Given that the temperature at which water changes from a liquid to a gas is 100°C, how can a puddle
of water completely evaporate at 28°C?

Answer :

Certainly! Let's explore how a puddle of water can completely evaporate at 28°C even though the boiling point of water is 100°C.

### Evaporation vs. Boiling
Firstly, we need to understand the difference between evaporation and boiling:

- Boiling: This is the process when water changes from liquid to gas not only at the surface but throughout the liquid. Boiling occurs at a specific temperature (100°C at standard atmospheric pressure).

- Evaporation: This process only occurs at the surface of the liquid, and it can happen at any temperature.

### Factors Influencing Evaporation
Evaporation rate depends on several conditions:

1. Surface Area: Larger surface areas allow more water molecules to escape into the air.

2. Temperature: Higher temperatures provide more energy to the water molecules, increasing evaporation even if the temperature is below boiling point. At 28°C, the molecules have enough energy to escape, although fewer than at higher temperatures.

3. Humidity: Lower relative humidity in the air allows more water molecules to evaporate, as there is less water vapor in the air to return into the liquid state.

4. Air Movement: Wind or air movement removes the humid air from over the surface and replaces it with drier air, enhancing evaporation.

5. Pressure: Lower atmospheric pressure can increase the rate of evaporation, though this is less significant compared to the other factors at typical earthly conditions.

### Detailed Explanation

Even at 28°C, a significant number of water molecules will have enough energy to escape the liquid state. Here's a detailed step-by-step breakdown of how water can evaporate at this temperature:

1. Energy Requirement: For a water molecule to evaporate, it needs to overcome the hydrogen bonds holding it in the liquid. While average kinetic energy of water molecules at 28°C is less than at 100°C, some molecules still have high enough energy to break free.

2. Surface Escape: Molecules at the surface are less tightly bound than those within the body of the liquid. Thus, even at lower temperatures, it's the surface molecules that primarily evaporate.

3. Humidity Role: If the surrounding air is dry (low relative humidity), the evaporated molecules do not easily condense back into the liquid. They dissipate into the atmosphere.

4. Wind’s Impact: Wind or any air movement sweeps away the gaseous water molecules from above the puddle, preventing saturation of the air above the water surface and continuously allowing more water to evaporate.

5. Continuous Process: This process continues until the entire puddle evaporates. Depending on the conditions, this might take a longer time compared to when it is at higher temperatures.

### Conclusion
In summary, a puddle of water can indeed completely evaporate at 28°C due to the phenomenon of evaporation, which is influenced by surface area, humidity, air movement, and ambient temperature. The key is that evaporation can occur at any temperature, given suitable environmental conditions, even though it happens more slowly compared to temperatures closer to the boiling point.