Topic 3.3 Notes – Solids, Liquids, and Gases

1. How the Three Phases Differ at the Particulate Level

Everything here comes back to three ideas: spacing between particles, strength of intermolecular forces (IMFs), and particle motion.

Solids

In a solid, particles are:

• In close contact
• Held by strong IMFs
• Limited to vibrational motion only (they jiggle in place, no overall translation)

Because of that:

• Definite shape and definite volume
• Nearly incompressible (no empty space to squeeze out)
• Extremely slow diffusion

Crystalline vs Amorphous Solids

Crystalline solids

• Particles arranged in a regular, repeating 3D pattern
• Structure depends on packing and interparticle forces

Think of a crystalline solid as a tiny building block repeated over and over in three dimensions.

Unit cell and repeating crystal lattice

The small cube represents a unit cell, the smallest repeating portion of the structure. When that unit cell repeats in all directions, it forms the full crystal lattice.

Amorphous solids

• No long-range repeating pattern
• Still rigid overall, but particles are arranged more randomly

On a test, if they show a neat repeating array of spheres, that’s crystalline. A jumbled but tightly packed diagram is amorphous.

Liquids

Liquids look similar to solids at the particle level in one key way: particles are still in close contact.

But now:

• IMFs are weaker than in solids
• Particles are in constant motion and collisions
• They can slide past one another

Macroscopic results:

• Definite volume
• No definite shape (takes shape of container)
• Nearly incompressible
• Slow diffusion, but faster than solids

Important connection: since solids and liquids both have particles in close contact, they usually have similar molar volumes for the same substance.

Gases

Gases are the big contrast.

• Particles are far apart
• IMFs are minimal or negligible
• Motion is constant, random, straight-line motion

Because of this:

• No definite shape
• No definite volume
• Highly compressible
• Rapid diffusion

Collision frequency and spacing depend on temperature, pressure, and volume. Higher temperature means faster particles and more forceful collisions.

On the AP exam, when they ask why gases are compressible but liquids are not, the answer is almost always about empty space between particles.

2. Comparing the Three Phases

Here’s how to line them up side by side:

Energy trend:
Solid → Liquid → Gas = increasing kinetic energy and particle motion

Density trend (generally):
Solid ≥ Liquid ≫ Gas

3. How Intermolecular Forces and Temperature Control Phases

Phase depends on the competition between:

• IMFs pulling particles together
• Thermal energy pushing particles apart

Stronger IMFs:

• Favor solid or liquid
• Increase surface tension and viscosity

Higher temperature:

• Increases kinetic energy
• Helps particles overcome IMFs
• Favors liquid or gas

If they describe a substance with strong hydrogen bonding, expect higher surface tension, higher viscosity, and a greater tendency to be liquid or solid at a given temperature.

4. Special Properties of Liquids

These are all IMF stories.

Surface Tension

Surface molecules experience a net inward force because they aren’t surrounded on all sides.

Surface molecules experience a net inward force

Interior molecules are pulled equally in all directions. Molecules at the surface are pulled inward, which creates surface tension along the top layer.

Trends:

• Stronger IMFs → higher surface tension
• Higher temperature → lower surface tension

Cohesive vs Adhesive Forces

• Cohesive = liquid-liquid attraction
• Adhesive = liquid-container attraction

These determine the meniscus.

Concave vs. convex meniscus

• Adhesive > Cohesive → Concave
• Cohesive > Adhesive → Convex

Capillary action happens when adhesion pulls liquid up a surface and cohesion drags more molecules along.

Viscosity

Viscosity = resistance to flow.

• Stronger IMFs → higher viscosity
• Higher temperature → lower viscosity

If they compare glycerol and hexane, think IMF strength.

5. What You Must Be Able to Do

You should be able to:

• Draw and interpret particulate diagrams of each phase
• Explain macroscopic properties using spacing + motion + IMFs
• Justify why solids and liquids have similar molar volume
• Explain why gases expand and are compressible
• Predict how temperature or IMF strength affects surface tension and viscosity

Everything in this topic reduces to particle-level reasoning.