"hardness" in the noun sense
the property of being rigid and resistant to pressure not easily scratched measured on Mohs scale
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. unfeelingness, callousness, callosity, hardness, insensibility
devoid of passion or feeling hardheartedness
4. hardness, ruggedness
the quality of being difficult to do
"he assigned a series of problems of increasing hardness"
"the ruggedness of his exams caused half the class to fail"
5. severity, severeness, harshness, rigor, rigour, rigorousness, rigourousness, inclemency, hardness, stiffness
"severity of character"
"the harshness of his punishment was inhuman"
"the rigors of boot camp"
Source: WordNet® (An amazing lexical database of English)
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