C1 States of Matter: The Amazing World of Particles
Welcome to the first chapter of Chemistry! Everything you see and touch—from your textbook to the air you breathe—is made of tiny particles. These particles decide whether the substance is a solid, a liquid, or a gas.
This chapter is fundamental because understanding how particles behave explains almost all chemical and physical processes. Don't worry if this seems tricky at first; we will use the amazing Kinetic Particle Theory to make sense of it all!
Quick Review: The Three States
The three common states of matter are Solid, Liquid, and Gas. They have very distinct properties:
Distinguishing Properties of Solids, Liquids, and Gases (C1.1 Core 1)
- Solids: Have a fixed shape and a fixed volume. They cannot be easily compressed. (e.g., Ice cube, rock)
- Liquids: Do not have a fixed shape (they take the shape of their container) but they do have a fixed volume. They are also very difficult to compress. (e.g., Water, juice)
- Gases: Do not have a fixed shape and do not have a fixed volume (they fill their container completely). They can be easily compressed. (e.g., Air, steam)
The Kinetic Particle Theory (C1.1 Core 2 & Supplement 5)
The Kinetic Particle Theory is the key to understanding the states of matter. It states that all matter is made up of tiny particles (atoms or molecules) that are constantly moving. The differences between solids, liquids, and gases depend on the arrangement, separation, and motion of these particles.
1. Structure of Solids
- Arrangement: Particles are arranged in a fixed, regular pattern (a lattice).
- Separation: Particles are very close together.
- Motion: Particles vibrate around fixed positions. They do not move from place to place.
Analogy: Think of people standing shoulder-to-shoulder in straight lines at a busy concert. They can only shake and wobble, they can't walk past each other.
2. Structure of Liquids
- Arrangement: Particles are randomly arranged.
- Separation: Particles are still close together (similar distance to solids).
- Motion: Particles move randomly and slide past each other. This allows the liquid to flow and take the shape of its container.
Analogy: Imagine people leaving the concert. They are still crowded together, but they can move around and push past their neighbours.
3. Structure of Gases
- Arrangement: Particles are totally random.
- Separation: Particles are very far apart from each other.
- Motion: Particles move very quickly and randomly in all directions. They frequently collide with each other and the walls of the container.
Analogy: Imagine people spread out across a massive, empty football field, running freely.
Quick Review Box: Summary of Structures
Key Takeaway: The forces between particles are strongest in solids and weakest in gases. The amount of energy the particles have determines their state.
Changes of State (C1.1 Core 3 & Supplement 5)
A change of state is a physical change, meaning the particles themselves do not change, only their energy, arrangement, and motion change.
Terminology for Changes of State (C1.1 Core 3)
Here are the terms you must know for the transitions between states:
- Melting (Solid to Liquid): Occurs at the melting point. (e.g., Ice melting into water)
- Freezing (Liquid to Solid): Occurs at the freezing point. (e.g., Water turning into ice)
- Boiling (Liquid to Gas): Occurs rapidly throughout the liquid at the boiling point. (e.g., Water boiling to become steam)
- Condensing (Gas to Liquid): Occurs when a gas is cooled. (e.g., Steam turning back into water droplets on a cold window)
- Evaporating (Liquid to Gas): Occurs slowly only at the surface of the liquid, below its boiling point. (e.g., A puddle drying up on a sunny day)
Explaining Changes of State using Kinetic Theory (C1.1 Supplement 5)
Changes of state happen when a substance gains or loses thermal energy.
Energy Gain (e.g., Melting and Boiling)
- Heating up: When a solid is heated, the particles absorb energy and their kinetic energy increases (they vibrate more vigorously).
- Melting: When enough energy is supplied (at the melting point), the particles vibrate so strongly that they overcome the fixed forces holding them in position. They break free from the lattice structure and can slide past each other, becoming a liquid.
- Boiling: As the liquid is heated further, particle energy increases until the boiling point is reached. The particles gain enough energy to overcome all attractive forces and escape completely from the liquid, becoming a gas.
Energy Loss (e.g., Freezing and Condensing)
- Cooling down: When a gas is cooled (energy is removed), the particles lose kinetic energy and slow down.
- Condensing: When particles slow down sufficiently, the attractive forces between them pull them closer together, forming a liquid.
- Freezing: When cooling continues, the particles slow down even more until they settle into fixed positions and regular arrangements, forming a solid.
Memory Aid: Think of energy as the particles’ excitement level!
- Low energy (calm) = Solid (stuck in place)
- Medium energy (wiggly) = Liquid (sliding past each other)
- High energy (crazy) = Gas (flying around everywhere)
The Effect of Temperature and Pressure on Gases (C1.1 Core 4)
Gases are special because they are the only state that is easily compressed. This is because gas particles are far apart, leaving lots of empty space.
1. Effect of Temperature (at Constant Pressure)
When you increase the temperature of a gas while keeping the pressure the same, the volume of the gas increases.
- Why? Heating the gas increases the kinetic energy of the particles.
- The faster-moving particles hit the walls of the container with greater force and more frequently.
- To keep the pressure constant, the container must expand, meaning the volume increases.
Did you know? This is why hot air balloons float! Heating the air makes it expand (increase volume) and become less dense than the cooler surrounding air.
2. Effect of Pressure (at Constant Temperature)
When you increase the pressure on a gas while keeping the temperature the same, the volume of the gas decreases.
- Why? Increasing the pressure means you are applying a greater external force on the gas.
- Since there are huge spaces between gas particles, the external force easily pushes the particles closer together.
- This results in a decrease in volume.
Real-World Example: When you push down the plunger of a syringe filled with air (but block the tip), the volume decreases because you are increasing the pressure on the gas particles.
Common Mistake to Avoid!
Don't confuse the density of a liquid and a gas. When liquids are heated, they expand only slightly, so their density changes very little. However, when gases are heated, they expand greatly, leading to a large drop in density.
Key Takeaway Summary
We use the Kinetic Particle Theory to describe solids, liquids, and gases based on their particle arrangement, separation, and motion. Changes of state are caused by the gain or loss of energy, which affects particle movement. Gases are highly compressible because of the large separation between their particles, and their volume is highly sensitive to changes in temperature and external pressure.