Unit 3 Properties of Substances and Mixtures

AP Chemistry Solids, Liquids, and Gases

How particles are spaced and how they move in each phase, why a mole of gas takes up so much more room, and how to draw and justify it on the exam.

AP Chemistry treats the three phases as a question about particles, not as a list of properties to memorize. Topic 3.3 asks you to represent each phase at the particle level and connect that picture to what you can observe. The forces behind the picture come from intermolecular forces, and the classification of solids by what holds them together is in types of solids. This page covers spacing and motion, the numbers that show how different a gas really is, and the particle diagrams you may be asked to draw.

Solids, Liquids, and Gases Compared at the Particle Level

Fig. 1 · Particle diagrams of the phases of one substance

	Solid	Liquid	Gas
Spacing	Close contact	Close contact	Far apart; depends on temperature, pressure, and volume
Arrangement	Regular if crystalline, irregular if amorphous	No long-range pattern	Random
Motion	Vibration only; particles do not move past each other	Continually moving and colliding, sliding past neighbors	Constant, fast, and mostly independent
Effect of forces between particles	Strong enough to hold positions	Keep particles together but allow movement	Minimal under most conditions
What you observe	Definite shape and volume	Definite volume, takes the shape of its container	No definite shape or volume; compresses easily

Solids can be crystalline, like table salt, or amorphous, like glass. Which one forms depends on how well the particles pack together and on the forces between them; types of solids sorts ionic, molecular, network, and metallic solids by those forces. Moving from one phase to another takes or releases energy without changing the temperature during the change, which is what the flat sections of a heating curve show. For a molecular substance, the molecules stay whole throughout, the point made in intermolecular vs intramolecular forces.

Not tested: the course description excludes understanding or interpreting phase diagrams, the pressure-temperature graphs with triple and critical points. Spend your time on the particle picture instead.

Why Solids and Liquids Have Similar Molar Volumes but Gases Do Not

Molar volume is the space one mole takes up: molar mass divided by density for a solid or liquid. Because particles touch in both phases, solid and liquid molar volumes are close. A gas is almost entirely empty space, so its molar volume is far larger and depends on conditions.

Substance	Solid (mL/mol)	Liquid (mL/mol)	Gas at its boiling point and 1 atm (mL/mol)
Water, H₂O	19.7	18.1	about 30,600
Aluminum, Al	10.0	11.4	about 229,000

Solid and liquid values come from densities of 0.917 and 0.997 g/mL for water and 2.70 and 2.375 g/mL for aluminum. Gas values are estimates from the ideal gas law at 100°C and 2519°C. Aluminum shows the usual pattern, with the liquid slightly larger than the solid. Water is the well-known exception: hydrogen bonding holds ice in an open structure, so ice has the larger molar volume and floats.

Worked Example 1: Comparing the Molar Volumes of Liquid Water and Steam

Liquid water has a density of 0.997 g/mL. How many times more space does one mole of steam take up at 100°C and 1.00 atm?

Liquid: 18.02 g/mol ÷ 0.997 g/mL = 18.1 mL/mol.
Gas: V = nRT/P = (1.00 mol)(0.08206 L·atm/(mol·K))(373 K) ÷ 1.00 atm = 30.6 L = 30,600 mL.
Ratio: 30,600 ÷ 18.1 = about 1,700 times.

The molecules themselves did not grow; the space between them did. That is why gases compress easily and liquids barely do, and why a particle diagram of a gas should show far fewer particles in the same frame as a liquid.

Worked Example 2: Explaining the Phases of Chlorine, Bromine, and Iodine

At 25°C, Cl₂ is a gas, Br₂ is a liquid, and I₂ is a solid. Explain this in terms of particles and forces.

All three are nonpolar, so London dispersion forces are the only forces between molecules. From Cl₂ (34 electrons) to Br₂ (70) to I₂ (106), the electron cloud gets larger and more polarizable, so dispersion forces get stronger.

