Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

A drop of liquid acquires a spherical shape because of (a) its tendency to maximize its surface area (b) its tendency to acquire minimum surface area (c) its viscous nature (d) none of these

Step-by-Step Solution

Answer

The drop acquires a spherical shape due to its tendency to acquire minimum surface area, so the answer is (b).

Step 1: Understand the Concept of Surface Tension

To begin with, we should understand that a liquid drop tends to minimize its potential energy. Surface tension is the property that causes liquids to contract to the smallest possible surface area for a given volume, resulting from cohesive forces within the liquid.

Step 2: Analyze Each Option

(a) Maximizing surface area is contrary to the liquid's behavior, as it would lead to higher potential energy. (b) Minimizing surface area aligns with the property of surface tension, aiming for the least surface area to minimize energy. (c) Viscosity is related to the resistance to flow and not directly to the shape of a liquid drop. (d) None of these would imply none of the options are correct, but option (b) is valid.

Step 3: Select the Correct Option

Given the liquid's tendency to minimize potential energy through surface tension, the correct answer is the option that mentions minimizing the surface area, which is option (b).

Key Concepts

Cohesive Forces

Cohesive forces are the attraction forces between like molecules. In the context of liquids, it's these forces that cause molecules to 'stick' together.
Think of water; its molecules are attracted to each other thanks to hydrogen bonds, which are a type of cohesive force. This attraction is what gives water its unique properties, like its ability to form droplets.
Surface tension, the phenomenon seen in liquid drops forming into spheres, results directly from these cohesive forces.
To help you visualize how cohesive forces work, imagine a blanket being pulled tightly over a group of people. It tries to minimize the space used, just like surface tension does with liquids. Here are key points to remember about cohesive forces:

• They are strong between like molecules.
• They create surface tension by trying to minimize surface area.
• These forces dictate much of the liquid's behavior, including droplet formation.

Potential Energy Minimization

Potential energy minimization is the driving force behind the spherical shape of liquid drops.
When in a droplet form, liquids aim to achieve the lowest energy state possible. A sphere naturally has the smallest surface area for any given volume, which means it involves the least amount of potential energy.
This urge to minimize potential energy, when combined with cohesive forces, is why we see drops settle in a spherical shape.
Breaking this concept down:

• Potential energy in liquids is lower when the surface area is minimized.
• This is because less surface area means fewer opportunities for disruptive forces or 'disturbances.'
• By forming into a sphere, the liquid is trying to reach its most stable structure.

Just imagine how you might curl into a ball to keep warm and conserve energy. It's similar to how liquids behave when minimizing potential energy.

Viscosity

While surface tension and potential energy minimization are related to shape, viscosity concerns liquid movement. It describes a fluid's internal resistance to flow.
In simpler terms, if you think of liquids as ants on the move, viscosity is the crowding of ants when they're trying to get from one place to another. The more crowded, the slower the movement.
Different liquids have varying viscosities. For instance, honey is much more viscous than water, which is why it flows more slowly.
Key aspects of viscosity include:

• It is a measure of a fluid's resistance to deformation or flow.
• Higher viscosity means a thicker fluid, like syrup or oil.
• While it affects flow and movement, it doesn't directly determine the shape of a liquid.

Understanding these properties of liquids can help explain various practical phenomena, such as why water flows quickly from a bottle, but ketchup needs a bit of help.