Which of the following ions will cause hardness in water?

Which of the following is a bad conductor of electricity?

Which of the following methods is the best test to detect the presence of water in a liquid?

Find the number of $CORRECT$ order against mentioned property :-
$(i)$ $D_2O = H_2O$ (number of electrons)
$(ii)$ $D_2O > H_2O$ $(|\Delta H_f|)$
$(iii)$ $D_2O > H_2O$ $(|\Delta H_{vaporization}|)$
$(iv)$ $D_2O > H_2O$ (density)
$(v)$ $D_2O > H_2O$ (Molecular mass)

Which pairs of ions are responsible for hardness of water?

Which of the following is a good scientific method to detect the presence of water in a liquid?