Everyone knows the picture: on a stove on fire there is a pot of water. Water from the cold gradually becomes hot, here the first bubbles appear on its surface, and soon all of it is bubbling cheerfully. 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 that they observe safety measures.
But how can this be? After all, fire still burns 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 spent on turning water into steam.
Where does the energy go
In ordinary life, we are accustomed to three states of matter surrounding 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, if only it is at least a degree above −273 ° C (this is an absolute zero), the molecules vibrate. Moreover, the amplitude of the vibration depends on the temperature. When heated, energy is transferred to particles of the substance, and these chaotic movements become more intense, and then reach such a force at a certain moment that the molecules leave the lattice nests - the substance becomes 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 the further accumulation of heat by the substance, the chaotic vibrations of part of the molecules become so great that the force of attraction of the molecules to each other is overcome, and they scatter. The temperature of the substance stops growing, 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 energy to carry out this process. The heat of vaporization of water, like other liquids, is finite and has specific meanings.
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, energy of 1 J can be obtained if you push a certain body with a force of 1 Newton at a distance of 1 meter.
Previously, the term calorie was used to measure heat. It was believed that heat is such a physical substance that can flow in or out of any body. The more it “flowed” into the physical body, the hotter it is. In old textbooks you can still find this physical quantity. But it is not difficult to translate it into joules, just 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 is the steam engine tank of a huge ship.
Therefore, for example, for 2 , in heat engineering they operate with the concept of "specific heat of water vaporization" (J / kg - unit). And the key word here is “specific”. The amount of energy that is needed to convert 1 kg of liquid substance into steam is considered.
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 water vaporization is measured as follows: the amount of N is poured into the tank, 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 equal to, the scientists were slightly surprised. To convert to gas, water requires more energy than all liquids distributed 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:
- mercury - 0.282 mJ / kg;
- in lead - 0.855 mJ / kg.
But what if the opposite?
And what happens if you reverse the process, make the liquid condense? Nothing special, the law of energy conservation is confirmed: when one kilogram of liquid is condensed, exactly the same amount of heat is released from the steam that it takes to turn it back into steam. Therefore, the term “specific heat of vaporization and condensation” is more often found in look-up tables.
By the way, the fact that heat is absorbed during evaporation is successfully used in household and industrial equipment to create artificial cold.