And another question on the same topic - why is it that when you boil water there is a little steam above the pot, but as soon as you turn off the fire, there is much more steam?
Answer to the first question:: Most of the substances we use every day (gases in the atmosphere, water and colds in air conditioners) behave in this way. The basic diagrams of thermodynamics are given as bell-shaped curves. In the case of a graph of pressure as a function of enthalpy (internal energy), a HP graph, it can be clearly seen that for every increase in pressure the energy between gas and liquid is small, that is, less heat is required to invest in vaporizing the substance. It could be expected that boiling water in a place with higher pressure (and where the temperature is higher) would require more energy because the substance requires boiling at a higher temperature. In addition, a thermally stimulating substance will make additional heating more difficult (heat transfer depends directly on the temperature difference between the donor and the recipient). You can think of this in terms of charging a capacitor. When the voltage difference between the charging source and the capacitor is high (at the beginning) the charging is fast, and it decreases over time, while reducing the voltage difference. If we return to the example of boiling, when the molecules of the water are at a higher temperature, there is a greater waste of energy during heating due to the refusal of the molecules to receive more heat (the rate of heat transfer decreases and therefore heat investment over a longer period of time is required). For the water, two opposite processes occur. The first is the compression action. The compression increases the pressure and tries to bring the water molecules closer together, which will make boiling more difficult. In order to boil the substance, we had to invest additional work in moving the molecules away (work equal to the pressure applied multiplied by the volume on which the work is applied). But saturated water is not compressible. Conversely, the compression raised the temperature of the water, making it easier for it to evaporate. And so as the pressure increases, the amount of energy required to be invested (after compression) decreases. Of course, the trick lies in the fact that we have already invested a large amount of energy in bringing the material to high pressure
Why is it that when you boil water there is a little steam above the pot, but as soon as you turn off the heat there is much more steam?
Answer: While the gas is turned on, heat enters through the pot into the water and evaporates it. The most energetic molecules in the upper layer leave the liquid and become a gas. Why does water evaporate throughout the process even though it has not reached 100 degrees? The liquid water feels a pressure of one atmosphere pushing it from below. The pressure above them is also one atm, but the partial pressure of the water in the air is lower than one atm, so it evaporates. Heat leaves the water and rises to the top as hot air. This heat partially enters the evaporated water vapor and corrodes it. Annealing means raising the temperature of water (or any other substance) after it has passed into a gas phase. Since they are in open air they cannot increase in pressure. Until complete evaporation, water at atmospheric pressure will not exceed XNUMX degrees Celsius, but as a gas it can reach a much higher temperature, and maintain atmospheric pressure. When this happens (as a result of the heat absorbed by the water) they become heated and the gas molecules move away from each other, and it looks like there is a little steam. However, after the fire is extinguished, there is no more heat source and the water that evaporates - condenses back into the air. It is more correct to say that they return to a saturated state. In this situation, the water molecules are clearly visible. An example of non-transparent vapor can be seen in cold and damp mornings, when moisture floats on the surface of the ground at a height of several meters. This is water vapor on the verge of condensation.
Another point about boiling water. The reason water in a kettle makes a lot of noise while boiling but becomes quiet when it comes to a boil is that the bubbles during boiling explode from the great pressure they are contained in. The pressure inside the bubble is low because of the low temperature of the water. But when all the water boils (when we reached 100 degrees) the pressure in the bubble is strong enough to hold the journey up to the front of the liquid and it gently explodes in the air. And if we are dealing with bubbles, we will add and say that bubbles explode in the air for no apparent reason because of the force of gravity. The liquid at the top of the bubble wants to slide down. The crust then becomes thin and the bubble cannot support the pressure exerted on it by the environment.
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Hello. I would be happy to expand on the first question and if possible a reference to additional reading material. What is the energy difference if I boil water in the Dead Sea compared to on the summit of Everest? Assuming that I took water at a temperature of 20 degrees, how much energy would I have to invest in order to boil it at 101 degrees in the Dead Sea and how much would I have to invest in order to boil it at 71 degrees in Everest?
To which scenario does the sentence "on the other hand, the compression raised the temperature of the water, making it easier for it to evaporate" fit? If I took water at 20 degrees to the Dead Sea, would it heat up due to the change in barometric pressure?
Thank you
It does not agree with what is written here https://he.wikipedia.org/wiki/ About the boiling point.
If I boiled a dish in a pressure cooker that is mostly water, without covering it with a lid while boiling, and when it boils I take it off the heat and close the pressure cooker with the lid (the almost "hermetic" one). Now the pressure in the pot will increase greatly due to the suffocated steam. Does the temperature of the water in the pot rise above because of the pressure? Will it even rise above 100 degrees in the first step (until the pot cools down)? In other words - does compressing a material heat it up?
Response to life:
Regarding the first claim, you are wrong. Quote: "But water in a saturated state is not compressible".
Regarding the second claim: water at thousands of atm has already passed the triple critical temperature - it is not saturated water at all.
Regarding the third claim: there is no other reason, heat investment raises the temperature in any material and in any situation according to Maxwell's equations of state (not the radiation equations but the equations of state).
Regarding the fourth claim: open a book and check a factor called transition enthalpy, h fg . It is smaller as a function of power 4 (slightly less). That is, in order to lower the amount of heat of the vapor by a lot, it is necessary to increase the entire pressure-temp scale. The enthalpy does decrease (as I claimed) but slightly. Certainly not.
Liquid water does not change its volume if you try to compress it.
(They will change their volume significantly only at a pressure of thousands of atmospheres).
So in practical terms, increasing the pressure on the surface of the water raises the boiling temperatures but for a completely different reason.
And of course it takes more energy to boil them.
I guess what you wrote is correct.
From here to explaining a scientific topic to the audience - who, for that matter, has not completed a thermodynamics course nor an electrical engineer,
there is room for improvement.
Answer to my people
Your guess is definitely correct. At first the bubbles that come out are air. The metal of the pot does not have a perfect surface route and air hides in the small crevices in the pot. This air does not come out at first because of surface tension between the air and the metal. But when the temperature rises, the air expands and rises like a bubble. After that the boiling metal creates evaporation of water at the bottom and the bubbles are of water vapor only. By the way, heat transfer using bubbles is the most efficient form of heat removal. The transition coefficient is sometimes over 20000 while for sweating in the wind it is about 50 (and can grow up to several hundreds).
Cool!!!
I've always wondered about the second question.
But I convinced myself that I was just imagining that only after I turn off the heat there is suddenly "more" steam. Now I understand that it is not that there is more steam - there is simply a greater condensation and therefore I see more steam. Thanks!
And I wanted to ask a short question about this:
The bubbles that come out of the bottom of the pot - what are they made of? Water gas bubbles? After all, there is no air there and I assume that the hot metal is not the one that creates the gas. Apparently these are areas so hot on the metal that liquid water touches them and evaporates and creates a bubble of water gas. I'm right?