What phase change is happening on the outside of the dry ice? How do you know?
But because dry ice is cold (its sublimation point is
Often when you brush a block of card ice with your fingers, you brush off water ice, which melts on your fingers to give liquid water.
So how do you know? Good question. Well, when you put a block of dry ice in a bag, upon sublimation there is no solid material in the bag. Clearly the solid mass has gone somewhere, and sublimation is a likely hypothesis.
Here is a simple experiment that could test this proposition. Weigh a mass (say