👋 Welcome to the Particle Model of Matter!

Hello future physicist! This chapter is super important because it explains the foundation of everything around you—how matter behaves. We will be looking at the tiny, invisible particles that make up solids, liquids, and gases, and how energy affects them.

Don't worry if some terms seem new; we will break them down using everyday examples. Let's get started on understanding how the world works at a microscopic level!

1. The Three States of Matter: A Particle Perspective

The Particle Model of Matter states that all substances are made up of tiny particles (atoms, molecules, or ions) that are constantly moving. The difference between a solid, a liquid, and a gas comes down to three things:

  • The arrangement of the particles.
  • The forces between the particles.
  • The movement of the particles.
1.1. Comparing Solids, Liquids, and Gases

Imagine the particles are people on a dance floor.


Solid (Example: Ice)

  • Arrangement: Regular, fixed pattern (like soldiers standing in formation).
  • Forces: Very strong forces hold them tightly together.
  • Movement: They can only vibrate around their fixed positions. They cannot move past each other.
  • Properties: Fixed shape and fixed volume. Very difficult to compress.

Liquid (Example: Water)

  • Arrangement: Random arrangement. They are still close together, but not in a pattern.
  • Forces: Weaker forces allow them to slide past one another (like people slow dancing).
  • Movement: They constantly move and change position.
  • Properties: Fixed volume, but take the shape of the container. Difficult to compress.

Gas (Example: Steam)

  • Arrangement: Completely random, far apart (like people sprinting across a stadium).
  • Forces: Very weak forces—almost zero interaction except during collisions.
  • Movement: Move rapidly and randomly in all directions.
  • Properties: No fixed shape or volume (they fill the container). Easy to compress.
Quick Review: The Strength of Forces

Solid (Strongest forces) → Liquid (Medium forces) → Gas (Weakest forces)

2. Internal Energy, Temperature, and Density

2.1. What is Internal Energy?

Every substance has Internal Energy. This is the total energy stored inside the system, which is split into two parts:

  1. Kinetic Energy (KE): The energy due to the movement (vibration or motion) of the particles.
  2. Potential Energy (PE): The energy stored in the bonds/forces between the particles due to their position or separation.

Temperature is directly related to the average Kinetic Energy of the particles.

  • If you heat something up, its particles move faster (higher KE), so its temperature increases.
  • If you cool something down, its particles slow down (lower KE), so its temperature decreases.
Did you know? Even in a solid that seems perfectly still, the particles are vibrating extremely quickly!
2.2. Density (\(\rho\))

Density is a measure of how much mass is squeezed into a certain volume. Think of it as how "packed" the particles are.

The formula for density is:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
Or, using symbols:
\[ \rho = \frac{m}{V} \]

  • Units of density are typically \(\text{kg}/\text{m}^3\) or \(\text{g}/\text{cm}^3\).
  • Generally, solids are denser than liquids, which are denser than gases. This is because particles are much closer together in solids.
  • Exception: Ice is less dense than water (which is why ice floats!)—but this is unusual for most substances.

Key Takeaway: Temperature measures particle movement (KE). Density measures how tightly packed the mass is (\(m/V\)).

3. Changes of State: The Role of Latent Heat

When a substance changes state (e.g., melting), you must supply energy. This energy does not make the temperature rise; instead, it provides the Potential Energy needed to break or overcome the forces between the particles.

Let’s review the key changes:

  1. Melting: Solid to Liquid (Energy is taken in).
  2. Freezing: Liquid to Solid (Energy is released).
  3. Boiling/Evaporating: Liquid to Gas (Energy is taken in).
  4. Condensing: Gas to Liquid (Energy is released).
  5. Sublimation: Solid directly to Gas (Energy is taken in - Example: Dry ice turning into gas).
3.1. Specific Latent Heat (SLH)

When a substance is melting or boiling, you can continue heating it, but the temperature stays constant. This "hidden" energy is called Latent Heat (Latent means 'hidden').

Specific Latent Heat (\(L\)) is the amount of energy (heat) required to change the state of 1 kg of a substance without changing its temperature.

We use different terms depending on the change:

  1. Specific Latent Heat of Fusion (\(L_f\)): Energy needed to melt (or freeze) 1 kg of a substance. This breaks the bonds of the solid structure.
  2. Specific Latent Heat of Vaporisation (\(L_v\)): Energy needed to boil (or condense) 1 kg of a substance. This completely separates the particles to form a gas.

Analogy: Imagine heating water in a pan. Once it hits 100°C, all the energy you add is used to turn liquid water into steam (breaking the water bonds), not to make the steam hotter than 100°C (Kinetic Energy).

The formula for calculating the energy (E) needed for a change of state is:


\[ \text{Energy (J)} = \text{Mass (kg)} \times \text{Specific Latent Heat (J}/\text{kg)} \]
In symbols:
\[ E = m L \]
⚠️ Common Mistake Alert!

Do NOT confuse Specific Latent Heat (\(L\)) with Specific Heat Capacity (\(c\)).

\(L\) is used when the state changes (temperature is constant).
\(c\) is used when the temperature changes (state is constant).

Key Takeaway: During a change of state, energy input changes Potential Energy (breaks forces) and the temperature remains flat.

4. Gases and Pressure

Pressure in a gas is caused by the billions of tiny particles moving randomly and colliding with the walls of their container.

4.1. The Origin of Gas Pressure

Every time a particle hits the wall and bounces off, it exerts a small force on the wall. Since there are so many particles hitting the walls constantly, this results in an overall outward push known as Gas Pressure.

Pressure is calculated as:
\[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} \]

4.2. Pressure, Temperature, and Volume

The particle model helps us understand how changing temperature or volume affects gas pressure.

1. Effect of Temperature (at constant volume):

  • When you heat a gas, the particles gain kinetic energy and move faster.
  • Faster particles hit the walls more frequently and with more force.
  • Therefore, increasing temperature increases pressure. (If a container is sealed, this can be dangerous!)

2. Effect of Volume (at constant temperature):

  • If you squeeze a gas into a smaller volume (decrease V), the particles have less space to move around in.
  • The particles will hit the container walls much more frequently.
  • Therefore, decreasing volume increases pressure.
Memory Aid: Think of a sealed spray can. If you heat it (increase T), the internal pressure builds up massively because the particles hit the inside walls much harder and faster.

Key Takeaway: Gas pressure is caused by particle collisions. Heating speeds up the particles, increasing pressure; reducing volume increases collision frequency, increasing pressure.

🎉 Summary and Final Thoughts

You have now covered the core concepts of the particle model! Remember that physics often relies on visualising the invisible. Always picture the particles—are they packed tight (solid), sliding around (liquid), or zooming everywhere (gas)?

  • Temperature is all about Kinetic Energy (movement speed).
  • Changes of state use Latent Heat to change Potential Energy (breaking bonds).
  • Density tells you how packed the mass is (\(\rho = m/V\)).
  • Pressure is generated by particles hitting container walls.

Keep practising the calculations for density and specific latent heat, and you'll master this chapter! Good luck!