If you leave a mug of water for a long enough time, the water will evaporate from it. Evaporation is the transformation into vapor, that is, water in a gaseous state. But how can water turn into vapor at room temperature? We know that this is only possible at 100 degrees Celsius.
Why water turns into steam
A water molecule consists of an Oxygen atom and two Hydrogen atoms. Each water molecule is a small magnet with a “plus” on the Hydrogen atoms and a “minus” on the Oxygen atoms. The plus of one water molecule is attracted to the minus of the other molecule. This is how a bond called a “polar bond” is formed between two water molecules. Exactly the same bond is formed between the rest of the water molecules, and as a result, they are all connected to each other. This bond is not the strongest. The bonds are constantly breaking and new ones are being formed. Because of this, water is fluid.
To tear a magnet away from another, you have to exert a force. Similarly, to tear one molecule away from another, you have to exert a force. That is, to transfer enough energy to a molecule. This is what happens when you heat water. If you heat water to 100 °C, the molecules have enough force to break their bonds with other molecules and go on a free journey. That is, to move into a gaseous state, vapor.
What is temperature
We are used to thinking of temperature as a property of a particular object, like its color. The temperature of green tea is 65 degrees. White steamed milk is 30 degrees. Black plastic inside a car standing in the sun is 70 degrees.
When we say white steamed milk, we mean that most of it is white. But in addition to white, milk also has creamy pink tones caused by the pigment carotene. And yellowish tones, due to the presence of fat in the milk. So, on average, milk is white. But some parts of it are different in color.
It’s the same with temperature. If the temperature of green tea is 65 degrees, that doesn’t mean that every molecule of tea is 65 degrees. One molecule is 65 degrees, another is 30 degrees, another is 80 degrees. But on average they are all 65 degrees. And the molecules are constantly hitting each other, transferring energy to the neighboring one, that is, raising its temperature. Like billiard balls. If you hit a white ball against a black ball, the white ball loses energy and slows down. And the black ball you hit, on the contrary, will gain energy and fly away, gaining acceleration.
In water, energy (temperature) redistribution is also constantly taking place. And in room-temperature water, molecules with a temperature of 100 degrees or even more constantly appear. That is, they have enough energy to break the bond with other molecules and fly away. And they fly away. But this happens quite slowly.
Why water doesn’t evaporate instantly
For a molecule to separate from the others, it is not only the amount of energy that is important. The molecule must not lose this energy either. Because if the molecule is at the very bottom, there could be millions of other molecules between it and the surface. It will bump into them and gradually lose all the energy.
In order for a molecule to separate, it must be on the surface. Especially since molecules on the surface itself need less energy to separate from the others. Because the molecules that are in the thickness are surrounded by molecules that bind them on all sides. And the surface molecules “hold” only at the bottom and sides. There is no water above them, so they don’t need to break additional bonds.
Where does evaporated water go?
Let’s assume that all this happened in a closed jar. The free molecule would meet on its way the atoms of the gases that make up the air. And atoms on the walls and the lid of the jar. And hitting those atoms, the water molecule will lose energy. Until it slows down enough to form a bond with other water molecules (or any other polar molecules). This is how condensation forms on the lid and walls of the jar. As it accumulates, it drains back to the bottom. The water cycle in the jar.
If the jar is not closed, then the water molecules can settle anywhere in the apartment, not in the jar. And if you also have a window open, then the water molecule can fly into it. And go out into the big world and have adventures.
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