You have probably heard that a watched pot doesn't boil. Or does it? If watched pots didn't boil, scientists wouldn't be able to study boiling. Well, scientists have studied boiling - even in space! You would think something common like boiling would be completely understood, but boiling is complex and there is much to learn about boiling on Earth and in space.

When you open a bottle of soda, what do you see? Do you remember the bubbles rising from the bottom to the top? If you don't remember, watch the next time you open a bottle of soda. The same thing happens to the bubbles when liquids are boiled on Earth. Now imagine you are going to boil some water on Earth.

Some of the liquid water gets hot enough to change to gas (also called water vapor or steam). This water vapor forms a bubble. Since a gas is less dense than the liquid it came from, this means that for the same volume, the gas will have less mass and weigh less than the liquid. In boiling water, the bubble rises to the surface because it weighs less than a chunk of liquid water nearby of the same volume. We call this buoyancy.

How does the heat energy from the burner spread throughout the liquid in the pot? Again, weight plays a role. As you heat fluids (liquids or gases), they usually become less dense. The hotter liquid water will be less dense than the cooler liquid and rise just like the bubbles and the cooler liquid will fall. This causes circulation in the fluid. Thus, the heat energy gets distributed throughout the heated water in this process we call convection. Convection is often used in heating homes.

Two other energy processes also play a role in boiling. Have you ever been warmed by a campfire? Heat energy is transferred from the campfire to you through the process of radiation. You can also feel the radiation of energy from the top of a pot of boiling water, but radiation doesn't help boil the water.

Have you ever picked up something that is too hot? Heat energy was transferred from the hot object to your hand through the process of conduction. Heat energy is conducted through the bottom of the metal pot to raise the temperature of the water and begin the boiling process. Heat energy is conducted through the liquid water to warm the water at the top of the pot, but conduction plays a minor role compared to convection in distributing the heat throughout the water in the pot.

What's different in space? On the space shuttle or International Space Station (ISS) everything is weightless (see our KSNN unit: Why do astronauts float around inside the ISS?). Buoyancy and convection don't affect liquids in space and the complex process of boiling is much simpler. Now you should be ready to see the results! What do you think will happen? Click on the image below, after you make your prediction, to see a movie of the results of the boiling experiment on Earth compared to space.

From 1992-1996, a team of scientists studied boiling in Freon¿* (Freon¿ coolant boils at a much lower temperature so it's easier and safer to work with in space) in a weightless environment from experiments aboard the shuttle. Click the movie below to see a comparison of boiling on Earth and in space.

The difference is spectacular! Since buoyancy and convection don't play a role, the many little bubbles we associate with boiling on Earth aren't seen. Instead, one giant bubble forms because there is no buoyancy present to drive the bubbles upward as they form. On the shuttle, the giant bubble forms out of the many small bubbles at the heated surface because it takes less energy to form one big round bubble (see our KSNN science unit: Why are bubbles round?).

What does the lack of convection in the space environment do to the boiling? The Freon¿ at the top stays cool because there is no convection. Sometimes a big bubble will remain attached to the heating surface and it insulates the heater from the cool Freon¿ above. In that case, with nothing to take its heat away, the heater's temperature can soar.

These observations of boiling in space have very important applications. Power plants on Earth boil liquid water to produce steam to turn a turbine blade and generate energy. Understanding boiling better can help engineers design more efficient power plants, which would save energy and fuel and reduce pollution for the equivalent energy output. It would get intolerably hot on the ISS without a cooling system. Part of the process in one proposed system will boil liquid ammonia and then condense it back to a liquid. The studies of boiling in space helped engineers design and build a better cooling system for our astronauts on the ISS. I hope you now realize how important boiling can be.