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2.20: Boiling

Difficulty Level: At Grade Created by: CK-12
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Why is steam rising from the spring in this picture? It’s because the water is boiling hot. The bubbles in the water show that it is boiling. The water in the spring is hot enough to boil because it comes from an underground source near hot molten rock.

All Steamed Up

Steam actually consists of tiny droplets of liquid water. What you can’t see in the picture is the water vapor that is also present in the air above the spring. Water vapor is water in the gaseous state. It constantly rises up from the surface of boiling hot water. Why? At high temperatures, particles of a liquid gain enough energy to completely overcome the force of attraction between them, so they change to a gas. The gas forms bubbles that rise to the surface of the liquid because gas is less dense than liquid. The bubbling up of the liquid is called boiling. When the bubbles reach the surface, the gas escapes into the air. The entire process in which a liquid boils and changes to a gas that escapes into the air is called vaporization. You can watch an animation of the process at this URL: http://www.visionlearning.com/library/flash_viewer.php?oid=321&mid=120

Q: Why does steam form over the hot spring pictured above?

A: Steam forms when some of the water vapor from the boiling water cools in the air and condenses to form droplets of liquid water.

Vaporization vs. Evaporation

Vaporization is easily confused with evaporation, but the two processes are not the same. Evaporation also changes a liquid to a gas, but it doesn’t involve boiling. Instead, evaporation occurs when particles at the surface of a liquid gain enough energy to escape into the air. This happens without the liquid becoming hot enough to boil.

Boiling Point

The temperature at which a substance boils and changes to a gas is called its boiling point. Boiling point is a physical property of matter. The boiling point of pure water is 100°C. Other substances may have higher or lower boiling points. Several examples are listed in the Table below. Pure water is included in the table for comparison.

Substance Boiling Point (°C)
Hydrogen -253
Nitrogen -196
Carbon dioxide -79
Ammonia -36
Pure water 100
Salty ocean water 101
Petroleum 210
Olive oil 300
Sodium chloride 1413

Q: Assume you want to get the salt (sodium chloride) out of salt water. Based on information in the table, how could you do it?

A: You could heat the salt water to 101°C. The water would boil and vaporize but the salt would not. Instead, the salt would be left behind as solid particles.

Q: Oxygen is a gas at room temperature (20°C). What does this tell you about its boiling point?

A: The boiling point of oxygen must be lower than 20°C. Otherwise, it would be a liquid at room temperature.


  • Vaporization is the process in which a liquid boils and changes to a gas.
  • Vaporization is easily confused with evaporation, but evaporation doesn’t involve boiling.
  • The temperature at which a liquid boils and starts changing to a gas is called its boiling point. The boiling point of pure water is 100°C.


  • vaporization: Process in which a liquid boils and changes to a gas.


Read about boiling at the following URL, and then answer the questions below.


  1. Name two factors that affect the boiling point of a substance.
  2. Compared with the molecules of many other substances, molecules of water are strongly attracted to each other. How is this likely to affect the boiling point of water?


  1. What is vaporization?
  2. Outline how vaporization occurs.
  3. Make a table comparing and contrasting vaporization and evaporation.
  4. Define boiling point. What is the boiling point of pure water?
  5. Suppose you place an aluminum pot containing water over the flame on a stovetop. Before long, the water starts boiling and turning to water vapor. The pot becomes very hot but otherwise appears to be unchanged by the increase in temperature. Based on these observations, what can you conclude about the boiling point of aluminum?




Process in which a liquid boils and changes to a gas.

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Difficulty Level:
At Grade

Concept Nodes:

7 , 8
Date Created:
Oct 31, 2012
Last Modified:
May 23, 2016
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