[selmic28]

Hello everyone!
I became interested in the effects of ammonium nitrate dissolving in water. I know it is an endothermic reaction, but have a few questions about it, since I am far from an expert in chemistry 
I watched a lot of YouTube videos, and the temperature drop was significant, but in all of them quite large ammounts of water and ammonium nitrate were used. I was wondering how does the reaction "behave" when there is just a few ml of water and a very small amount of ammonium nitrate? (for example 5 ml and a gram of ammonium nitrate?). I don't have an opportunity to try it myself, so I hoped someone could tell me. Is the temperature drop the same? Or significantly reduced? Also, in a closed environment, like those instant cold packs, does the reaction produce any pressure? How come the packs dont break?
Once again, please excuse my ignorance, I am really a chemist amateur 
Thank you in advance!

[Borek]

I suggest you start by learning about the difference between intensive and extensive properties of matter, they play an important role here.

https://en.wikipedia.org/wiki/Intensive_and_extensive_properties

[Enthalpy]

I don't expect any big volume change from the dissolution, because no gas evolves. But the volume of the solid+liquid may change, by the dissolution or because the temperature of the gas between the solid grains changes, so it's useful if the container can adapt. For instance, oval package can become less or more round to fit a volume change.

[selmic28]

Thank you for your answer. What about the temperature drop? Would it be the same when smaller quantites are used?

[Enthalpy]

Borek already gave hints about the temperature drop...

The amount of absorbed heat increases like the amount of substances. If the substances, ammonium nitrate and water, cool only themselves, the absorbed heat cools a quantity of matter that increases equally, so the temperature drop remains the same.

But if you use the reaction to cool the same amount of target material, say a bottle of champagne, then a lower amount of absorbed heat from the same target amount produces a smaller temperature drop.