Eli5 why water doesn’t go back to liquid ? It need to be 100°C to evaporate but once in the air it is not 100°C anymore so how is it that it stay gaz?


Eli5 why water doesn’t go back to liquid ? It need to be 100°C to evaporate but once in the air it is not 100°C anymore so how is it that it stay gaz?

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Water always evaporates and always condenses back into liquid, but at different speeds dependent on temperature and the availability of liquid and gaseous water. If evaporation is currently faster than condensation then the net effect is that liquid water becomes water vapor. Conversely, if condensation is currently faster than evaporation then water vapor becomes liquid. 100 degrees is the temperature at which evaporation overtakes condensation no matter how much water vapor there already is (at atmospheric pressure). This means that even within a volume of liquid water evaporating molecules will stay gaseous and not condense back – the process which we can witness as boiling.

When water changes to gas, its molecules break the intermolecular forces that hold them together as liquid. Thus forming water vapor. Once in the air, the surrounding temperature and pressure are such that they do not have enough energy to condense back into liquid.
But when comes into contact with a surface that is cooler than the surrounding air vapor can condense back into liquid form.

Water do not need to be at 100°C to evaporate, it needs to be at 100°C to boil at standard atmospheric pressure.

Water will evaporate if it is above 0°C that is because that is the temperature it freezes at in normal atmospheric pressure. Water can still change to gas if it is colder, it is called sublimation. Evaporation is the change from a liquid to a gas, sublimation is solid to a gas.

Boiling is a rapid chang from a liquid to a gas. It occurs when water can produce gas at the same pressure as the atmosphere, which means bubbles can form it is and are not compressed back into a liquid. The bubbles can then rise and push away the air around them. There is not limit to how much water can build if it is not enclosed and can push away the air and water vapor that is produced.

You can build water at room temperature you just need to reduce the pressure. There is room-temperature water boiling in a vacuum chamber. https://www.youtube.com/watch?v=glLPMXq6yc0 Water still needs the energy to become a gas and the result is the temperature of the remaining water drops. If you continue to pump away the vapor it will boil until only a block of ice remains.

Water will the time change from a gas to a liquid and from a liquid to a gas. The rate depends on the temperature so the pressure of the gas the water can produce depend on temperature. The pressure is the same at 1 atmosphere at 100°C In lower pressure like on a mountain peek the boiling temperature is lower, it a pressure cooker the pressure is higher so water can be warmed and remain a liquid

If you boil water in an encoded container the pressure will increase when the water evaporates and you need a higher and higher temperature to boil the water.

There is a graph of the amount of water air can contain at a given temperature.

If you could down moist air so there is more water in it that it can hold it will start to condensate into a liquid. Fog, mist and clouds is just that, air that has got so cold that it can no longer hold all water as gas and some condensate into small liquid droplet. Water as a gas is invisible, if you see something white it is water as small liquid droplets or even solid ice crystals.

If you boil water in a pot is can contender back to a liquid. Do it outdoors in cold air and you see the droplets as a white mist just above the pot. If it is cold outdoors and you build water indoors is can condensate on thr windows that are cooler the walls. It can also condensate on the stove vent, walls etc. If there is enough dry air and enough air motion it might not constant at all, it can just remain as water vapor in the air.

You will have more water as a vapor if you take a warm shower then when you cook food, a foggy mirror, window or another part of a bathroom is an example of air that get cooled down and water condensates back to a liquid.

What keeps water as water is surface tension. When a water molecules has sufficient energy to break free of the surface tension (whether the water is boiling or not) it breaks free of the water and exists in a gaseous state far away from other water molecules. For water to condense back into water it must meetup with some other water molecules. This starts to happen at the dew point. At this temperature there is enough water in the air and the heat energy is low enough that water molecules are likely to run into each other and stick together. Other water molecules meet up and decide to join. If enough meet up and stick together they’ll get large enough that they can’t remain suspended in the air and will fall to the ground as rain.