Physical value: heat of water vaporization

Everyone knows the picture: on the stove on fire there is a pan with water. The water from the cold gradually becomes hot, here on its surface appear the first bubbles, and soon all of it is full of fun. What is the heat of vaporization of water? Some of us remember from the school program that the water temperature at natural atmospheric pressure can not exceed 100 ° C. And who does not remember or does not believe, can use the appropriate thermometer and make sure, observing the security measures.

But how can this be? After all, the fire is still burning under the pot, it gives its energy to the liquid, and where does it go if it does not heat the water? Answer: The energy is spent on the transformation of water into steam.

Where does the energy go?

In ordinary life, we are accustomed to the three states of the surrounding matter: 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 was at least a degree higher than -273 ° C (this is an absolute zero), the molecules vibrate. And the amplitude of vibration depends on temperature. When heated, energy is transferred to the particles of matter, and these chaotic motions become more intense, and then reach at a certain moment such a force that the molecules leave the grids - 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, chaotic vibrations of a part of the molecules become so great that the force of attraction of molecules to each other is overcome and they fly apart. The temperature of the substance ceases to grow, all energy is now transferred to the next and next batch 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 it measured

Any energy (although motion, even heat) is measured in joules. Joule (J) is named after the famous scientist James Joule. Numerically, energy in 1 J can be obtained if a distance of 1 meter is pushed by a certain body with an effort of 1 Newton.

Previously, the concept of calorie was used to measure heat. It was believed that heat - this is such a physical substance that can flow in or out of any body. The more "flowed" into the physical body, the hotter it is. In old textbooks, you can still meet this physical value. But it's easy to translate into joules, multiply by 4.19.

Energy, however, necessary for the conversion of liquids into gases, is called the specific heat of vaporization. But how to calculate it? It's one thing to turn steam into a test tube of water and another thing - the tank of the steam engine of a huge vessel.

Therefore, for example, for H ₂ O, in heat engineering operate with the concept of "specific heat of water vaporization" (J / kg - unit of measurement). And the key word here is "specific". It is considered that the amount of energy that is needed to convert 1 kg of liquid substance into steam.

The value is denoted by the Latin letter L. The value in joules is measured by 1 kg.

How much energy does water require?

Specific heat of steam formation of water is measured as follows: the amount of N is poured into the container, it is brought to a boil. The energy spent on steam generation of a liter of water will be the desired value.

Measuring what the specific heat of steam formation is equal to, the scientists were slightly surprised. To convert into a gas, water requires more energy than all the fluids on the Earth: the whole range of alcohols, liquefied gases and even more than metals such as mercury and lead.

So, the heat of evaporation of water turned out to be equal to 2.26 mJ / kg. For comparison:

• In mercury, 0.282 mJ / kg;
• Lead - 0.855 mJ / kg.

And what if on the contrary?

And what happens if you reverse the process, cause the liquid to condense? Nothing special, there is a confirmation of the law of conservation of energy: when one kilogram of liquid is condensed from the vapor, exactly the same amount of heat is allocated, which it takes to convert it back into steam. Therefore, more often in the reference tables, the term "specific heat of vaporization and condensation" occurs.

By the way, the fact that during evaporation heat is absorbed is successfully used in domestic and industrial equipment to create artificial cold.