7.3 States of Matter

Matter is all around us—the air we breathe, the water we drink, and the ground we stand on. But not all matter is the same. It exists in different forms, called states of matter. The three most common states are solid, liquid, and gas. Understanding the differences between these states—how their particles are arranged, how they move, and how they respond to forces—is fundamental to understanding the physical world.

Definition

States of Matter are the distinct forms that different phases of matter take on. The three fundamental states are:

• Solid: A state in which matter has a definite shape and volume.
• Liquid: A state in which matter has a definite volume but takes the shape of its container.
• Gas: A state in which matter has neither a definite shape nor a definite volume; it expands to fill its container.

Main Idea

The main idea is that the state of matter is determined by the arrangement and motion of its particles, and by the strength of the forces between them. In solids, particles are closely packed and vibrate about fixed positions, giving the solid a definite shape and high density. In liquids, particles are still close but can move past each other, so the liquid has a definite volume but no fixed shape. In gases, particles are far apart and move freely at high speeds, so the gas fills its container and has low density.

Core Concept

The text describes the properties of each state of matter.

• Solids are made up of particles (atoms or molecules).
• There is strong attractive force between particles.
• Due to strong attractive force, particles of solids are closely packed and have a fixed lattice pattern.
• Solid particles cannot freely move like gas particles or slide past one another like liquid particles due to their fixed location. Solids' particles can only vibrate about their mean positions.
• Solids have fixed shape.
• Solids have high density.

• Liquids are also made up of particles (molecules).
• The attractive force between particles is stronger than that of gases but weaker than that of solids.
• The distance between particles in liquids is greater than that in solids due to an intermediate attractive force, yet they are still close together.
• Liquid particles flow and glide over each other. Liquid particles keep changing their position.
• Due to flowing particles, a liquid has no fixed shape and can adopt the shape of the container.
• A liquid has intermediate density (smaller than a solid and higher than a gas).

• Gas is also composed of particles (atoms and molecules).
• There is negligibly weak attractive force between gas particles.
• Due to weak attractive force, the distance between particles is larger as compared to the size of the particles.
• Particles are in constant random motion and they constantly collide with each other and with the walls of the container.
• Forces between molecules are negligible, except during collisions.
• Gas has no fixed shape and volume. Gas spreads out in the container and fills it, therefore its volume is equal to the volume of the container.
• Gases are less dense than liquids and solids.

Detailed Explanation

The differences between solids, liquids, and gases can be explained by the behavior of their particles and the strength of the interparticle forces.

• Solids: Particles are in a regular, repeating pattern (lattice). They vibrate about fixed positions. The strong forces between particles keep them locked in place.
• Liquids: Particles are randomly arranged but still close together. They can slide past each other, allowing the liquid to flow. The forces are weaker than in solids.
• Gases: Particles are far apart and arranged randomly. They move at high speeds in straight lines until they collide with other particles or the container walls. The forces are negligible.

Comparison Table | Property | Solid | Liquid | Gas | | :--- | :--- | :--- | :--- | | Density | High | Medium | Low | | Arrangement of Particles | Regular pattern | Randomly arranged | Randomly arranged | | Movement of Particles | Vibrate around a fixed position | Move around each other | Move quickly in all directions | | Energy of Particles | Low energy | Greater energy | Highest energy | | Shape | Fixed | Takes shape of container | Fills container | | Volume | Fixed | Fixed | Fills container | | Compressibility | Almost incompressible | Slightly compressible | Highly compressible |

Plasma: The Fourth State The text also mentions plasma as the fourth state of matter (Section 7.4), which consists of ionized gas with free electrons and positive ions.

• Melting: Solid → Liquid (e.g., ice melting).
• Freezing: Liquid → Solid (e.g., water freezing).
• Evaporation/Boiling: Liquid → Gas (e.g., water boiling).
• Condensation: Gas → Liquid (e.g., steam condensing).
• Sublimation: Solid → Gas (e.g., dry ice).

Intermediate Level

In physics, the states of matter are distinguished by the balance between the kinetic energy of the particles and the potential energy of the interparticle forces.

• Solids: The interparticle forces are strong enough to hold the particles in fixed positions. The particles have low kinetic energy and can only vibrate. This gives solids a definite shape and volume, and high density.
• Liquids: The interparticle forces are weaker, allowing particles to move past each other while remaining in close contact. The particles have intermediate kinetic energy. Liquids have a definite volume but no fixed shape, and intermediate density.
• Gases: The interparticle forces are negligible compared to the kinetic energy of the particles. The particles move freely at high speeds, colliding with each other and the container walls. Gases have no fixed shape or volume, and low density.

The state of a substance depends on temperature and pressure. Increasing temperature increases the kinetic energy of the particles, which can overcome the interparticle forces and cause a change of state (e.g., melting or boiling). Increasing pressure can force particles closer together, also causing changes of state (e.g., condensation).

Important Points

• - Solid: Fixed shape and volume, high density, particles vibrate about fixed positions.
• - Liquid: Fixed volume, takes shape of container, intermediate density, particles slide past each other.
• - Gas: No fixed shape or volume, low density, particles move freely at high speeds.
• - Interparticle forces: Strongest in solids, weaker in liquids, negligible in gases.
• - Particle motion: Vibrational in solids, flowing in liquids, random and fast in gases.
• - Changes of state occur when energy is added or removed.

Remember

• - Solid: Fixed shape + fixed volume.
• - Liquid: Fixed volume + takes shape of container.
• - Gas: Fills container (no fixed shape or volume).
• - Particles: Closely packed (solid) → less closely packed (liquid) → far apart (gas).
• - Forces: Strong (solid) → weaker (liquid) → negligible (gas).
• - Density: Solid > Liquid > Gas.

