Can ionic substances have high or low melting points?

All ionic compounds form crystals. So far as I know, there are no exceptions to this. Again, salts like to form crystals because when you have a whole bunch of little electrical positive and negative charges all stuck together, they seem to like to bunch into little stacking groups. The arrangement that these ions like to stack into is different, and is referred to as the "unit cell". There are ten or so different general shapes of unit cells. When you get to graduate school, ask me about them. For high school classes, it's really not all that important. Ionic compounds tend to have high melting and boiling points. When I say "high", what I mean is "very, very high." Most of the time, when you work with ionic compounds in a chemistry class, the melting point is hot enough that you can't melt them with a Bunsen burner. So, why are these temperatures so high? Well, it has to do with the way that ionic materials are held together. Remember how we said above that ionic compounds form crystals? These crystals are basically just great big blocks of positive and negative charges all stuck together. To break the positive and negative charges apart, it takes a huge amount of energy. This means that if we heat up the compound to add energy, it takes a huge amount of energy to break it apart.