hardness

noun

1. the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale
2. a quality of water that contains dissolved mineral salts that prevent soap from lathering"the costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present"
3. devoid of passion or feeling; hardheartedness
4. the quality of being difficult to do"he assigned a series of problems of increasing hardness"
5. excessive sternness"severity of character"