Complete the following statements about the effect of intermolecular forces on the physical properties of a substance. (a) The higher the boiling point of a liquid, the (stronger, weaker) are its intermolecular forces. (b) Substances with strong intermolecular forces have (high, low) vapor pressures. (c) Substances with strong intermolecular forces typically have (high, low) surface tensions. (d) The higher the vapor pressure of a liquid, the (stronger, weaker) are its intermolecular forces. (c) Because nitrogen, \(\mathrm{N}_{2}\), has (strong, weak) intermolecular forces, it has a (high, low) critical temperature. (f) Substances with high vapor pressures have correspondingly (high, low) boiling points. (g) Because water has a high boiling point, it must have (strong, weak) intermolecular forces and a correspondingly (high, low) enthalpy of vaporization.

The boiling point of a liquid is a telltale sign of the strength of its intermolecular forces. In simple terms, it's the temperature at which a liquid turns to gas. This process happens when the vapor pressure of the liquid matches the pressure of the surrounding environment. Strong intermolecular forces mean that the molecules in the liquid hold on to each other tightly, making it harder for them to move into the gas phase. As a result, a higher temperature (thus more energy) is needed to boil the liquid. That's why substances with strong intermolecular forces, like water, have a higher boiling point.

For students puzzling over why a substance boils at a certain temperature, think of it as a tug-of-war between the molecules and heat. When the forces among the molecules are very strong, heat has to work harder (i.e., you need more of it) to get those molecules to let go and vaporize.

Vapor pressure might sound complex, but it's essentially a measure of a substance's eagerness to transform from liquid to vapor. When the intermolecular forces within a liquid are weaker, molecules can escape more easily, leading to higher vapor pressure. On the flip side, stronger forces mean fewer molecules can break away, resulting in lower vapor pressure.

If you imagine the surface of a liquid as a tightly packed party, a high vapor pressure means it's very easy for guests (molecules) to leave the party (the liquid), whereas a low vapor pressure indicates that the partygoers are having too much fun together to leave easily.

Picture trying to break the surface of a still pond with your hand; the slight resistance you feel is due to surface tension. It's a liquid's ability to behave like an elastic sheet. This occurs due to strong intermolecular forces acting between molecules at the liquid's surface. They're a bit like a team of acrobats holding hands to form a strong net. The stronger the intermolecular forces, the higher the surface tension, making it harder to penetrate the surface.

For example, water beads on a waxy car hood because water's strong hydrogen bonds create high surface tension, preventing the liquid from spreading out.

At the critical temperature, no matter how much you squeeze a gas (increase the pressure), you can't make it a liquid; it's the point of no return. Substances with weak intermolecular forces like nitrogen have lower critical temperatures. That's because their molecules can easily wander away from each other, and at a certain temperature, no amount of pushing or compressing can bring them back together into a liquid.

Thus, the critical temperature gives us a clue about the strength of a substance's intermolecular forces: low critical temperature equals weak forces, and vice versa.

The enthalpy of vaporization is a fancy term for the heat energy required to turn a liquid into a gas at a given pressure. It's closely tied to the boiling point and intermolecular forces. A high enthalpy of vaporization means it takes a lot of energy to vaporize the liquid, which in turn means the liquid must have strong intermolecular forces holding it together.

To understand this, you can think of it as the energy needed to get all the molecules to 'un-stick' from each other. Water, with its strong hydrogen bonds, has a high enthalpy of vaporization compared to many other liquids because those bonds take a lot of energy to break.