back. That's it.
That's all doesn't seem to be an expression all by itself, but means that there's nothing more to it, that's all you have to do, etc.
So in some cases, as in the second example from the Cambridge Dictionary, they could be used the same way with a slight difference in meaning.