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Changes of State

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Both of these photos show the famous Golden Gate Bridge near San Francisco, California. The pictures were taken from about the same point of view, but they look very different. In the picture on the left, the deck of the bridge is almost completely hidden by a thick layer of fog. In the picture on the right the fog has disappeared, and the deck of the bridge—as well as the water below it—is clearly visible. Fog consists of tiny droplets of liquid water. The fog in the picture is like a cloud at ground level. Where did the fog come from, and where did it go?

What Are Changes of State?

The water droplets of fog form from water vapor in the air. Fog disappears when the water droplets change back to water vapor. These changes are examples of changes of state. A change of state occurs whenever matter changes from one state to another. Common states of matter on Earth are solid, liquid, and gas. Matter may change back and forth between any two of these states.

Changes of state are physical changes in matter. They are reversible changes that do not change matter’s chemical makeup or chemical properties. For example, when fog changes to water vapor, it is still water and can change back to liquid water again.

Processes that Cause Changes of State

Several processes are involved in common changes of state. They include melting, freezing, sublimation, deposition, condensation, and evaporation. The Figure below shows how matter changes in each of these processes.

Phase transitions between solid, liquid, and gas

Credit: Christopher Auyeung
License: CC BY-NC 3.0

[Figure1]

Q: Which two processes result in matter changing to the solid state?

A: The processes are deposition, in which matter changes from a gas to a solid, and freezing, in which matter changes from a liquid to a solid.

The Role of Energy in Changes of State

Suppose that you leave some squares of chocolate candy in the hot sun. A couple of hours later, you notice that the chocolate has turned into a puddle like the one pictured in the Figure below .

Chocolate melting from solid to liquid

Credit: Image copyright Anteromite, 2013
License: CC BY-NC 3.0

[Figure2]

Q: What happened to the chocolate?

A: The chocolate melted. It changed from a solid to a liquid.

In order for solid chocolate to melt and change to a liquid, the particles of chocolate must gain energy. The chocolate pictured in the Figure above gained energy from sunlight. Energy is the ability to cause changes in matter, and it is always involved in changes of state. When matter changes from one state to another, it either absorbs energy—as when chocolate melts—or loses energy. For example, if you were to place the melted chocolate in a refrigerator, it would lose energy to the cold air inside the refrigerator. As a result, the liquid chocolate would change to a solid again.

Q: Why is energy always involved in changes of state?

A: The energy of particles of matter determines the matter's state. Particles of a gas have more energy than particles of a liquid, and particles of a liquid have more energy than particles of a solid. Therefore, in order for matter to change from a solid to a liquid or from a liquid to a gas, particles of matter must absorb energy. In order for matter to change from a gas to a liquid or from a liquid to a solid, particles of matter must lose energy.

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 from liquid to solid

License: CC BY-NC 3.0

[Figure3]

 

License: CC BY-NC 3.0

[Figure4]

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/img/app/library/objects/Flash/VLObject-321-030317040343.swf

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.

From Liquid to Gas Without Boiling

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. 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 .

Motion of particles causes evaporation

License: CC BY-NC 3.0

[Figure5]

 

From Gas to Liquid

License: CC BY-NC 3.0

[Figure6]

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 . You can read more about condensation at this URL:  http://www.kidsgeo.com/geography-for-kids/0107-condensation.php

Water as Three States of Matter

Water is the only substance on Earth that is present in all three states of matter. This means that water is present as a solid, liquid, and gas. Earth is the only planet where water is present in all three states. All three phases may be present in a single location or in a region. The three phases are solid (ice or snow), liquid (water), and gas ( water vapor ). See ice, water, and clouds pictured below ( Figure   below ).

(A) Ice floating in the sea. Can you find all three phases of water in this image? (B) Liquid water. (C) Water vapor is invisible, but clouds that form when water vapor condenses are not.

Water has remarkable properties. Water expands when it freezes, even though all other substances contract. This means that in the winter ice on a pond or lake will float because its expansion as a solid makes it less dense than liquid water. Without water, life might not be able to exist on Earth. If it did exist, it would not be as diverse or as complex.

Summary

A change of state occurs whenever matter changes from one state to another. Changes of state are physical changes in matter. They are reversible changes that do not change matter’s chemical makeup or chemical properties.

  • Processes involved in changes of state include melting, freezing, sublimation, deposition, condensation, and evaporation.
  • Energy is always involved in changes of state. Particles of matter either absorb or lose energy when matter changes from one state to another.
  • 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.

Vocabulary

  • Freezing:  process in which a liquid changes to a solid. 
  • Vaporization : process in which a liquid boils and changes to a gas.
  • Melting :  process in which a solid changes to a liquid. 
  • Evaporation : process in which a liquid changes to a gas without boiling.  
  • Condensation : process in which a gas changes to a liquid.

Practice

Take the changing states quiz at the following URL. Be sure to check your answers.

http://www.bbc.co.uk/bitesize/ks2/science/materials/changing_states/quiz/q69393075/

Review

  1. Define change of state, and give an example.
  2. Identify processes that change matter to a liquid state.
  3. Why must energy be absorbed to change a liquid to a gas?

Missouri Standards

  1. GLEs: 1.1.G.a

Image Attributions

  1. [1]^ Credit: Christopher Auyeung; License: CC BY-NC 3.0
  2. [2]^ Credit: Image copyright Anteromite, 2013; License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0
  5. [5]^ License: CC BY-NC 3.0
  6. [6]^ License: CC BY-NC 3.0

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