Which of the following pairs of ions makes the water hard?
A.Na + , SO42 +
B.Ca2 + , HCO3 -
C.Ca2 + , NO3
D.NH4 - , Cr

The pair of ions that typically contribute to water hardness are calcium (Ca2+) and magnesium (Mg2+) ions, commonly found in the form of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3).

Looking at the options provided:

A. Na+ and SO42+ do not contribute to water hardness.
B. Ca2+ and HCO3- are indicative of temporary hardness due to the presence of calcium bicarbonate.
C. Ca2+ and NO3- do not contribute to water hardness.
D. NH4- and Cr do not contribute to water hardness.

Therefore, none of the given pairs precisely represent the ions responsible for water hardness. However, if you were to choose the closest option, it would be B. Ca2+ and HCO3-.