<Li> A French degree (° fH or ° f) is defined as 10 mg / L CaCO, equivalent to 10 ppm . </Li> <P> Because it is the precise mixture of minerals dissolved in the water, together with the water's pH and temperature, that determine the behavior of the hardness, a single - number scale does not adequately describe hardness . However, the United States Geological Survey uses the following classification into hard and soft water, </P> <Table> <Tr> <Th> Classification </Th> <Th> hardness in mg - CaCO3 / L </Th> <Th> hardness in mmol / L </Th> <Th> hardness in dGH / ° dH </Th> <Th> hardness in gpg </Th> <Th> hardness in ppm </Th> </Tr> <Tr> <Td> Soft </Td> <Td> 0--60 </Td> <Td> 0--0.60 </Td> <Td> 0 - 3.37 </Td> <Td> 0 - 3.50 </Td> <Td> 0 - 60 </Td> </Tr> <Tr> <Td> Moderately hard </Td> <Td> 61--120 </Td> <Td> 0.61--1.20 </Td> <Td> 3.38 - 6.74 </Td> <Td> 3.56 - 7.01 </Td> <Td> 61 - 120 </Td> </Tr> <Tr> <Td> Hard </Td> <Td> 121--180 </Td> <Td> 1.21--1.80 </Td> <Td> 6.75--10.11 </Td> <Td> 7.06 - 10.51 </Td> <Td> 121 - 180 </Td> </Tr> <Tr> <Td> Very hard </Td> <Td> ≥ 181 </Td> <Td> ≥ 1.81 </Td> <Td> ≥ 10.12 </Td> <Td> ≥ 10.57 </Td> <Td> ≥ 181 </Td> </Tr> </Table> <Tr> <Th> Classification </Th> <Th> hardness in mg - CaCO3 / L </Th> <Th> hardness in mmol / L </Th> <Th> hardness in dGH / ° dH </Th> <Th> hardness in gpg </Th> <Th> hardness in ppm </Th> </Tr>

When is a sample of water said to be hard
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