Problem 979 - JEE Physics

Question

Soap helps in cleaning because (A) chemicals of soap change (B) It increase the surface tension of the solution. (C) It absorbs the dirt. (D) It lowers the surface tension of the solution

Step-by-Step Solution

Answer

Soap helps in cleaning by lowering the surface tension of the solution, allowing water molecules to spread out and penetrate dirt and grease more easily. This is due to soap being a surfactant, with molecules composed of a hydrophilic head and hydrophobic tail that form micelles to trap and wash away dirt. Therefore, the correct answer is (D) It lowers the surface tension of the solution.

Step 1: Define surface tension

Surface tension is a property of liquids that results from the cohesive forces between its molecules. It is defined as the force per unit length acting perpendicular to an imaginary line drawn on a liquid's surface. This causes the liquid to behave like a stretched elastic membrane.

Step 2: Discuss the properties of soap

Soap is a surfactant, meaning it reduces the surface tension of liquid solutions. Soap molecules are composed of a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. When dissolved in water, they arrange themselves in such a way that the hydrophilic heads face towards the water, while the hydrophobic tails face away from the water, forming micelles.

Step 3: Explain the role of soap in cleaning

As a surfactant, soap lowers the surface tension of the water, making it easier for the water molecules to spread out and penetrate the surfaces of dirt and grease. These particles then get surrounded by the soap molecules, with the hydrophobic tails facing towards the dirt and the hydrophilic heads facing away. This structure, called a micelle, traps the dirt and allows it to be washed away along with the soap and water.

Step 4: Choose the correct answer

Based on our discussion, soap helps in cleaning by lowering the surface tension of the solution. So, the correct answer is: (D) It lowers the surface tension of the solution.

Key Concepts

Soap Properties

Soap is a wonder when it comes to cleaning, and it owes its effectiveness to its unique properties. A soap molecule has two distinct parts:

• Hydrophilic Head: This part loves water, meaning it is attracted to and easily mixes with water.
• Hydrophobic Tail: This part avoids water and tends to bind with grease and oil.

When soap is added to water, these molecules arrange themselves. The hydrophilic heads stay close to the water, while the hydrophobic tails try to get away. Because of this arrangement, soap can interact with both water-soluble and water-insoluble substances. This gives soap its ability to capture dirt and grease, making it easier for these impurities to be rinsed away with water.

Surfactants

Surfactants are substances that significantly reduce the surface tension of the liquid they are dissolved in. The term "surfactant" stands for surface-active agent. When you add a surfactant like soap to water, it disrupts the cohesive forces between water molecules. This means that the water molecules no longer hold onto each other as tightly as they did before. As a result, the water is able to spread out more easily and penetrate surfaces better.
This property of surfactants aids in wetting and spreading across surfaces, which is why they are so vital in cleaning products. With lower surface tension, water can reach and clean more effectively, making surfactants essential in household and industrial cleaning.

Micelles

Micelles are fascinating structures formed by soap molecules in water. They come into play when soap molecules cluster together. When enough soap is dissolved, the water-repelling tails of soap molecules tuck themselves inside a spherical formation, while the water-loving heads face outward toward the water. This sphere, called a micelle, effectively traps oils, greases, and dirt particles inside it. The structure of micelles is key in cleaning. Once dirt is trapped inside, it can be easily washed away when rinsed with water. This makes micelles incredibly useful for removing oily stains and residue, as they encapsulate the dirt, preventing it from redepositing on surfaces.