Your Name:

What Does it Mean
for a Liquid to Boil?

Temperature and Pressure and the Boiling Point

Phase.Diagram.Water (73K)

What are the conditions under which boiling happens? This question and others will be investigated in two classroom demonstrations.

In the first demonstration a flask of water containing boiling stones is heated to a temperature below the normal boiling point of water. The flask is then stoppered and connected to a sink aspirator or a vacuum pump. Despite the fact that it is not on a heat source, the water comes to a boil. As long as the air is pumped out of the flask the water continues to boil for quite a few minutes. When the flask is disconnected and the temperature is measured, the temperature is found to be quite a bit below the normal boiling point of water.

In the second demonstration a flask containing water and some boiling stones is set on the heat to boil. After a few minutes, and the production of a large amount of steam, the flask is sealed with a stopper. Next, ice-cold water is poured onto the flask. The water in the flask, clearly cooled now below the normal boiling point, comes back to a boil.

Here is a fuller description of what's going on. This section will not print.
  1. Heat water in a filter (side-arm) flask containing a few boiling stones or small glass beads. Get the temperature to somewhere between 60° and 70°C. Stopper tightly and connect to a vacuum pump or sink aspirator.The water will boil merrily as soon as the pressure is reduced enough. It will continue for a while but as it cools it will eventually stop boiling unless there is a very good vacuum. Boiling can be accomplished even at 25°C if the pressure can be reduced enough. Check the temperature after disconnecting the flask from the vacuum.
  2. Now heat water to boiling in an ordinary Erlenmeyer flask (include some boiling stones again). Let it fill completely with water vapor. Stopper with a one-hole stopper that has a glass tube through it which is connected to a short length of rubber tubing. Clamp the tubing tightly to make the flask air-tight. Unclamping the rubber tube later allows the pressure to equalize and makes it possible to get the stopper out. Next, using tongs, hold the stoppered flask over a big pan and pour ice-cold water over the vapor-filled part of the flask. This causes the water vapor to condense and reduces the pressure inside the flask by a large amount. This reduction in pressure leads to renewed boiling in the liquid. This can be repeated over and over again for quite a while.

Answer the following questions based on the presentation and class discussion.

  1. Consider the phase diagram of water pictured above. Use one or two complete sentences to describe what happens to a small amount of ice when it is heated at 1 atm to a temperature above 100°C. Use the diagram by tracing across the dashed line marking 1 atm of pressure.
  2. What is meant by the phrase “the normal boiling point of water”?
  3. Describe what happened in the first demonstration in which a flask with hot but not boiling water was hooked up to an aspirator to reduce the pressure inside it.
  4. How would you plot the events of the first demonstration on the phase diagram of water?
  5. Describe what happened in the flask full of just-off-the-boil water that had the cold water poured on it.

page break
  1. How would you plot the events of the second demonstration on the phase diagram of water?
  2. What do these two demonstrations have in common?
  3. Bubbles in a boiling liquid are filled with the vapor phase of the liquid. They can only form when the liquid evaporates fast enough. If the external pressure is 1 atm, what pressure is required for a bubble to avoid collapsing or in fact to exist at all?
  4. Evaporation rate depends on temperature. The hotter it is, the faster the liquid will evaporate. With this in mind, what partial pressure of water vapor do you think is produced by water at 100°C? Why?
  5. The partial pressure of water vapor produced by liquid water at temperatures below 100°C is less than the pressure inside the bubbles in water boiling at 1 atm and 100°C. Why does reducing the external pressure on the water lead to the formation of bubbles in water at temperatures below 100°C?
  6. Using one or more complete sentences, write an explanation for why it is that reducing the pressure on a liquid can bring it to a boil at temperatures below the normal boiling point.
  7. Why did the hot water in the flask cooled with ice water come back to a boil in the sealed flask?
  8. Why does it take longer to hard boil an egg in Boulder, Colorado or in the Himalayan mountains? Remember, air pressure is a function of elevation: the higher you go, the lower the air pressure.
  9. How does a pressure-cooker make it possible to cook food faster? (A pressure cooker is a sealed-up pot that allows you to greatly increase the air pressure on the food you are cooking).
Last updated: Jan 29, 2021       Home