Difference between "Can't you" and "Can you not"?

I've been wondering about the difference between questions that use can't you and can you not. Like:

• Can't you tell just by looking? [I read this from a comic-detective series]
• Can you not hear me? [I heard this from an animation movie]

So, does it make a difference when you use can't you and can you not? Or anything else like it, such as is it not and isn't it?

I also find that question tags often use those two types of tags, which in my mind are somewhat not interchangeable, such as:

• It's surprisingly hard to find, is it not?
• It's surprisingly hard to find, isn't it?

I think both forms are grammatically correct, but I can't figure how they differ each other.

A simple stress of the NOT in the sentence, that's all.

Can't you hear me? - am I not getting through?

Can you not hear me? - is it really true you can NOT hear me?

The is it not? is getting on a bit. A little archaic. I would not expect to hear it outside a '50s boarding school movie

Another user for not is this:

Can't you do this? - are you not able to do it?

Can you NOT do this? - please stop doing it.

In Cumbria we often use the 'is it not'/'would you not'/'can you not' etc. version where people from the south of the Uk would say 'isn't it'/'wouldn't you'/'can't you' etc.

With parts of be and modals (can, should etc), the two forms

Isn't he, won't you, shouldn't they

and

Is he not, will you not, should they not

are in free variation. Some people rarely use the uncontracted forms, but others use both, with sometimes the uncontracted form being a little stronger, or just more formal.

In the case of the modals, and especially can, there is the possibility of ambiguity, because

can you not go?

which usually means "is it impossible for you to go", can also mean "is it possible for you not to go": this meaning will generally be expressed by a strong stress on not, and probably a break before it.