Question

Which of the following phenomena does not involve surface tension? (a) Mercury drops acquire spherical shape. (b) Liquids tend to rise in the capillary. (c) A liquid flows over a fixed surface. (d) Moist soil grains are pulled together.

Short answer

(c) A liquid flows over a fixed surface does not primarily involve surface tension.

Step-by-step solution

Understanding Surface Tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules. It is responsible for the phenomena where the liquid surface behaves like a stretched elastic membrane.

Analyzing the Phenomena

Examine each phenomenon to determine if surface tension plays a significant role. Surface tension is typically involved when the shape of the liquid is a factor, or when forces between liquid molecules and other surfaces or particles come into play.

Option (a) - Mercury drops acquire spherical shape

Surface tension causes mercury drops to minimize their surface area, resulting in a spherical shape. Therefore, this phenomenon involves surface tension.

Option (b) - Liquids tend to rise in the capillary

Surface tension is responsible for the capillary action where liquid rises or falls in a narrow tube. So, this phenomenon involves surface tension.

Option (c) - A liquid flows over a fixed surface

The flow of a liquid over a fixed surface is mainly governed by viscosity and gravity instead of surface tension. This liquid flow could be affected by surface tension but not in a dominant way, so this phenomenon does not primarily involve surface tension.

Option (d) - Moist soil grains are pulled together

Capillary bridges formed by water molecules can pull soil grains together due to surface tension. So, this phenomenon involves surface tension.

Determining the Correct Answer

Based on the analysis of each option, option (c) 'A liquid flows over a fixed surface' is the phenomenon that does not primarily involve surface tension, making it the correct answer.

Key concepts

Mercury Spherical Shape

When observing a drop of mercury, one can't help but notice its striking spherical shape. This is due to the effects of surface tension. Mercury, a unique liquid metal, exhibits a high surface tension, meaning the cohesion between its molecules is exceptionally strong.

Surface tension acts to minimize the surface area of a liquid. In the absence of other forces, the shape with the smallest surface area for a given volume is a sphere. Thus, when mercury drops are free from external forces, they naturally form into tight, almost perfectly round spheres. The phenomenon can be easily observed when mercury is dropped on a surface, where it does not flatten out like water but maintains its rounded shape.

In the educational context, connecting this everyday observation to the molecular interactions in liquids can reinforce students' understanding of the underlying scientific principles at play.

Capillary Action

Capillary action is a fascinating display of surface tension at work, often seen when a liquid travels up a narrow tube, against the force of gravity. It's a result of the adhesive forces between the liquid and the tube material, as well as the cohesive forces within the liquid itself.

As molecules of liquid adhere to the sides of the narrow passage, they pull other molecules along due to the intermolecular cohesion, leading to the liquid’s ascent. This mechanism is central to the movement of water in plants, from the roots to the leaves. In a classroom setting, demonstrations using water and thin tubes or paper towels can effectively showcase this phenomenon and help bridge the gap between theoretical concepts and real-world observations.

Cohesive Forces in Liquids

The concept of cohesive forces in liquids explains the tendency of a liquid's molecules to stick together. These intermolecular attractions are what give a liquid its shape and are crucial to the phenomenon of surface tension. In essence, cohesive forces are the 'glue' that holds a liquid together, resisting external forces.

Cohesive forces are responsible for the liquid being able to contract and form droplets, as well as the reason why some insects can walk on water without sinking. Without cohesive forces, liquids would not have the ability to form into defined shapes and could not exhibit behaviors such as beading up or forming waves. Instructional experiments, such as dropping different liquids onto a surface to compare how they spread or bead can be instructive and visually impactful ways to demonstrate these principles to students.