Question

rank the elements or compounds in the table below in decreasing order of their boiling points. that is, choose 1 next to the substance with the highest boiling point, choose 2 next to the substance with the next highest boiling point, and so on.

substance	a	b	c	d
boiling point	select <	select <	select <	select <

Explanation:

Step1: Analyze intermolecular forces

Boiling - point is related to the strength of intermolecular forces. $H_2O$ has hydrogen - bonding, dipole - dipole forces and London dispersion forces. $HCl$ has dipole - dipole forces and London dispersion forces. $HgCl_2$ has London dispersion forces and some ionic character due to the electronegativity difference between $Hg$ and $Cl$. $H_2$ has only London dispersion forces.

Step2: Rank based on intermolecular forces strength

Hydrogen - bonding is the strongest among these intermolecular forces in this set. So, $H_2O$ has the highest boiling point. Among $HCl$ and $HgCl_2$, $HCl$ has a smaller molar mass compared to $HgCl_2$. Generally, for non - hydrogen - bonded substances, the higher the molar mass, the stronger the London dispersion forces. So, $HgCl_2$ has stronger London dispersion forces than $HCl$. And $H_2$ has the weakest intermolecular forces due to its non - polar nature and low molar mass.

Answer:

1. $H_2O$
2. $HgCl_2$
3. $HCl$
4. $H_2$