Question

Heavy water ${D_2}O$ freezes at?
a.) $ - 3.8^\circ C$
b.) $3.8^\circ C$
c.) $0^\circ C$
d.) $38^\circ C$

Answer 1

Hint: Freezing point of any liquid is the temperature at which the given liquid freezes. It is the temperature for a particular liquid when the liquid turns into the solid state when it undergoes cooling.

Complete step by step solution:
The temperature for the heavy water at which it will undergo freezing (i.e. it will start to convert itself into the solid state) is $3.8^\circ C$ and in the kelvin scale it is $275.8K$.
Hence option (B) is the correct answer.

This freezing point of the heavy water is higher than the freezing point of the ordinary water, this is because of the molecular structure of the deuterium found in the heavy water.
The physical and chemical properties of the heavy water and water are quite different. The molecular structure of the heavy water and the ordinary water is also quite different. In the ordinary water each hydrogen atom has just a single proton in its nucleus while in the heavy water, each hydrogen water is heavier, having a neutron as well as a proton in its nucleus. If we name the heavy water more scientifically then the heavy water is called deuterium oxide because it is having the isotope of the hydrogen called deuterium.
So, the reason behind the freezing point of the heavy water at higher temperature is that the more massive bound deuterons have less zero-point energy as compared to the hydrogen atom of the ordinary water.

Note: The extra neutron in the isotope of the hydrogen atom (deuterium) makes the heavy water denser which can be seen in the by doing some practical activity. If we drop a cube of ice of heavy water in a glass full of the ordinary water then we will see that the ice cube of the heavy water is sinking in the glass of the ordinary water.