$D_2O$ is used more in

Choose the correct statement about various water softening methods.

Give an example of a substance that can exist in different physical states. State its physical properties,chemical composition,and characteristics.

The products formed when heavy water is reacted with magnesium nitride are:

Give reasons.
$(i)$ Lakes freeze from top towards bottom.
$(ii)$ Ice floats on water.

Permanent hardness due to $Mg^{2+}$ ions is best removed by