Particle model

1. Overview

The particle model (or kinetic theory) describes matter as being made of tiny, moving particles. This model allows us to explain the physical properties of solids, liquids, and gases, as well as how substances respond to changes in temperature and pressure.

Key Definitions

• Kinetic Particle Model: The theory that all matter is made of tiny particles in constant motion.
• Absolute Zero: The lowest possible temperature (-273°C), where particles have the least possible kinetic energy and are practically motionless.
• Pressure: The force exerted by gas particles colliding with the surface area of their container.
• Brownian Motion: The random, jerky motion of microscopic particles (like smoke or pollen) suspended in a fluid.
• Internal Energy: The total energy stored by the particles that make up a system (kinetic energy + potential energy).

Core Content

States of Matter

Matter exists in three primary states, distinguished by the arrangement, separation, and motion of its particles.

State	Arrangement	Separation	Motion
Solid	Regular lattice	Touching (very close)	Vibrate about fixed positions
Liquid	Irregular/Random	Touching (close)	Slide over each other
Gas	Random	Far apart	Move rapidly and randomly

Temperature and Motion

• Relationship: As the temperature of a substance increases, the average kinetic energy of its particles increases. This means the particles move faster (in liquids/gases) or vibrate more violently (in solids).
• Absolute Zero: At -273°C, particles reach their minimum possible kinetic energy. You cannot get colder than this because you cannot have less than zero motion.

Gas Pressure

• Gas particles move randomly at high speeds.
• When they collide with the walls of a container, they exert a force on that wall.
• The sum of these many tiny collisions creates pressure.
• Increasing Pressure: If you heat a gas, particles move faster and hit the walls more often and with more force, increasing the pressure.

Brownian Motion

• Evidence: If you look at smoke particles or pollen grains in water under a microscope, they move in a zig-zag, random way.
• Explanation: This happens because the microscopic particles are being hit by much smaller, invisible, fast-moving atoms or molecules of the air or water.
• Because the collisions occur from all sides at different times, the larger particle is knocked around randomly.

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Extended Content (Extended Only)

Forces and Properties

• Solids: Strong intermolecular forces hold particles in a fixed lattice, giving solids a definite shape and making them difficult to compress.
• Liquids: Forces are strong enough to keep particles close together but weak enough to allow them to flow and take the shape of a container.
• Gases: Intermolecular forces are negligible. Particles are far apart, explaining why gases are easily compressed and expand to fill any volume.

Pressure as Force per Unit Area

Pressure is not just a "push"—it is specifically the force exerted by particles divided by the area they hit.

• When a gas particle hits a surface, its momentum changes, which exerts a force on the surface.
• $Pressure = \frac{Force}{Area}$

Detailed Brownian Motion

• Microscopic Particles: These are the visible ones (smoke, pollen). They are relatively large and heavy.
• Atoms/Molecules: These are the invisible ones (air, water). They are very light but move at extremely high speeds.
• The Mechanism: Even though a water molecule is much lighter than a pollen grain, many high-speed collisions provide enough total force to move the grain.

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Key Equations

• Pressure: $P = \frac{F}{A}$
  • $P$ = Pressure (Pascals, Pa or $N/m^2$)
  • $F$ = Force (Newtons, N)
  • $A$ = Area ($m^2$)
• Temperature Conversion: $T (K) = \theta (^\circ C) + 273$
  • $T$ = Kelvin (Absolute temperature)
  • $\theta$ = Degrees Celsius

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that smoke particles or pollen grains move because they are "alive" or moving on their own.
• ✓ Right: They move because they are being bombarded by invisible, fast-moving air/water molecules.
• ❌ Wrong: Assuming that particles themselves expand or get bigger when a substance is heated.
• ✓ Right: The particles stay the same size; only the space between them increases (or they vibrate more).
• ❌ Wrong: Thinking that heating a gas always increases its volume.
• ✓ Right: If the container is rigid (like a metal tank), the particles cannot spread out; instead, the pressure increases.
• ❌ Wrong: Using -273 to calculate a change in temperature (an interval).
• ✓ Right: A change of 10°C is the same as a change of 10 K. You only add/subtract 273 when converting specific temperature points.
• ❌ Wrong: Thinking heavy particles are needed to cause Brownian motion.
• ✓ Right: Very light molecules (like $H_2O$) can move larger particles because they travel at very high speeds.

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Exam Tips

1. Use the correct terminology: In Brownian motion questions, distinguish between the "microscopic particles" (which you can see) and the "atoms/molecules" (which you cannot see but cause the movement).
2. Pressure Descriptions: When explaining why pressure increases, always mention two things: particles hit the walls more often (frequency) and harder (greater force).
3. State Changes: Remember that during a state change (like melting), the temperature stays constant even though you are adding heat, because the energy is being used to break bonds/overcome forces between particles, not to increase their speed.