Common Mistakes

• - Thinking particles in a solid don't move: They do move—they vibrate about fixed positions. They just can't move from place to place.
• - Thinking gases have no mass: Gases have mass; they just have low density because their particles are far apart.
• - Confusing the states of matter with the shape of the container: A liquid takes the shape of its container, but it does not fill it completely (unless the volume of liquid equals the volume of the container). A gas fills the entire container.

Quick Revision

The three states of matter are solid, liquid, and gas. Solids have a fixed shape and volume because their particles are closely packed and held by strong forces, vibrating about fixed positions. Liquids have a fixed volume but no fixed shape because their particles are still close but can slide past each other. Gases have no fixed shape or volume because their particles are far apart and move freely at high speeds, with negligible forces between them.

60-Second Revision

• Solid: Fixed shape + fixed volume. Particles vibrate in fixed positions. High density.
• Liquid: Fixed volume, takes shape of container. Particles slide past each other. Intermediate density.
• Gas: No fixed shape or volume. Particles move freely at high speeds. Low density.
• Forces: Strong (solid) → Weaker (liquid) → Negligible (gas).

Frequently Asked Questions (FAQs)

• Q1. What are the three states of matter?
• Answer: The three states of matter are solid, liquid, and gas.

• Q2. How do particles in a solid move?
• Answer: Particles in a solid vibrate about fixed positions. They do not move from place to place.

• Q3. Why do liquids take the shape of their container?
• Answer: Because the particles in a liquid can slide past each other, allowing the liquid to flow and take the shape of its container.

• Q4. Why do gases fill their container?
• Answer: Because the particles in a gas move freely at high speeds in all directions and are far apart, so they spread out to fill the available space.

• Q5. Which state of matter has the highest density?
• Answer: Solids generally have the highest density because their particles are closely packed.

• Q6. Which state of matter is the most compressible?
• Answer: Gases are the most compressible because there is a lot of empty space between their particles.

Q7. What causes a change of state? Answer: A change of state is caused by adding or removing energy (heat). This changes the kinetic energy of the particles and can overcome or strengthen the interparticle forces.

Q8. What is plasma? Answer: Plasma is the fourth state of matter, consisting of ionized gas with free electrons and positive ions. It exists at very high temperatures or under strong electromagnetic fields.

Multiple Choice Questions (MCQs)

• Q1. Which state of matter has a fixed shape and fixed volume?
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: A
• Explanation: The text states that solids have a fixed shape and volume.

• Q2. In which state of matter do particles vibrate about fixed positions?
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: A
• Explanation: The text states that solid particles vibrate about their mean positions.

• Q3. Which state of matter takes the shape of its container but has a fixed volume?
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: B
• Explanation: The text states that liquids have no fixed shape and can adopt the shape of the container, but they have a fixed volume.

• Q4. In which state of matter are particles far apart and move freely at high speeds?
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: C
• Explanation: The text states that gas particles are in constant random motion and the distance between them is larger.

• Q5. Which state of matter generally has the highest density?
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: A
• Explanation: The text states that solids have high density because their particles are closely packed.

• Q6. Which state of matter has negligible attractive forces between particles?
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: C
• Explanation: The text states that there is negligibly weak attractive force between gas particles.

• Q7. The particles in a liquid:
• A. Are fixed in position
• B. Can slide past each other
• C. Move freely at high speeds
• D. Are ionized

• Correct Answer: B
• Explanation: The text states that liquid particles flow and glide over each other.

• Q8. Which of the following is NOT a characteristic of gases?
• A. No fixed shape
• B. No fixed volume
• C. High density
• D. Particles move randomly

• Correct Answer: C
• Explanation: Gases have low density, not high density. The other options are characteristics of gases.

• Q9. The state of matter with the highest kinetic energy of particles is:
• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

• Correct Answer: C
• Explanation: The text states that gas particles have the highest energy and move quickly in all directions.

• Q10. Which of the following is an example of a change of state?
• A. Ice melting
• B. Water flowing
• C. Air moving
• D. A rock sitting

• Correct Answer: A
• Explanation: Ice melting is a change from solid to liquid, which is a change of state.

Frequently Asked Questions (FAQs)

Q1. What is the difference between a liquid and a gas? Answer: A liquid has a fixed volume but no fixed shape. A gas has neither a fixed volume nor a fixed shape. Gas particles are much farther apart and move much faster than liquid particles.

• Q2. Why do solids have a fixed shape?
• Answer: Because the particles in a solid are held in fixed positions by strong interparticle forces. They can only vibrate about these positions.

Q3. Why are gases compressible? Answer: Gases are compressible because there is a large amount of empty space between the gas particles. When pressure is applied, the particles can be pushed closer together, reducing the volume.

Q4. What is the difference between boiling and evaporation? Answer: Boiling is a rapid change from liquid to gas that occurs at a specific temperature (the boiling point) throughout the liquid. Evaporation is a slow change from liquid to gas that occurs at the surface of the liquid at any temperature.

Q5. What is sublimation? Answer: Sublimation is the change of state directly from solid to gas without passing through the liquid state. An example is dry ice (solid carbon dioxide) turning into gas.

Q6. How does temperature affect the state of matter? Answer: Increasing temperature increases the kinetic energy of particles, which can overcome interparticle forces and cause a change of state (e.g., solid → liquid → gas). Decreasing temperature has the opposite effect.

Q7. What is plasma? Answer: Plasma is the fourth state of matter, consisting of ionized gas with free electrons and positive ions. It exists at very high temperatures or under strong electromagnetic fields. Examples include the Sun, stars, lightning, and neon lights.

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