Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Surface tension of water is very high (72.7dynes/cm) compared to other liquids.
Reason (R) : High attraction of water molecules for each other is because of H bond.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Water's high surface tension and cohesive properties are essential for soil water movement. Together with adhesion to soil mineral surfaces, they drive the capillary rise of water through narrow soil pores.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Surface tension is the cohesive force at the surface of a liquid that causes its surface layer to behave like an elastic sheet.
This property is determined by the strength of the intermolecular attractive forces between the liquid's molecules.

Step 3: Detailed Explanation:
Let us evaluate both statements:
- Assertion (A): This statement is correct.
Water has an exceptionally high surface tension of $72.7 \text{ dynes/cm}$ at $20^\circ\text{C}$ compared to most other common organic liquids (such as ethanol, which has a surface tension of around $22 \text{ dynes/cm}$).
- Reason (R): This statement is also correct.
Water molecules ($\text{H}_2\text{O}$) are highly polar.
The highly electronegative oxygen atom draws electron density away from the hydrogen atoms, creating a strong partial negative charge on the oxygen and partial positive charges on the hydrogens.
This polar structure allows water molecules to form strong hydrogen bonds with up to four neighboring water molecules.
These strong cohesive hydrogen bonds hold the water molecules together tightly, requiring significant energy to break them at the surface, which produces the high surface tension.
Therefore, the presence of hydrogen bonding (R) directly explains why water has such a high surface tension (A).
Both statements are true, and (R) is the correct explanation of (A).

Step 4: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A), corresponding to option (A).