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.
This graphic shows the 6 phase changes that can occur between solids, liquids, and gases.
Q: Which two processes result in matter changing to the solid state?
The Role of Energy in Changes of State
What could cause this chocolate to change from its solid form to a liquid one?
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.
Notice the difference in the arrangement of the particles in liquids versus solids.
What states of matter do you see in this picture?
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 (you must click on "Magnify 109x"):
Q: Why does steam form over the hot spring pictured on the previous page?
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.
The surface molecules that have enough energy to overcome their attraction to other particles will pull away from the liquid and move into the air. This is what occurs during evaporation.
From Gas to Liquid
Condensation causes the drops of water to form on the spider web.
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:
The condensation on this cup of tea is due to water vapor in the air changing into liquid water as it cools near the cold cup.
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. ll 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 3 phases of water in this image? (B) Liquid water (C) Water vapor is invisible, but clouds that form when water condenses on particles in the atmosphere are not.
(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.
Up until 2015, it was believed that Earth was the only planet where water is present in all three states. Recent discoveries of liquid water on Mars may add it to the list with Earth. Read more about this discovery at: https://newsela.com/articles/mars-liquidwater/id/12172/
- 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.
Take the changing states quiz at the following URL. Be sure to check your answers.
- Define change of state, and give an example.
- Identify processes that change matter to a liquid state.
- Why must energy be absorbed to change a liquid to a gas?
- 1.1.G.a: Identify and classify changes in matter as chemical and/or physical