Boiling Point of Water?

Question

The boiling point of water is lower when atmospheric pressure decreases. Theoretically, then, you could attach a suction pump to a pot and suck out the air above the water level, and the water would come to a boil faster, thereby saving energy. Do you see anything wrong with this argument?

Answer 1

Everyone seems interested in practicalities. As asked, there is absolutely nothing wrong with this argument and it would, in fact, work exactly so. Almost exactly — actually, the biggest reason it takes less energy is that the water boils at a lower temperature (internal energy) and so does not need as much energy added to reach that point.

That said, there are some practical (i.e.: secondary) concerns (having nothing to do with WHY you are doing this) for actually doing it or calculating expectations before doing it. Start with your vacuum. As you draw it, the amount of water entering the "air" space will rise. This will increasingly impede your drawing of the vacuum and in the real world, may limit how much of a vacuum you can draw. Of course, your seals may also limit you. Once you begin boiling, the seals may face another serious challenge due to the expansion of the water. You may choose to continue drawing the vacuum and evacuate the steam as it forms (in which case, as it entrains water on its way out of the water, this non-boiled water will be evacuated as well; depending on whether you want the boiling water or the steam, this may be good or bad). If so, and if you can maintain the vacuum, you do not have to consider the rising pressure due to the expansion of the water as it becomes steam — as much. If you do not continue drawing it off, that will change the pressure markedly and you will find yourself boiling water at a higher, rising pressure and not the lower one you hoped for. Even if your experimental apparatus is large enough to lessen this effect, it is almost certainly closed or you would have needed the mother of all vacuum pumps. Leave it so and the pressure rises. Open it and suddenly you have at least atmospheric pressure again. So, unless you continuously draw off the steam/water vapor mixture rising from the water, you will be back to at least your starting point. One more important point the first person addressed is that, while you may use less energy for the boiling of the water, it is possible the other equipment may consume as much or more doing their parts. As an experiment, that probably fails to achieve the goal. As an industrial process, so long as those costs are not prohibitive, your only real concern might be fitting into limitations in the boiling portion of the process while still achieving the boiling.

Answer 2

  Sadly the answer is just No. Water will not just boil as any other liquid.   Some other liquids will just boil (with a small temperature) when taken to outer space. But water molecules have Strong Hydrogen bonds to keep them together. That is why water has such a high boiling point (that is too high for its molecular mass). It is that molecules with higher molecular mass boil at lower temperatures and vice versa.   This wonderful hydrogen bond is the very reason for the life on earth. Without that all water will boil and steam on atmospheric temperature.   Apart from hydrogen bonds there are many other factors like VanderWaals bonds. Sadly you cannot apply vapour pressure since it is used for closed systems. The outer space is not a closed system...   When you say without application of heat energy, we can’t apply that. A substance needs energy for its molecules to jump away from the rest. The Kelvin zero is considered as the lowest possible temperature. When you take something outer space it can get heat energy from things far away from radiation, light of stars &c. You cannot cool anything to Kelvin zero where even air will freeze. It is hypothetical.

Answer 3

This is a valid point. IT has something to do with the pressure in the water. I have actually seen this done in person. Another example of this is the reason that there is high altitude directions on cooking prducts.

It saves energy, but the point of boiling would be do add head to whatever is being cooked. If you were trying to cook spaghetti in water that was boiling at 70 degrees, the spaghetti would only be 70 degrees. The reason we boil it at standard pressure is because that is the warmer, and ideal for eating it, unless you like cold spaghetti.

Answer 4

Only that the energy you saved in boiling the water at a lower temperature, would be lost to the pump used to induce a lower pressure. You may end up expending more energy on the vacuum than what you save while boiling.

Answer 5

The problem with this is the energy you save is also the same energy you need to cook your food. Just because the water is boiling doesn't mean that it's hot enough to cook anything if you have the pressure around the water low enough. The water still needs to be around 100 celcius. So don't try doing this or you'll never cook anything.

Answer 6

are you serious