I am trying to dissolve popcorn butter and milk into water. I notice the milk dissolves, but the popcorn butter does not. Why is this? *

Because butter is an oil-like substance. Water is a combination of hydrogen and oxygen, oil on the other hand is not. For the two to mix the oil would have to break the hydrogen chain but that is not possible because then the water won't be water.

So most fats repel water, just think about it being hydrophobic :)

Hope that helps and have a great day!