Question

Pick out the wrong statement(s). (i) Vapour pressure of a liquid is the measure of the strength of intermolecular attractive forces. (ii) Surface tension of a liquid acts perpendicular to the surface of the liquid. (iii) Vapour pressure of all liquids is same at their freezing points. (iv) Liquids with stronger intermolecular attractive forces are more viscous than those with weaker intermolecular force. (a) (ii), (iii) and (iv) (b) (ii) and (iii) (c) (i), (ii) and (iii) (d) (iii) only

Short answer

(b) (ii) and (iii)

Step-by-step solution

Analyze statement (i)

Statement (i) says that vapour pressure of a liquid is the measure of the strength of intermolecular attractive forces. This statement is true. Lower vapour pressure indicates stronger intermolecular forces because fewer molecules are able to escape into the vapor phase.

Analyze statement (ii)

Statement (ii) claims that surface tension of a liquid acts perpendicular to the surface of the liquid. This statement is incorrect, as surface tension actually acts parallel to the surface of the liquid and is due to the cohesive forces between molecules.

Analyze statement (iii)

Statement (iii) asserts that vapour pressure of all liquids is the same at their freezing points. This statement is incorrect. Vapour pressure is a property that depends on the nature of the liquid; different liquids generally have different vapour pressures at their freezing points.

Analyze statement (iv)

Statement (iv) says that liquids with stronger intermolecular attractive forces are more viscous than those with weaker intermolecular forces. This statement is correct; stronger intermolecular attractive forces tend to result in a higher viscosity because the molecules resist flow more effectively.

Pick out the incorrect statement(s)

Based on the analysis of individual statements, statements (ii) and (iii) are incorrect. Therefore, the choices that include both of these statements should be considered.

Determine the correct choice

Choice (b) 'Statement (ii) and (iii)' is the one that correctly identifies the wrong statements.

Key concepts

Vapour Pressure

Understanding vapour pressure is crucial for comprehending the behavior of liquids in various conditions. It is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. In simpler terms, it's a measure of how much the molecules of a liquid tend to escape into the gas phase. A lower vapour pressure indicates that the liquid has stronger intermolecular forces because the molecules are held together more tightly and find it harder to escape into a gas form.

When analyzing the exercise statement, it is essential to grasp that vapour pressure is not only a measure of intermolecular forces but also varies with temperature. For instance, when you warm up a liquid, its vapour pressure increases because the molecules have more energy to overcome the attractive forces between them. Different liquids will exhibit different vapour pressures even at the same temperature due to varying intermolecular strengths.

In the context of the exercise improvement advice, remember that vapour pressure does not become equal for all substances at any given temperature, including their freezing points. Substances with stronger intermolecular forces generally have lower vapour pressures at the same temperature compared to those with weaker forces.

Surface Tension

Surface tension is a fascinating phenomenon found in liquids. If you've ever seen a water droplet bead up on a surface or observed an insect like a water strider glide across a pond, you've witnessed surface tension in action. It arises from the attractive forces among molecules at a liquid's surface. Because the liquid molecules at the surface are not surrounded by similar molecules on all sides, they cohere more strongly to those directly associated with them on the surface.

This cohesive force creates a 'film' at the surface, which resists external force and acts parallel to the surface, not perpendicular as erroneously stated in one of the exercise statements. A liquid with higher surface tension will create more pronounced droplets and resist objects penetrating its surface, illustrating the 'tightening' effect these intermolecular forces can have.

Always bear in mind that surface tension is an essential factor in capillary action, where a liquid can flow in narrow spaces without the assistance of, and even in opposition to, external forces like gravity.

Viscosity

Now, let's talk about viscosity, which can be thought of as a liquid's resistance to flow. Have you ever compared how honey pours out of a bottle to how water flows? Honey moves much slower because it has a higher viscosity than water. The main factor behind this resistance is, again, intermolecular forces.

Liquids with stronger forces between molecules, like honey, will exhibit higher viscosities because the molecules do not slide past one another easily. In contrast, liquids like water, with weaker intermolecular forces, have lower viscosities and flow more freely.

Regarding the exercise, stronger intermolecular attractive forces leading to higher viscosity was correctly identified. When addressing the viscosity of a substance, it's valuable for students to consider both the type and the strength of the intermolecular forces present, as well as temperature effects, since heating up a viscous liquid generally reduces its viscosity, making it flow more easily.