Question

Read the assertion and reason carefully and mark the correct option given below. (a) If both assertion and reason are true and the reason is the correct explanation of the assertion. (b) If both assertion and reason are true but reason is not the correct explanation of the assertion. (c) If assertion is true but reason is false. (d) If the assertion and reason both are false. Assertion: The concept of surface tension is held only for liquids. Reason: Surface tension does not hold for gases. (A) a (B) \(b\) (C) \(\mathrm{c}\) (D) \(\mathrm{d}\)

Short answer

The correct option is (a) because both the assertion and reason are true, and the reason is the correct explanation of the assertion. Surface tension is held only for liquids due to the attractive forces between liquid molecules, while gases do not have a well-defined surface and no significant cohesive forces between their molecules.

Step-by-step solution

Defining surface tension

Surface tension is a property of liquids that relates to the cohesive forces between liquid molecules at the surface of a liquid. At the surface, the attractive forces of the liquid molecules cause the surface to contract, creating a force that appears as if the surface were covered by a stretched elastic film.

Surface tension in liquids

Surface tension is an important property in liquids because it causes liquids to contract to form spherical shapes, which minimizes their surface area. Surface tension plays a crucial role in various everyday phenomena, such as the formation of water droplets and soap bubbles.

Surface tension in gases

Gases do not have a well-defined surface, and their molecules are in constant motion with no significant cohesive forces between them. Due to the lack of attractive forces between gas molecules, surface tension is not a relevant concept in gases.

Evaluating the assertion and reason

The assertion is correct—the concept of surface tension is held only for liquids. The reason is also correct—surface tension does not hold for gases. The reason provides a correct explanation for the assertion.

Choosing the correct option

Since both the assertion and reason are true, and the reason is the correct explanation of the assertion, the correct option is (a).

Key concepts

Cohesive Forces

Cohesive forces are the attractive forces that occur between like molecules in a substance. Imagine the way magnets pull together; cohesive forces work similarly but at a molecular level. In liquids, these forces are especially important because they hold the molecules together, allowing the liquid to maintain a consistent structure. These cohesive forces are responsible for phenomena like water droplets forming beads on a surface.

In simple terms, the molecules within a liquid like to "stick" to each other, and this attraction is what holds them together. This 'stickiness' is stronger at the surface, where the liquid meets the air, leading to the interesting effects of surface tension. In comparison, gases experience much weaker cohesive forces because their molecules are often moving freely and are much further apart.

Understanding cohesive forces helps explain why liquid follows the shape of a container but can form droplets when spilled. It points to how molecular attraction varies between different states of matter.

Liquids and Surface Tension

Surface tension is an intriguing property of liquids. It can almost be visualized like a thin skin on the liquid’s surface. This 'skin' forms because of the cohesive forces between the liquid's molecules. When the molecules at the surface experience attractive forces inward, the liquid tends to contract into shapes with the smallest possible surface area, like spheres.

Here’s a practical example: when you see beads of water on a car bonnet, you are witnessing surface tension. The cohesive forces pull the surface molecules tightly together, making the water try to minimize its surface area by forming a rounded bead shape. Surface tension is critical in other situations too: think about soap bubbles or insects that can walk on water.

Cohesive forces ensure that the surface tension only applies to liquids, as these forces are significant enough to create a noticeable effect, unlike in gases. This idea shows how different properties of liquids create observable behaviors around us every day.

Gases and Surface Tension

Unlike liquids, gases do not exhibit surface tension because they lack the strong cohesive forces necessary to create a defined surface. In gases, molecules move rapidly and are spread far apart compared to liquids, which means they rarely interact with enough strength to create any "surface."

Gases are more about freedom of movement. The lack of a fixed shape or surface means gases can't generate surface tension like liquids do. This absence is why the assertion—"surface tension is held only for liquids"—is true, and the reason—"surface tension does not hold for gases"—follows logically.

This difference in behavior between gases and liquids illustrates the diverse nature of states of matter and shows how specific molecular structures and interactions can completely alter physical properties. Understanding this helps to appreciate the unique ways that molecules behave in different environments.