Question

Which of the following is heavy water?
A.${H_2}{O_{18}}$
B.${H_2}{O_{16}}$
C.${H_2}{O_3}$
D.${D_2}O$

Answer 1

All the options look similar or related to the chemical formula of water but deuterium is an isotope of hydrogen. Isotopes are atoms having the same atomic number but have different atomic masses as the number of neutrons are different.

Complete answer:
Let us take a complete look at heavy water but before that let us understand the isotopes of hydrogen. There are certain isotopes of hydrogen. Isotopes are elements having the same atomic number but different mass. There are three isotopes of hydrogen which are protium, deuterium and tritium. All have the same atomic number one. They all have one proton but protium has no neutron deuterium has one and tritium has two. So atomic masses of protium, deuterium and tritium are $1,2,3$ respectively.
Heavy water is a substance similar to normal water but it is made up of deuterium and oxygen. As we have seen deuterium is a heavier isotope than hydrogen that is why this similar looking water molecule is known as heavy water. It is also called deuterium oxide just like water is called hydrogen oxide. So its chemical formula is ${D_2}O$ . Its molar mass is effectively greater than water. Because of deuterium isotope, heavy water has different chemical and physical properties than water. Heavy water is colorless in appearance at standard temperature and pressure. It is an odorless liquid at room temperature. An ice cube made up of heavy water will sink in normal water as the density of heavy water is $11\%$ greater than normal water.

So, option (D) is the correct answer.

Note:
Heavy water is considered not safe for drinking. In nuclear reactors, heavy water is used to slow down neutrons. Out of every twenty million water molecules one of them is heavy water. Heavy water is not radioactive though.