The Particle Model

• Solids: Particles are packed tightly in a regular, repeating pattern called a lattice. Because the attractive forces are very strong, particles can only vibrate in fixed positions, giving solids a definite shape and volume.

• Liquids: Particles remain close together but are arranged randomly and can slide past one another. The attractive forces are strong enough to maintain a definite volume but weak enough to allow the liquid to flow and take the shape of its container.

• Gases: Particles are very far apart with negligible attractive forces between them. They move rapidly in straight lines until they collide, allowing gases to expand to fill any available volume and making them highly compressible.

Comparison of Physical States

• Physical vs. Chemical Change: In the context of the particle model, a physical change (like melting) involves changing the arrangement or energy of particles without altering the particles themselves. A chemical change involves breaking and forming bonds to create entirely new types of particles.

• Keywords for Success: When describing gases, always use terms like 'random motion', 'collisions', and 'negligible forces'. For solids, focus on 'fixed positions' and 'vibrations'.

• Explaining Pressure: If asked why pressure increases with temperature, follow this logical chain: Higher temperature $\rightarrow$ higher kinetic energy $\rightarrow$ faster moving particles $\rightarrow$ more frequent and forceful collisions with walls $\rightarrow$ higher pressure.

• Conservation of Mass: Always remember that during a state change, the number of particles remains constant. If a diagram shows 10 particles in a solid state, the corresponding liquid or gas diagram must also contain exactly 10 particles to be accurate.

• Density Logic: Recognize that density usually decreases from solid to gas because the same number of particles occupies a much larger volume (with the notable exception of water/ice).

• Particle Expansion Myth: A very common error is believing that individual particles expand when heated. In reality, the particles stay the same size; it is the space between them that increases due to increased kinetic energy.

• The 'Stillness' Error: Students often assume particles in a solid are not moving. They are actually vibrating; particles only theoretically stop moving at absolute zero ($0$ K or $-273.15$ $^{\circ}$C).

• Disappearing Matter: When a liquid evaporates, it might seem to disappear. The model clarifies that the particles have simply spread out into the air as a gas and are still present, maintaining the total mass of the system.