READ: Changes of State

1. Evaporation

The clothes on this clothesline are soaking wet, but before too long they will be completely dry. How does this happen? Where does the water go? And what factors might help the clothes on the line dry more quickly?

From Liquid to Gas Without Boiling

Evaporation explains why clothes dry on a clothesline. Evaporation is the process in which a liquid changes to a gas without becoming hot enough to boil. It occurs when individual liquid particles at the exposed surface of the liquid absorb just enough energy to overcome the force of attraction with other liquid particles. If the surface particles are moving in the right direction, they will pull away from the liquid and move into the air. This is illustrated in the Figure below.

Factors that Affect the Rate of Evaporation

Many factors influence how quickly a liquid evaporates. They include:

• temperature of the liquid. A cup of hot water will evaporate more quickly than a cup of cold water.
• exposed surface area of the liquid. The same amount of water will evaporate more quickly in a wide shallow bowl than in a tall narrow glass.
• presence or absence of other substances in the liquid. Pure water will evaporate more quickly than salt water.
• air movement. Clothes on a clothesline will dry more quickly on a windy day than on a still day.
• concentration of the evaporating substance in the air. Clothes will dry more quickly when air contains little water vapor.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

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.

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/

3. Condensation

This beautiful beaded net is something very common in nature: a spider web. The “beads” on the spider web are actually drops of water. The drops of water are not raindrops, and they weren’t there on the previous day. Where did they come from?

From Gas to Liquid

The drops of water on the spider web are dewdrops. They formed overnight when warm moist air came into contact with the cooler spider web. Contact with the cooler web cooled the air. When air cools, it can hold less water vapor, so some of the water vapor in the air changed to liquid water. The process in which water vapor—or another gas—changes to a liquid is called condensation. Another common example of condensation is pictured in the Figure below. 

Dew Point

When air is very humid, it doesn’t have to cool very much for water vapor in the air to start condensing. The temperature at which condensation occurs is called the dew point. The dew point varies depending on air temperature and moisture content. It is always less than or equal to the actual air temperature, but warmer air and moister air have dew points closer to the actual air temperature. That’s why glasses of cold drinks “sweat” more on a hot, humid day than they do on a cool, dry day. 

Q: What happens when air temperature reaches the dew point?

A: When air temperature reaches the dew point, water vapor starts condensing. It may form dew (as on the spider web above), clouds, or fog (Shown in the Figure below). Dew forms on solid objects on the ground. Clouds form on tiny particles in the air high above the ground. Fog is a cloud that forms on tiny particles in the air close to the ground.

4. Evaporation and Condensation

The videos below show both condensation and evaporation:

5. Freezing

The man in this photo is climbing a frozen waterfall. Ice climbing is a dangerous sport that should be attempted only by highly experienced climbers. For one thing, ice is very slippery, which makes it harder than rock to grip. Clinging to the slippery vertical surface takes strength, training, and the right equipment.

From Liquid to Solid

You don’t have to be an ice climber to enjoy ice. Skating and fishing are two other sports that are also done on ice. What is ice? It’s simply water in the solid state. The process in which water or any other liquid changes to a solid is called freezing. Freezing occurs when a liquid cools to a point at which its particles no longer have enough energy to overcome the force of attraction between them. Instead, the particles remain in fixed positions, crowded closely together, as shown in the Figure below.

Freezing Point

The temperature at which a substance freezes is known as its freezing point. Freezing point is a physical property of matter. The freezing point of pure water is 0°C. Below this temperature, water exists as ice. Above this temperature, it exists as liquid water or water vapor. Many other substances have much lower or higher freezing points than water. You can see some examples in the Table below. The freezing point of pure water is included in the table for comparison.

Q: What trend do you see in this table?

A: Substances in the table with freezing points lower than water are gases. Substances in the table with freezing points higher than water are solids.

Q: Sodium is a solid at room temperature. Given this information, what can you infer about its freezing point?

A: You can infer that the freezing point of sodium must be higher than room temperature, which is about 20°C. The freezing point of sodium is actually 98°C.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

6. Melting

The fiery red-orange “river” flowing downhill in this photo isn’t water. It’s molten (melted) rock, and it’s erupting from a volcano. Most rocks on Earth’s surface are solid objects. Why is the rock that erupts from a volcano in a liquid state? How does rock change from a solid to a liquid?

