Everyone is familiar with the picture: there is a pot of water on the stove on the fire. Water from cold gradually becomes hot, here the first bubbles appear on its surface, and soon all of it is merrily seething. What is the heat of vaporization of water? Some of us remember from the school curriculum that the temperature of water at natural atmospheric pressure cannot exceed 100 °C. And those who do not remember or do not believe can use the appropriate thermometer and make sure, observing safety measures.
But how can this be? After all, the fire is still burning under the pan, it gives up its energy to the liquid, and where does it go if it does not heat the water? Answer: Energy is used to turn water into steam.
Where does the energy go
In ordinary life, we are accustomed to the three states of the matter around us: solid, liquid and gases. In the solid state, the molecules are rigidly fixed in the crystal lattice. But this does not mean their complete immobility, at any temperature, as long as it is at least a degree higher than -273 ° C (this is absolute zero), the molecules vibrate. Moreover, the amplitude of vibration depends on temperature. When heated, energy is transferredparticles of a substance, and these chaotic movements become more intense, and then reach such a force at a certain moment that the molecules leave the nests of the lattice - the substance becomes a liquid.
In the liquid state, the molecules are closely related to each other by the force of attraction, although they are not fixed at a certain point in space. With further accumulation of heat by the substance, the chaotic vibrations of a part of the molecules become so great that the force of attraction of the molecules to each other is overcome, and they fly apart. The temperature of the substance stops rising, all the energy is now transferred to the next and next batches of particles, and so, step by step, all the water from the pan fills the kitchen in the form of steam.
Each substance requires a certain amount of energy to carry out this process. The heat of vaporization of water, like other liquids, is finite and has specific values.
In what units is measured
Any energy (even movement, even heat) is measured in joules. Joule (J) is named after the famous scientist James Joule. Numerically, an energy of 1 J can be obtained if a certain body is pushed at a distance of 1 meter with a force of 1 Newton.
Previously, to measure heat, they used such a concept as “calorie”. It was believed that heat is such a physical substance that can flow in or out of any body. The more it "leaked" into the physical body, the hotter it is. In old textbooks, you can still find this physical quantity. But it is not difficult to convert it to joules, it is enough to multiply by 4,19.
The energy required to convert liquids into gases is called the specific heat of vaporization. But how to calculate it? It's one thing to turn a test tube of water into steam, and another thing to turn a huge ship's steam engine tank.
Therefore, for example, for H2O, in heat engineering they operate with the concept of "specific heat of vaporization of water" (J / kg - unit of measurement). And the key word here is "specific". It is considered the amount of energy that is necessary to turn 1 kg of liquid substance into steam.
The value is indicated by the Latin letter L. The value is measured in joules per 1 kg.
How much energy does water require
The specific heat of vaporization of water is measured as follows: an amount of N is poured into the container, brought to a boil. The energy spent on the vaporization of a liter of water will be the desired value.
Measuring what the specific heat of vaporization of water is, the scientists were slightly surprised. To turn into a gas, water requires more energy than all liquids common on Earth: the entire line of alcohols, liquefied gases, and even more than metals such as mercury and lead.
So, the heat of vaporization of water turned out to be 2.26 mJ/kg. For comparison:
- for mercury - 0.282 mJ/kg;
- lead has 0.855 mJ/kg.
What if it's the other way around?
What happens if you reverse the process, make the liquid condense? Nothing special, there is a confirmation of the law of conservation of energy: when condensing oneof a kilogram of liquid from steam, exactly the same amount of heat is released as it takes to turn it back into steam. Therefore, the term “specific heat of vaporization and condensation” is more often found in reference tables.
By the way, the fact that heat is absorbed during evaporation is successfully used in household and industrial appliances to create artificial cold.
