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Question: Which one has the lowest boiling point? (A). \({H_2}O\) (B). \({H_2}Se\) (C). \(N{O_2}\) (D)...

Which one has the lowest boiling point?
(A). H2O{H_2}O
(B). H2Se{H_2}Se
(C). NO2N{O_2}
(D). All have same boiling point

Explanation

Solution

The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure which is surrounding the liquid. Also, at this pressure, the liquid changes into the vapor.

Complete step by step answer:
(A). Water (H2O)\left( {{H_2}O} \right) molecules from hydrogen bonds with each other. The partial negative charge at 0 atom of one molecule can form a hydrogen bond with the partial positive charge on the hydrogen atom of the other molecule. However, the hydrogen and oxygen atoms are attached to each other by covalent bonds within a molecule. The boiling point of water is 100∘c100^\circ c.
(B). Hydrogen selenide (H2Se)\left( {{H_2}Se} \right) is an inorganic compound. H2Se{H_2}Se exhibit dipole-dipole intermolecular forces between its two molecules.
(C).The molecules of (NO2)\left( {N{O_2}} \right) nitrogen chloride are bonded to each other by covalent bonds.
(D). Since, all these molecules have different intramolecular bonding, thus their boiling points cannot be same.
However, the dipole-dipole interactions formed between H2Se{H_2}Se molecules are weak as compound to that hydrogen bonding and covalent bonding in H2O{H_2}O and NO2N{O_2} respectively. We already know that, weaker is the molecular forces, easier is it to break the bond even with less amount of energy and thus lesser will be its boiling point.

Hence, the answer is option (B).

Note: As we increase temperature of a liquid, the molecules tend to gain more kinetic energy and thus random movement and vibrations of atoms also increases within a molecule. Due to these vibrations, the intermolecular forces vary considerably and thus the strength of the forces is reduced. Hence, the temperature at which this process takes place is known as boiling point.
From this discussion, it can be concluded that, greater are the intermolecular forces, higher will be the boiling point of the liquid and vice-versa.

The boiling point of a liquid depends on temperature, atmospheric pressure and the vapor pressure, when the atmospheric pressure becomes equal to the vapor pressure of the liquid, then boiling will begin.