Question

Gases have

• A. infinite number of free surfaces
• B. only one free surface
• C. two free surfaces
• D. no free surface

Answer 1

The correct option is D

no free surface

Correct option (d): no free surface.

Free surface of gases

1. Gases do not have definite shape and volume.
2. There is almost no force of attraction between the molecules of a gas.
3. The dispersed nature of gas molecules does not allow the gases to have a free surface.
4. The molecules of gases possess translatory motion and move in a random direction.
5. Therefore, they do not have a fixed position and no free surface can be observed.
6. In terms of a solid, liquid, or gas, the free surface is the surface where surface tension occurs.

Explanation for incorrect options

Option (a)

1. Solids do have a fixed shape and fixed volume.
2. The intermolecular force of attraction between the molecules of solids is high.
3. As the molecules of solid are fixed at their positions due to the high intermolecular force of attraction.
4. They do not possess translatory motion.
5. Due to the fixed position of molecules, solids have many free surfaces.
6. In terms of a solid, liquid, or gas, the free surface is the surface where surface tension occurs.
7. The surface that is free or in contact with air has a balance of force to maintain it.
8. For example, brick is made up of mud and this mud is made up of small particles like dry dirt and some amount of water.
9. When masonry makes brick, by which we can initiate the building design.
10. This small brick will convert into a big building with an infinite number of surfaces.

Option (b)

1. Liquids have fixed volume but they do not have a fixed shape.
2. Intermolecular force of attraction between liquid molecules is less as compared to solids but the molecules are bound by a force of attraction.
3. Due to the presence of this intermolecular force, liquid has only one free surface at its top layer.

Thus, gases have no free surface.

Hence, option (d) is correct.