READ: Changes of State
2. Vaporization
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.
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 theTable 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.
CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/