From Solid to Liquid

The process in which rocks or other solids change to liquids is called melting. Meltingoccurs when particles of a solid absorb enough energy to partly overcome the force of attraction holding them together. This allows them to move out of their fixed positions and slip over one another. Melting, like other changes of state, is a physical change in matter, so it doesn’t change the chemical makeup or chemical properties of matter.

Q: The molten rock that erupts from a volcano comes from deep underground. How is this related to its liquid state?

A: It is always very hot deep underground where molten rock originates. The high temperatures give rock enough energy to melt and remain in a molten state. Underground rock in this state is called magma.

Q: What happens to magma after it erupts and starts flowing over the surface of the ground?

A: After magma erupts, it is called lava. On the surface, lava eventually cools and hardens to form solid rock.

Other substances that are normally solids on Earth can also be heated until they melt. You can see an example in the Figure below. The photo shows molten gold being poured into a mold. When the gold cools, it will harden into a solid gold bar that has the same shape as the mold.

Melting Point

The temperature at which a substance melts is called its melting point. Melting point is a physical property of matter. The gold pictured above, for example, has a melting point of 1064°C. This is a high melting point, and most other metals also have high melting points. The melting point of ice, in comparison, is much lower at 0°C. Many substances have even lower melting points. For example, the melting point of oxygen is -222°C.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

7. Sublimation

Rock bands often use special stage effects, like the fake fog in this picture. Real fog forms when water vapor in the air condenses into tiny droplets of water. The fake fog shown here formed when solid carbon dioxide changed directly to carbon dioxide gas.

How Sublime!

Solid carbon dioxide is also called dry ice. That’s because when it gets warmer and changes state, it doesn’t change to a liquid by melting. Instead, it changes directly to a gas without going through the liquid state. The process in which a solid changes directly to a gas is called sublimation. It occurs when energy is added to a solid such as dry ice. You can watch dry ice changing directly to a gas in the video below.

Q: Alyssa’s mom put some mothballs in her closet in the spring to keep moths away from her wool clothes. By autumn, the mothballs were much smaller. What happened to them?

A: Mothballs are made of naphthalene, a substance that undergoes sublimation at room temperature. The solid mothballs slowly changed to a gas during the summer months, explaining why they were much smaller by autumn.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

8. Summary and Vocabulary

Summary:

• Evaporation is the process in which a liquid changes to a gas without becoming hot enough to boil. It occurs only at the exposed surface of a liquid.
• Many factors affect how quickly a liquid evaporates, including the temperature of the liquid and air movement.
• When particles evaporate from the surface of a liquid, the remaining liquid is cooler because it has less energy. This is called evaporative cooling.
• 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.
• The process in which water vapor or any other gas changes to a liquid is called condensation.
• The temperature at which condensation of water vapor occurs is called the dew point. The dew point varies depending on air temperature and moisture content.
• Freezing is the process in which a liquid changes to a solid. It occurs when a liquid cools to a point at which its particles no longer have enough energy to overcome the force of attraction between them.
• The freezing point of a substance is the temperature at which it freezes. The freezing point of pure water is 0°C.
• Melting occurs when particles of a solid absorb enough energy to partly overcome the force of attraction holding them together. This allows them to move out of their fixed positions and slip over one another, forming a liquid.
• The temperature at which a substance melts is called its melting point. The melting point of ice is 0°C.
• Sublimation is the process in which a solid changes directly to a gas without going through the liquid state. Solid carbon dioxide is an example of a substance that undergoes sublimation.
• Snow and ice undergo sublimation under certain conditions. This is most likely to happen where there is intense sunlight, very cold temperatures, and dry winds.

Vocabulary:

• evaporation: Process in which a liquid changes to a gas without boiling.
• vaporization: Process in which a liquid boils and changes to a gas.
• condensation: Process in which a gas changes to a liquid.
• freezing: Process in which a liquid changes to a solid.
• melting: Process in which a solid changes to a liquid.
• sublimation: Process in which a solid changes directly to a gas without going through the liquid state.

 CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/