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Name: Devin
Status: student
Age: 7
Location: N/A
Country: N/A
Date: Around 1999


Question:
Can water boil at room temperature? Mom says that you can do it if you put the container of water in a vacuum chamber.


Replies:
Dear Devin,

Yes, your mom is right. Water can boil at room temperature if the pressure is reduced enough (a vacuum means zero pressure).

In general as the pressure decreases, so does the temperature required to boil a liquid. The boiling point at the top of a mountain is lower than it is at the bottom, because the air pressure is lower at the top. I used to live in Colorado, and when I was there I had to steep my teabag to make tea longer than here in NEw York. This is because boiling water in Colorado, which is about a mile high in elevation, is cooler than boiling water in New York, which is at sea level.

Thanks for your question, hope this helps you understand boiling a little better.

Best wishes,
Prof. Topper


Mom is right. In fact, when you do this, the water will boil and freeze at the same time.

The reason is that at room temperature, water molecules are constantly going into the air as vapor and moving back into the liquid water. When you heat water, you are making the vapor that comes off the water have a higher pressure. When the pressure gets as high as the pressure of the surrounding air, bubbles of vapor can form inside the liquid and push against the air. When this happens, the liquid boils. If you remove the air and water vapor in a vacuum chamber, water vapor can't go back into the liquid. In fact, the vapor going off from the liquid will have enough pressure to actually make bubbles in the liquid.

That's why water will boil in a vacuum chamber. I also told you it will freeze at the same time. Why is this? It turns out that water molecules like to be next to each other in the liquid. It takes energy to move them into the vapor. When water molecules are moving back and forth between the liquid and vapor, the energy that is taken from the liquid to put molecules into the vapor is replaced by the energy returned when molecules move from the vapor into the liquid. In the vacuum chamber, though, the vapor is taken away, so none of the energy is returned to the water. As the water loses energy, it gets colder and finally freezes.

Richard Barrans Jr., Ph.D.



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