At first glance, we would be eager to jump to ionic bonding as the correct answer, as ionic bonding provides for very high boiling points. The correct answer, however, is a rare type of intermolecular force called network covalent bonding. Network covalent bonding is typically seen in diamond and quartz, and is a stronger intermolecular force than ionic bonding. Hydrogen bonding is the next strongest intermolecular force and also increases the boiling points of pure substances.

In general, increased intermolecular interraction and higher magnitude of intermolecular forces lead to an increase in a molecule's boiling point. Inversely, decreased intermolecular interraction and lower magnitude of intermolecular forces lead to a decrease in a molecule's boiling point.

In this case, the only intermolecular force exhibited by any of these molecules are London dispersion forces. The magnitude of London dispersion forces decreases with a decrease in molecule size (carbon chain length and molecular surface area). Therefore, the shortest, most branched molecule in this problem will have the lowest boiling point. The correct answer is isobutane, a four membered, branched hydrocarbon.

When discussing boiling points of hydrocarbons, it is important to remember that branching decreases a molecule's boiling point. We can first eliminate hexane and pentane as our answers, as neither are branched. From here, we can come upon 2,3-dimethylbutane as our answer because it is more branched than 2-methylpentane. Also important when ranking hydrocarbons in terms of boiling point is the number of carbons - more carbons means a higher boiling point.

Most polar (II) has highest boiling point due to hydrogen bonds. The other molecules: increasing boiling point with decreased branching of the molecule (the more branched, the less surface area, and the lower the boiling point due to molecular stacking).

The strongest of those listed s hydrogen bonding. This type of intermolecular force is the attraction that occurs between hydrogen atoms and the lone pairs on atoms of oxygen, nitrogen and/or fluorine. Hydrogen bonds are the strongest while dispersion forces are the weakest. The strength of hydrogen bonds is responsible for properties of water such as high specific heat capacity, high surface tension, cohesion, high boiling point, and other.

Intermolecular bonds occur between adjacent molecules (recall that ‘inter’ means ‘between’). There are several types of intermolecular bonds. Hydrophobic bonds, or van der Waals forces, are the weakest intermolecular forces and occur between every molecule; therefore, all of the listed molecules in the question have hydrophobic bonds. Polar interactions between molecules occur between charged species or polar molecules. Recall that polar molecules are molecules that contain two or more atoms with very different electronegativities. The more electronegative atom pulls the electrons closer to itself, causing polarity in the molecule. The more electronegative atom will have a partial negative charge (due to the proximity to electrons) and the less electronegative atom will have a partial positive charge. This polarity in molecule allows for dipole-dipole interactions, a type of intermolecular force.

To solve this question, we need to determine which molecules are polar. Hexane, or , has only carbon and hydrogen atoms. Carbon and hydrogen electronegativities are very similar; therefore, this molecule is nonpolar. Hydrofluoric acid () have two atoms with very different electronegativities; therefore, this molecule is polar and will have polar interactions. Similarly, hydrobromic acid () will also be polar and will have polar interactions.

Intermolecular bonds occur between adjacent molecules whereas intramolecular bonds occur within molecules. Examples of intermolecular bonds include hydrogen bonds, van der Waals interactions, and dipole-dipole interactions. The strongest intermolecular bond is hydrogen bond whereas the weakest is the van der Waals interactions. Covalent bonds and ionic bonds occur within molecules and are termed intramolecular bonds. Covalent bond is the strongest intramolecular bond.

As a molecule's mass increases, van der Waal forces also increases due to an increased area for fleeting charged interactions. Thus, the longer carbon chain length in octanal causes a higher boiling point. Because both octanal and butanal can participate in dipole-dipole interactions, this does not differentiate their boiling points, as it would if butane and butanal were compared. Both compounds participate in hydrogen bonding, which will account for their relatively high boiling points, but both molecules share the increased boiling point due to this type of intermolecular force.

Hydrogen bonds are strong intermolecular bonds between hydrogen and one of three atoms: nitrogen, oxygen and fluorine. A typical hydrogen bond occurs between a hydrogen atom on one molecule and one of the three atoms listed on another molecule. These bonds are reversible; however, they serve as strong interactions that stabilize a mixture of molecules.

Recall that boiling is the process of converting a liquid to a gas. This process involves the separation of molecules, which requires breaking the intermolecular bonds; therefore, boiling points depend on the strength of the intermolecular bonds. A stronger intermolecular bond will require more energy to break and, therefore, will have a higher boiling point. The question states that molecule A has the higher boiling point; therefore, molecule A must have stronger intermolecular interactions than molecule B. The strongest intermolecular bond is hydrogen bond, followed by dipole-dipole interactions and van der Waals interactions (weakest).

If we look at scenario I, molecule A is polar and has dipole-dipole interactions and van der Waals interactions (every molecule has van der Waals). It does not have hydrogen bonds because it does not contain nitrogen, oxygen, or fluorine (in addition the the hydrogen atom). Molecule B, on the other hand, has hydrogen bonds in addition to other intermolecular forces; therefore, molecule B has stronger intermolecular forces and a higher boiling point.

In scenario II, hydrogen peroxide has hydrogen bonds whereas diamond only has weak van der Waals interactions; therefore, molecule A has higher boiling point. In scenario III, diamond only has weak van der Waals interactions whereas nitric oxide can participate in dipole-dipole interactions as well (note that nitric oxide doesn’t have hydrogen atom and, therefore, cannot participate in hydrogen bonds). This means that molecule B has the higher boiling point in scenario III.