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

Permanent hardness of water can be removed by

Identify the incorrect statement regarding heavy water $(D_2O)$.

On strong heating,$ZnCl_2 \cdot 2H_2O$ gives .......

Observe the following statements:
$1.$ Heavy water is harmful for the growth of animals.
$2.$ Heavy water reacts with $Al_4C_3$ and forms $\text{deuterated acetylene}$.
$3.$ $BaCl_2 \cdot 2D_2O$ is an example of $\text{interstitial deuterate}$.
The correct statements are:

The maximum number of hydrogen bonds formed by a water molecule in ice is