Problem 180 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: Detergents with low CMC are more economical to use Reason: Cleansing action of detergents involves the formation of micelles. These are formed when the concentration of detergents becomes equal to CMC.

Step-by-Step Solution

Answer

The assertion and reason are correct, and the reason is the correct explanation of the assertion.

Step 1: Identify the Assertion and Reason

The assertion states that detergents with low critical micelle concentration (CMC) are more economical to use. The reason provided is that the cleansing action of detergents involves forming micelles, which occurs when the detergent concentration reaches the CMC.

Step 2: Understand the Concepts of CMC and Micelles

The critical micelle concentration (CMC) is the concentration at which detergents start forming micelles, which are aggregates of detergent molecules. Micelles are crucial for the cleansing process as they trap and remove dirt and grease. A lower CMC means that fewer detergent molecules are needed to form micelles, suggesting resource efficiency.

Step 3: Analyze the Connection Between Assertion and Reason

The assertion concludes that detergents with a low CMC are economical because fewer detergent molecules are required to achieve effective cleaning. The reason supports this by explaining the significance of micelle formation in the cleaning process, which directly depends on reaching the CMC.

Step 4: Evaluate the Combination of Assertion and Reason

Both assertion and reason are true. The reason (importance of micelle formation at CMC) directly explains why detergents with a low CMC are economical, as they require less detergent for effective cleaning, reducing the overall detergent use.

Key Concepts

Detergent Chemistry

Detergents are special cleansing agents we use every day. They are chemicals that help in removing dirt and grease, making things clean. Detergents contain surfactants, which are crucial because they interact with water and oils. These surfactants have two ends: one that likes water (hydrophilic) and one that repels water but likes oils (hydrophobic).

This unique structure allows detergents to break up grease and dirt. When you mix detergent with water, it can surround stubborn grease, making it easier to wash away. The hydrophobic end attaches to the oils while the hydrophilic end sticks to water molecules. This process traps dirt and carries it away when rinsed.

Therefore, understanding detergent chemistry helps comprehend how they function in everyday cleaning.

Micelles Formation

Micelles are like tiny spheres formed in a solution of soap or detergent. Imagine each detergent molecule as having a head and a tail. The head loves water, while the tail does not. When detergent concentration reaches a certain point, known as the Critical Micelle Concentration (CMC), these molecules group into micelles.

In this formation, the tails hide away from water inside the sphere, while the heads face the water outside. Micelles can capture oils and grease within these internal spaces, effectively making them easier to remove from surfaces. Thus, micelle formation is crucial for detergent effectiveness.

Importantly, a low CMC means this micelle structure forms with fewer detergent molecules. As a result, it is more efficient and economical, since less detergent is needed to achieve effective cleaning.

Cleansing Action in Chemistry

The cleansing action of detergents relies heavily on the formation of micelles. When you wash dishes, clothes, or your hands, the formation of micelles is what truly makes them clean. This action involves several key steps:

• When detergent is added to water, it reduces the water's surface tension, allowing better penetration.
• As the concentration reaches CMC, micelles form, trapping and dissolving dirt and grease.
• Finally, rinsing the surface takes away the micelles, along with the captured debris.

Without micelles, most dirt and grease would remain stuck on surfaces. So, the science of micelle formation is at the heart of why detergents are so effective. Lower CMC detergents, therefore, offer more economical solutions, as they can do the same job with less product.