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Thermodynamics 1



As long as the video at the bottom of the page is still available, it shows how water, at room temperature, can be be brought to the boil without heating. All one has to do, is to reduce the pressure. At this point, a look at the above diagram would be a good idea, because there you can see how much pressure reduction, at the respective temperature, is necessary to cause the water to boil.

One thing is clear, water boils at 100C° and 1,0 bar (1.000 hPa), this is the ambient pressure. Standard tyre-pressure gauges, e.g., would show a reading of 0 bar. So, if we assume an environment pressure of 1 bar, we speak of of the absolute pressure and this, as far as thermodynamics is concerned, is the more important value. Looking at the diagram, we see, that to achieve a water-temperature of 80°C, the pressure must be reduced to just under 0,4 bar (400 hPa).

Read more about the pressure here.

Further on the left of the curve is the confirmation, that water can indeed, evaporate at room temperature, however, the environment must then be made almost completely void of air. Basically, the same amount of energy is required as would be when the water is brought to the boil by heating it. And even more energy would be needed to transform all the water into steam.

What actually happens, when water is transformed into steam? The explanation is really quite simple, the formation of the H2O-molecules is broken down so that they now take up much more space, to be more precise, 1.700 times more space.

After all, the particular amount of heat that a substance has, is a form of energy. It manifests itself in oscillations around a central point, in ice e.g., within a crystal grid. Different from other substances however, the density of water is at it's highest at 4°C. It drops when the temperature is further increased, with the already indicated, enormous changes when boiling.

Due to the fact that water expands so much when evaporating, around five times as much energy is needed, to bring the water from a temperature of 0°C to 100°C. This is why it takes so long for a kettle full of boiling water to completely evaporate. By the way, the various changes that take place, when water changes from a liquid to ice or to steam, are called 'aggregate states'. 09/13



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