Ok, Comatose, thank you for explaining what you wanted.
It happens on a sliding scale (i.e. he could make something absolutely tasteless or overload your sense of taste, but couldn't make bread taste like fish) and only on physical properties (i.e. boiling/melting points, hardness, taste, etc.). Also, it gets harder to push the 'scale' the farther it goes; say, a doubling or halving a property makes it 5x harder to push. Say, making a person evaporate would be very hard, though it is possible. Making the water in a person is much easier, so is more likely to be used, though Chasmfiend likes evaporating people because it's flashy and usually scares the heck out of people. Also the mass matters; a double in mass would make it, say, 3x harder. So, while evaporating people is hard, evaporating a large tree is just about impossible without a considerable amount of time; he would have to continue pushing the slide, so the tree would have melted already anyways.
He refuses to believe he has a weakness. He has major Hubris.