Walrus's nucleation explaination is the correct one, but also the dissolved coffee wil elevate the boiling point( cf Raoults law)where the vapour pressure is raised depending on the amount of dissolved substance. Penny, that is also correct, difficult to produce but theoretically correct, The freezing point is also depressed, depending on the amount of dissolved solid, hence "salting" iced roads to depress the freezing point of the water so it liquifys and does not re-freeze( in theory!)