Can i use a class b fire extinguisher on a class a fire?

A class B extinguisher is usually a compressed dry chemical, where a class A is water. Using a class B on a class A fire may work but it would tend to scatter the fire which may cause greater danger depending on the surroundings. Besides, with the cost of re-charging, it makes better sense to use the right tool for the job.(water is a lot cheaper to replace than the dry chemical)