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पाणी उकळेपर्यंत (100° सेल्सियस) गरम केल्याशिवाय त्याची वाफ होत नाही. मग 20°-30° तापमानाच्या समुद्रातून वाफ वायुमंडलात कशी जाते?

Does water evaporate before it is boiled (100 degrees C)? If not, how does vapor from sea water enter the atmosphere at 20-30 degrees C?

Physics Maharashtra

Yes, water from the seas, rivers and oceans evaporates at the atmospheric temperature of 20o-30oC. Apart from water bodies, we can also observe washed utensils or clothes dry at normal room temperature. However, this is different from what we know about boiling of water, which happens at 100oC. So, if water changes its state from liquid to gas at 100oC, how is it that water around us evaporates at temperatures much lower than that? Let's try to understand this by observing the similarities and differences between boiling and evaporation. During both boiling and evaporation, water in the liquid state is changed to its gaseous state. 

Now, let's think about the differences: One major difference between these processes is the speed at which they take place. You might have noticed that if you leave a cup of water out it will evaporate in a couple of days. However, if you take the same amount of water and boil it, it might disappear in a few minutes. You can test this yourself by doing this simple experiment. 

You might have also observed that when you boil water, bubbles start appearing at the bottom and sides of the vessel. Have you wondered what are they made of? They are made of water vapour. Have you observed any such bubbles appearing in the water which is simply left to evaporate? What does this mean? 

Heat or energy is needed for water to change its state from liquid to water vapour. Water boils when heat is provided by a heat source, like a flame. The flame heats up the base of the vessel. This explains why bubbles appear at the bottom of the vessel - which heats up first during boiling. During evaporation, the heat is provided by the environment. A glass of water might look very still, but the molecules in the water are in continuous motion. They move at different speeds. The faster moving molecules, if they happen to be near the surface of water, can move away easily into the air. The sun's heat may also make the molecules at the surface of the water move slightly faster, causing them to easily escape the liquid water. This is what we call evaporation. Evaporation occurs at the surface of the liquid whereas boiling is a bulk phenomenon. This also explains why evaporation is a much slower process than boiling and also why we don't see any bubbles during evaporation. Evaporation is also faster if the air around has relatively less water vapour. 

Now that we know this, can we explain why clothes dry better in summer vs say winter or rainy season? Also, if you keep a glass of water for a few days and carefully note the drop in the water level due to evaporation with the help of a marker, do you think this drop will be the same every day? Why don't you try this experiment yourself and find out more. 



Mrinal Shah


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