Which of the following is the best scientific method to test the presence of water in a liquid?

Provide general information about water $(H_2O)$.

Hardness of water is due to the presence of salts of

Permanent hardness in water is due to the presence of which of the following ions?

When the first element of the periodic table is treated with dioxygen,it gives a compound whose solid state floats on its liquid state. This compound has an ability to act as an acid as well as a base. What products will be formed when this compound undergoes autoionisation?

Consider the following statements regarding heavy water $(D_2O)$. Which of the following statements is/are correct?
$(A)$ Heavy water is used as a moderator in nuclear reactors.
$(B)$ The power of association is higher in $D_2O$ than in ordinary water $(H_2O)$.
$(C)$ Heavy water is more effective as a solvent than ordinary water.