why evaporation is called a surface phenomenon

Question

Please answer for class 9th

Chapter- Matter

 

Answer 1

The basic principle of evaporation is that molecules of a liquid gain energy, usually through absorbing sunlight, and over time they get more excited as the energy from the sun is converted to kinetic energy. Once the molecules reach a threshold amount of kinetic energy, they evaporate into their gaseous phase. This is a surface phenomenon because more energy is absorbed by molecules closer to the surface, preventing it from being absorbed by molecules further down. Evaporation happens from the top down because molecules of liquid are only able to gain enough energy to evaporate when the molecules that were above them have already evaporated away. This differs from boiling, the other form of vaporization, because boiling involves heating all molecules to a uniform "boiling point" temperature, giving them all enough energy to vaporize, not just the ones on top. Evaporation can occur at room temperature if there is another source of energy, such as the sun, but it is a much slower process.

The rest of this is bonus information. This explanation ignores some complicated things like molecular motion and treats each molecule as if it remains in the same layer of molecules throughout the whole process. In reality, each molecule is constantly moving around, and the same molecules do not stay on the top layer the whole time. However, the simpler explanation (or "statistical model") is a good enough explanation for pretty much any practical application because we work in amounts of liquid that have around 10^23 molecules in them, erasing any inaccuracies of a statistical approach. If that was confusing, feel free to ignore the whole second paragraph.