I'll or I will for spontaneaous decision

[fredfred]

Good afternoon,

I have a question about the future form used to express a sudden and spontaneous decision.

Let's take the following interaction:

A: it's so warm in this room. I'm suffocating.
B: I’ll open the window.

In this dialogue, B who was not aware of the situation 10 seconds ago finds a solution. This is why B uses the form “I’ll” + the action that will be an (appropriate and quick) answer to the problem expressed by A.

My question is: can we use the full form ‘I will” in this situation? If I’m not mistaken, the contracted form and the full form are not interchangeable in this specific case.

Thank you very much for your help

[Alan]

Yes, the full form here would be unnatural.