Question

Which one has the lowest boiling point?
A. \[{{\text{H}}_{\text{2}}}{\text{O}}\]
B. \[{{\text{H}}_{\text{2}}}{\text{Se}}\]
C. \[{\text{N}}{{\text{O}}_2}\]
D. All have same boiling point

Answer 1

Hint: For this we need to know the type of interaction that these molecules have. Since boiling point depends upon the interaction between molecules. Greater the interaction between two molecules of the same compound, higher will be the boiling point.

Complete step by step answer:
- Boiling point is a point where the liquid and gas phase are present in equilibrium with each other. The boiling point depends on various factors one of them is the nature of interaction between the molecules. We will need to check the nature of interaction between these molecules. Greater the interaction between the molecules, more energy or heat needed to be provided to break the interaction. Hence higher will be the boiling point and the reverse is also true.
- In water molecules due to the presence of hydrogen attached with a more electronegative element that is oxygen, hydrogen bonding exists. While in the other two dipole-dipole interactions exists. Since hydrogen bonding is much stronger than dipole-dipole interaction, water has the highest boiling point.
- Now coming to \[{{\text{H}}_{\text{2}}}{\text{Se}}\] and \[{\text{N}}{{\text{O}}_2}\]. \[{\text{N}}{{\text{O}}_2}\] has more strong dipole-dipole interaction between their molecules because of the presence of more electronegative elements such as nitrogen and oxygen. So, it has a higher boiling point than \[{{\text{H}}_{\text{2}}}{\text{Se}}\].

So, option B is correct.

Note:
No doubt boiling point is a fixed point but only at constant external pressure. When external pressure changes, the boiling point of the same substance changes. We can understand this with the help of an example. Water boils or food cooks earlier in higher altitude regions compared to lower plains. This is because pressure decreases on higher altitude and hence boiling point decreases.