READ: Phases of Matter

1. Phases of Matter

The state of matter of a substance is a physical property. A given kind of matter has the same chemical makeup and the same chemical properties regardless of its state. As a result, when matter changes state, it doesn’t become a different kind of substance. For example, water is still water whether it exists as ice, liquid water, or water vapor.

Liquids have a fixed volume, but not a fixed shape. The particles of liquids are in close contact with each other but not as tightly packed as the particles in solids. The particles can slip past one another and take the shape of their container. However, they cannot pull apart and spread out to take the volume of their container, as particles of a gas can. 

Solids

Solids have a fixed shaped and volume. Solids have particles that are much more tightly packed together. The particles are held rigidly in place by all the other particles around them so they can’t slip past one another or move apart. However even though the particles can't move, they still vibrate in place.

Gases

Gases do not have a fixed volume, or a fixed shape. The particles of a gas can pull apart from each other and spread out. As a result, a gas does not have a fixed shape or a fixed volume. In fact, a gas always spreads out to take up whatever space is available to it. If a gas is enclosed in a container, it spreads out until it has the same volume as the container.

Plasma

Plasma is a state of matter that resembles a gas but has certain properties that gases do not have. Like a gas, plasma consists of particles of matter than can pull apart and spread out, so it lacks a fixed volume and a fixed shape. Unlike a gas, plasma can conduct electricity and respond to a magnetic field. That’s because plasma consists of electrically charged particles called ions, instead of uncharged particles such as atoms or molecules. 

Where Is Plasma Found?

The sun and other stars consist of plasma. Plasma is also found naturally in lightning and in the northern and southern lights. If you’ve never seen these shimmering colorful lights in the winter night sky, the Figure below shows what they can look like. Human-made plasmas are found in fluorescent lights, plasma TV screens, and plasma spheres like the one pictured below:

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2. Properties of Solids, Liquids, and Gases

Properties of Solids, Liquids, and Gases

The most common states of matter on Earth are solids, liquids, and gases. How do these states of matter differ? Their properties are contrasted in the Figure below. You can also watch videos about these three states of matter at the following URLs.

Properties of Matter in Different States

Q: The Figure below shows that a liquid takes the shape of its container. How could you demonstrate this?

A: You could put the same volume of liquid in containers with different shapes. This is illustrated below with a beaker (left) and a graduated cylinder (right). The shape of the liquid in the beaker is short and wide like the beaker, while the shape of the liquid in the graduated cylinder is tall and narrow like that container, but each container holds the same volume of liquid.

Q: How could you show that a gas spreads out to take the volume as well as the shape of its container?

A: You could pump air into a bicycle tire. The tire would become firm all over as air molecules spread out to take the shape of the tire and also to occupy the entire volume of the tire.

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

3. Kinetic Energy and Matter

Kinetic Energy

Energy is the ability to cause changes in matter. Any matter that is moving has energy just because it’s moving. The energy of moving matter is called kinetic energy. Scientists think that the particles of all matter are in constant motion. In other words, the particles of matter have kinetic energy. The theory that all matter consists of constantly moving particles is called the kinetic theory of matter. 

Pay attention to the animations of the molecules in each phase in the following video. The movement of molecules depends on the kinetic energy. You can just ignore the man talking about whipped cream at the end, although it is funny. :)

Kinetic Energy and States of Matter

Differences in kinetic energy explain why matter exists in different states. Particles of matter are attracted to each other, so they tend to pull together. The particles can move apart only if they have enough kinetic energy to overcome this force of attraction. It’s like a tug of war between opposing sides, with the force of attraction between particles on one side and the kinetic energy of individual particles on the other side. The outcome of the “war” determines the state of matter.

• If particles do not have enough kinetic energy to overcome the force of attraction between them, matter exists as a solid. The particles are packed closely together and held rigidly in place. All they can do is vibrate. This explains why solids have a fixed volume and a fixed shape.

• If particles have enough kinetic energy to partly overcome the force of attraction between them, matter exists as a liquid. The particles can slide past one another but not pull apart completely. This explains why liquids can change shape but have a fixed volume.

• If particles have enough kinetic energy to completely overcome the force of attraction between them, matter exists as a gas. The particles can pull apart and spread out. This explains why gases have neither a fixed volume nor a fixed shape.

Look at the Figure below. It sums up visually the relationship between kinetic energy and state of matter. 

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

4. Summary and Vocabulary

Summary

• States of matter are different phases in which any given type of matter can exist. There are four well-known states of matter—solid, liquid, gas, and plasma—but only the first three states are common on Earth.
• State of matter is a physical property of matter. A given kind of matter has the same chemical makeup and the same chemical properties, regardless of state.
• Solids have a fixed volume and a fixed shape. Liquids have a fixed volume but take the shape of their container. Gases take both the volume and the shape of their container.
• According to the kinetic theory, particles of matter are in constant motion. The energy of motion is called kinetic energy.
• The kinetic energy of particles of matter determines the state of matter. Particles of solids have the least kinetic energy and particles of gases have the most.
• A liquid is a state of matter in which particles can slip past one another and take the shape of their container. However, the particles cannot pull apart and spread out to take the volume of their container.
• A solid is a state of matter in which particles of matter are tightly packed together. This holds the particles rigidly in place and gives solids a fixed shape and fixed volume.
• Gas is a state of matter in which particles of matter can pull apart from each other and spread out. As a result, a gas does not have a fixed shape or a fixed volume.
• Gas particles are constantly moving and bumping into things, and this creates force. The amount of force pushing against a given area is called pressure.

Vocabulary

• state of matter: Different phase (solid, liquid, gas, and plasma) in which matter can exist without the chemical makeup of matter changing.
• kinetic theory of matter: Theory that all matter consists of constantly moving particles.
• liquid: State of matter that has a fixed volume but not a fixed shape.
• solid: State of matter that has a fixed volume and fixed shape.
• gas: State of matter that has neither a fixed volume nor a fixed shape.
• plasma: State of matter that has no fixed shape or volume that contains ions. It responds to magnetism and conducts electricity. 

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