How to Boil Water at Room Temperature

Boil Water at Room Temperature
It’s easy to boil water at room temperature when you lower atmospheric pressure.

The normal boiling point of water is 100 °C or 212 °F, but it’s possible to boil water at room temperature. Here are easy ways to demonstrate this and an explanation of how it works.

Two Ways to Boil Water at Room Temperature

Simple Syringe Demonstration

All you need is a large syringe and water. There’s no needle involved, so this demonstration is a safe way for kids to explore boiling point.

  1. Draw a small volume of water into the syringe using the plunger. Don’t fill the syringe, but add enough water that you’ll be able to see it boil.
  2. Seal the bottom of the syringe so it can’t get more air or water. Cap it (if a cap came with the syringe), place your finger over the opening, or seal it with tape.
  3. Now, boil the water at room temperature. All you do is pull back as quickly as possible on the syringe plunger.

It may take a couple of tries to perfect your technique. If you like, set up your phone to take video so you can concentrate on boiling the water. Then, watch it later.

Boil Water Using a Vacuum Pump

The classic demonstration uses a vacuum pump. The advantage is you boil a larger volume of water so it’s easier to observe. Of course, the disadvantage is that you need a vacuum pump!

  1. Fill a 250-ml beaker with about 150 ml of warm water.
  2. Cover the beaker with a bell jar.
  3. Connect and run the vacuum pump.
  4. Once the pressure becomes low enough, the water boils.

This demonstration works best with warm water because it already has a higher vapor pressure than cold water. This means the vacuum pump boils water more quickly. This is good because prolonged exposure to water vapor gets water in the hose and pump.

How It Works

Phase Diagram
A liquid boils at a lower temperature when pressure decreases.

Water (or any liquid) boils when its vapor pressure equals atmospheric pressure. The normal boiling point applies to 1 atm of pressure (sea level). So, water boils at a lower temperature at lower pressure. This is why there are high-altitude cooking instructions. Lowering the pressure further brings down the boiling point temperature. In fact, you can boil water at temperatures colder than room temperature. Water isn’t a liquid at all when the pressure approaches a vacuum. Ice undergoes sublimation directly into water vapor, in much the way dry ice turns into carbon dioxide gas.

What Pressure Does Water Boil at Room Temperature?

There pressure at which water boils at room temperature depends on the temperature of the water. Warmer water has a higher vapor pressure, so it boils at a higher pressure than cold water.

Aside from experimentation, there are two ways to find the pressure at which water boils at a given temperature. You can consult a water phase diagram or you can look up the vapor pressure of water as a function of temperature on a table. Here are some sample values:

Temperature (°C)Vapor Pressure (torr)
Vapor pressure of water at different temperatures

Converting units, water boils at room temperature at a pressure between 0.02 and 0.03 atm. In other words, water boils at room temperature when pressure is about 1/40th normal atmospheric pressure.


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