Question

Which of the following properties indicates very strong intermolecular forces in a liquid: (a) very low surface tension, (b) very low critical temperature, (c) very low boiling point, (d) very low vapor pressure?

Short answer

Very low vapor pressure indicates very strong intermolecular forces.

Step-by-step solution

Understand Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between neighboring molecules. Strong intermolecular forces result in characteristics such as high viscosity, high surface tension, high boiling point, and low vapor pressure.

Analyze Surface Tension

Surface tension is the force that causes the molecules on the surface of a liquid to be pushed together and form a layer. High surface tension indicates strong intermolecular forces because the molecules are strongly attracted to each other.

Analyze Critical Temperature

The critical temperature is the temperature above which a liquid cannot exist, regardless of the pressure. A high critical temperature indicates strong intermolecular forces because more energy (temperature) is required to break the forces keeping the liquid together.

Analyze Boiling Point

The boiling point is the temperature at which a liquid turns into vapor. High boiling points indicate strong intermolecular forces because these forces need to be overcome to transition to the gas phase.

Analyze Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid. Low vapor pressure indicates strong intermolecular forces because fewer molecules have enough energy to escape the liquid phase.

Conclusion Based on Analysis

Given the properties, very low vapor pressure (option d) indicates strong intermolecular forces. This is because strong forces keep the liquid molecules in place, preventing them from entering the vapor phase easily.

Key concepts

Surface Tension in Liquids

When we talk about surface tension, we are referring to the elastic tendency of liquids. This property allows them to acquire the least surface area possible, effectively acting like a stretched rubber sheet on the surface. Surface tension results from the cohesion among liquid molecules, **cohesion** meaning the attraction between like molecules. In water, the molecules are strongly attracted to each other through hydrogen bonds, creating a high surface tension. High surface tension in any liquid is a clear indicator of strong intermolecular forces. Liquids with weak intermolecular forces have molecules that are not as strongly attracted to each other, resulting in lower surface tension. Understanding surface tension helps us conceptualize how and why some liquids bead up on surfaces and can support small objects like insects on their surface without sinking.

Understanding Critical Temperature

Critical temperature is an essential concept when discussing phase changes in substances. It represents the temperature above which a liquid can't be liquefied no matter how much pressure is applied. At temperatures above this point, a substance exists only as a gas. Liquids with a high critical temperature possess strong intermolecular forces, as these forces require more energy, in the form of temperature, to break. Strong forces mean that molecules in the liquid state need a lot of energy to disperse into the gas state. Let's picture water, it has a relatively high critical temperature because of its strong hydrogen bonds. Understanding critical temperature is crucial in fields such as chemistry and engineering, especially in working with gases under varying pressures.

What Determines Boiling Point?

Boiling point is defined as the temperature at which a liquid turns into vapor. It is at this temperature that the liquid's vapor pressure equals the external atmospheric pressure. Liquids with strong intermolecular forces have higher boiling points. Why? Because more energy, in the form of heat, is necessary to overcome the strong attraction between molecules to change them from liquid to gas phase. For instance, compared to acetone, ethanol has a higher boiling point due to its stronger hydrogen bonding. The study of boiling points is foundational in understanding how substances react under heat and in designing processes like distillation in the industry.

Exploring Vapor Pressure

Vapor pressure is the pressure created by the vapor released by a liquid into the air above it, and is a measure of a liquid's tendency to evaporate. At equilibrium, this vapor exerts a pressure that is particular to the temperature and characteristics of the liquid. It helps us understand how volatile a liquid is, which means how easily it evaporates. Liquids with lower vapor pressures display strong intermolecular forces because their molecules are less likely to escape the liquid phase. For example, water has a lower vapor pressure than ether at room temperature, showcasing its strong hydrogen bonds. Recognizing vapor pressure aids in various applications, from designing airtight containers to predicting how a liquid behaves when exposed to different conditions.