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Chapter 11: Problem 11

Explain the surface tension of a liquid in molecular terms. How does the surface tension make a liquid act as though it had a "skin"?

Short Answer

Expert verified
Surface tension is due to cohesive forces pulling surface molecules tightly together, creating a 'skin' effect on the liquid.

Step by step solution

01

Understanding Molecules at the Surface

Surface tension arises due to the cohesive forces between liquid molecules, which are stronger at the surface. Molecules within the liquid experience attraction from all sides, but surface molecules only experience lateral and downward forces.
02

Cohesive Forces Explained

Cohesive forces are the intermolecular forces that hold molecules of the same substance together. At the surface, these forces make the surface molecules pull together more tightly since there are no molecules above to attract them upwards.
03

Resulting Surface 'Skin'

Due to unbalanced cohesive forces, the surface acts like a stretched elastic membrane, often referred to as 'skin'. This 'skin' resists external force and minimizes the surface area, giving liquids like water their characteristic dome shape when droplets form.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cohesive Forces at the Surface
Usually, molecules within a liquid are surrounded by neighbors. These molecules are pulled equally in all directions, which keeps everything in a balanced state. But things are different for molecules at the surface.
At the surface, cohesive forces pull molecules sideways and downwards as there are no molecules above them. This imbalance means the surface molecules are drawn closer together. They form a tighter bond with each other than with the air above.
  • Cohesive forces are strongest among similar molecules, contributing to surface tension.
  • This tight bond at the surface is why water can form droplets and resists spreading out.
Molecular Interactions and Surface Tension
Molecular interactions play a significant role in understanding surface tension. These interactions occur when molecules are close enough to attract each other due to their intermolecular forces.
In a liquid, molecules interact in ways that minimize their energy. At the surface, molecules have fewer neighbors, leading to fewer interactions. To stabilize, these molecules pull together more tightly.
  • The reduced number of interactions at the surface leads to greater cohesive force.
  • These interactions depend on the molecular structure and the type of liquid.
  • Stronger molecular interactions result in higher surface tension.
Understanding Intermolecular Forces
Intermolecular forces are the invisible threads that tie molecules together. These forces include things like hydrogen bonds and van der Waals forces. They are essential for keeping molecules in a liquid closely packed.
The strength of these intermolecular forces determines how a liquid behaves. More potent intermolecular forces mean stronger cohesive interactions. This is why some substances, like water, have high surface tension.
  • Hydrogen bonds are particularly strong and account for water's high surface tension.
  • Van der Waals forces also play a part but are generally weaker.
  • Different liquids have varying intermolecular forces, making their surface tensions different.

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Most popular questions from this chapter

Describe the distinguishing characteristics of a crystalline solid and an amorphous solid.

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