States of Matter
States of Matter
This lesson explains the differences between solids, liquids, gases, and plasmas. It also describes how a sample can change from one state of matter to another.
The following video will provide a review on the States of Matter.
States of Matter
On Earth, substances are found in four states of matter: solid, liquid, gas, and plasma. Many properties of these states of matter are familiar. For example, solids are rigid and hard, liquids can flow inside their containers, and gases can spread throughout an entire room. But what happens at the molecular level may not be as familiar. The differences among them can be explained by the amount of energy that the particles have and the strength of the cohesive forces that hold the particles together. Cohesion is the tendency of particles of the same kind to stick to each other and is an important property to consider when looking at states of matter. The motion and density of particles in a substance and the tendency of a substance to take the shape and volume of its container differentiate states of matter.
Connections
Gases and liquids are considered fluids because of their ability to “flow” and take the shape of their containers.
Solids have the lowest energy. The particles are packed close together, and their structure is relatively rigid. Strong cohesive forces prevent particles from moving very far or very fast. Therefore, both the shape and volume of a solid are fixed.
In liquids, particles have more energy than in solids and can overcome the cohesive forces to some degree. Since particles can move more freely, they flow and take the shape of their container. However, cohesive forces are strong enough to somewhat restrict the movement of particles. While the shape of the liquid is not fixed, the volume is.
Gases have more energy than solids or liquids. In a gas, the cohesive forces are very weak because the particles move very quickly. Gas particles move more freely than liquids, which means that gas particles can not only take the shape of the container, but also spread to occupy the entire volume of the container.
Did You Know?
While particles are generally more tightly packed in solids than in other states, water is an exception. When liquid water freezes, it expands. The molecules are pushed apart as strong intermolecular forces, known as hydrogen bonds, allow the particles to form crystals. This property of water is important in many processes on Earth.
In plasma, the particles have so much energy that the electrons separate from their nuclei. The result is a substance composed of moving positively and negatively charged particles. Although plasmas are less common in everyday life than the other states of matter, there are a few familiar examples. First, the hottest parts of the sun are made of plasma because of the high temperature (up to 15,000,000 K). Also, neon signs glow when plasma is produced by passing an electric current through a gas.
Did You Know?
Not all neon signs contain neon. Other noble gases (helium, argon, and xenon) can be used to produce different colors of light.
Phase Changes
Whenever a substance transforms from one state of matter to another, it undergoes a phase change. These processes are physical changes because the chemical composition of the substance remains the same; only its appearance is different. The six most common phase changes are summarized in the diagram and chart below. Note that the states of matter are arranged in order of increasing energy from left to right.
| Phase Change | Name | Absorb or Release Energy |
|---|---|---|
| solid to liquid | melting | absorb |
| liquid to gas | vaporization | absorb |
| solid to gas | sublimation | absorb |
| liquid to solid | freezing | release |
| gas to liquid | condensation | release |
| gas to solid | deposition | release |
All phase changes require the system to either absorb or release energy.
Any phase change that moves to the right in the diagram above requires energy to be added to the system because the substance has more energy at the end of the phase change. The phase changes are melting, vaporization (boiling), and sublimation. When energy is added, particles move faster and can break away from each other more easily as they move to a state of matter with a higher amount of energy. This is most commonly done by heating the substance.
Any phase change that moves to the left in the diagram requires energy to be removed from the system because the substance has less energy at the end of the phase change. These phase changes are freezing, condensation, and deposition. When the particles release energy, they move more slowly. The cohesive forces bring these particles closer together as they move to a state of matter with a lower amount of energy. This is most commonly done by cooling the substance.
The temperatures at which phase changes occur depends the strength of the cohesive forces between particles. For substances like metals that have high melting and boiling points, it takes a relatively large amount of energy to overcome the intermolecular forces enough to change states of matter. Similarly, substances with low melting and boiling points, like the gases that make up Earth’s atmosphere, do not require as much energy to overcome their intermolecular forces.
Heating and Cooling Curves
When studying phase changes, one can examine the heating or cooling curve of a substance. Heating and cooling curves are plots of temperature versus time that occur as energy is added to or removed from the system at a constant rate.
The heating curve for water is shown above. Notice that at the beginning of the experiment, the substance is a solid. As heat is added, the temperature of the solid increases until it reaches its melting point, 0°C. The temperature remains constant at the melting point until the entire sample has changed to a liquid. Note that even though heat is still being added, the temperature is not increasing. This is because the added energy is used to disrupt the cohesive forces in the solid, allowing the particles to move more freely as the substance changes to a liquid.
Keep In Mind
The temperature of a substance is a measure of the kinetic energy of the particles that make up a substance. In other words, temperature is related to how fast the particles are moving.
Once the sample is completely melted, the temperature increases again. It increases until the boiling point, 100°C, is reached. Heat is still being added, but the temperature remains constant as the substance boils. This time, the added energy is being used to break the intermolecular bonds in the liquid as the particles transform into a gas and move farther away from each other. It is not until the phase change is complete and the sample is entirely gas that the temperature starts increasing again.
A cooling curve has the opposite shape of a heating curve, as seen in the graph above. In these experiments, the sample starts as a high-temperature gas. As heat is removed, the temperature of the gas decreases to its boiling point. At this point, the temperature remains constant until the entire sample is liquid. The liquid then cools to a lower temperature until it reaches the freezing point. The temperature remains constant as the substance freezes, and once it is completely solid, the temperature decreases again.
Key Point
As a substance undergoes a phase change, its temperature remains constant. The only time a substance experiences an increase or decrease in temperature is when it is entirely in one state of matter.
Let’s Review
- Solids, liquids, gases, and plasmas differ from one another in the amount of energy that the particles have and the strength of the cohesive forces that hold the particles together.
- Cohesion is the tendency of particles of the same kind to stick to each other.
- A solid has the lowest amount of energy because its particles are packed close together. Liquids have more energy than a solid, and gases have more energy than solids or liquids because the cohesive forces are very weak.
- A substance can undergo a phase change if it either absorbs or releases enough energy.
- Heating and cooling curves show the temperature of a substance as heat is consistently added or removed.
- The only time a substance experiences an increase or decrease in temperature is when it is entirely in one state of matter.
- As a substance changes states, its temperature remains constant. Any energy that is absorbed or released is used to change the way in which the particles interact with one another.
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