Predict which substanoe has the greater viscosity in its liquid form at \(0^{\circ} \mathrm{C}\) : (a) ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), or dimethyl ether, \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\); (b) butane, \(\mathrm{C}_{4} \mathrm{H}_{10}\) or propanone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\).

When we describe how particles interact within a substance, we're talking about intermolecular forces, the forces that occur between molecules. These forces are responsible for many physical properties of substances, including boiling point, melting point, and as highlighted in the textbook exercise, viscosity.

Intuitively, you can think of these forces like the 'glue' holding molecules together: stronger 'glue' means it's harder for the molecules to move past each other, leading to higher viscosity. There are several types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Each has a distinct impact on how molecules behave in different states of matter.

Understanding these interactions allows us to predict the physical properties of substances without necessarily conducting experiments, which makes them invaluable in the disciplines of chemistry and materials science.

Let's delve into hydrogen bonding, which is a type of strong intermolecular force that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. This setup creates a significant polarity within the molecule, and the hydrogen atom becomes a partially positive end, which is then attracted to the negative end of another electronegative atom in a neighboring molecule.

Hydrogen bonds are the reason why water has such a high boiling point for its molar mass and also contribute to the 'stickiness' or high viscosity of substances like ethanol. They're much stronger than most other intermolecular forces and can greatly affect a molecule's physical properties. On a practical level, this is why you might observe that syrup, which forms hydrogen bonds, pours much slower than something like cooking oil, which does not.

Lastly, let's explore the London dispersion forces. These forces are the weakest of all intermolecular forces and are present in all molecules, regardless of their polarity. London dispersion forces arise from the fluctuations in the electron distribution within molecules, creating temporary dipoles that induce dipoles in adjacent molecules.

Even though they're individually weak, these forces can add up and be significant, especially in larger atoms or molecules where there are more electrons that can contribute to the effect. Substances with more of these forces can be stickier or thicker, meaning they have a higher viscosity. For instance, gasses at room temperature can be liquefied under high pressure, significantly increasing their London dispersion forces due to closer molecule proximity, resulting in higher viscosity.