Cl₂ (boils near −34°C): at 25°C, the kinetic energy of the molecules overcomes the weak attractions, so they move independently as a gas.
Br₂ (melts near −7°C, boils near 59°C): attractions keep the molecules in contact but do not hold them in fixed positions.
I₂ (melts near 114°C): attractions are strong enough to hold the molecules in a regular arrangement.

The justification needs all three links: the structure (number of electrons), the force (dispersion), and the balance between that force and particle motion at this temperature.

Worked Example 3: Correcting a Particle Diagram of Water

A student draws a sealed flask of liquid water with some vapor above it. In the liquid, the H₂O molecules are spread far apart. In the vapor, many H and O atoms are drawn separately. Identify the two errors.

Liquid spacing: liquid molecules should be in close contact, with no regular pattern, and more closely packed than anything in the vapor.
Vapor particles: evaporation does not break covalent bonds, so the vapor should show whole H₂O molecules, spaced far apart, with the same bent shape as in the liquid.

Particle-diagram questions reward the conventions: draw whole molecules, keep shapes and atom ratios consistent, and make spacing match the phase. More of these rules are in particulate diagrams, and the balance between evaporation and condensation in that sealed flask is covered in vapor pressure.

Worked Example 4: How Compressing or Heating a Gas Changes Its Particles

A gas at 1.00 atm is in a cylinder with a movable piston. Describe what happens to the spacing, collisions, and average kinetic energy of its particles in each case.

Volume halved at constant temperature: the spacing decreases and the particles hit the walls more often, so the pressure doubles to 2.00 atm. Average kinetic energy does not change, because temperature did not.
Temperature raised in a rigid, fixed-volume container: the spacing stays the same, but average kinetic energy increases, so the particles collide more often and with more force, and the pressure rises.

This is the particle-level reasoning behind the gas laws; the numbers can be checked with the gas law calculator, and the link between temperature and particle speed is in kinetic molecular theory. When the particles are crowded or slow, the forces and volumes this picture ignores start to matter, which is the subject of real gases.

Five Solids, Liquids, and Gases Practice Questions With Answers

In which phase or phases are the particles in close contact at all times?
Show answer
Solids and liquids. That is why their molar volumes are similar and why neither compresses much.
What is the main particle-level difference between a solid and a liquid of the same substance?
Show answer
Motion. Particles in a solid vibrate around fixed positions and do not move past each other; particles in a liquid continually move past their neighbors while staying in contact.
Glass has no regular arrangement of particles, yet it holds its shape. Classify it and explain why it is rigid.
Show answer
It is an amorphous solid. Even without a regular pattern, the motion of its particles is limited and they do not move past one another, so it keeps its shape.
Liquid nitrogen has a density of 0.807 g/mL. One mole of N₂ gas occupies 24.5 L at 25°C and 1 atm. Roughly how many times larger is the gas volume?
Show answer
The liquid is 28.02 g/mol ÷ 0.807 g/mL = 34.7 mL/mol. Then 24,500 mL ÷ 34.7 mL is about 700 times larger.
A sealed, rigid flask of neon is cooled from 300. K to 150. K. What happens to the spacing of the atoms and to their average kinetic energy?
Show answer
The spacing stays the same, because the volume and number of atoms are fixed. The average kinetic energy is cut in half, because it is proportional to absolute temperature.

Common Solids, Liquids, and Gases Mistakes

Drawing liquid particles spread out. Liquid particles are in close contact, almost as close as in the solid.
Saying particles in a solid do not move. They vibrate; they just do not move past each other.
Assuming all solids are crystalline. Amorphous solids such as glass have no regular arrangement.
Breaking molecules apart in a phase change. Melting and boiling a molecular substance leave its covalent bonds intact, so diagrams must show whole molecules.
Linking a higher temperature with larger particles. Particles move faster when heated; they do not expand.

This topic sits inside Unit 3, the most heavily weighted unit on the exam. Test the whole unit with the Unit 3 practice test, and to see how a strong unit would change your overall result, try the AP Chem Score Calculator.

Frequently Asked Questions

What is the difference between solids, liquids, and gases at the particle level?

Particles in a solid are in close contact and only vibrate around fixed positions. Particles in a liquid are also in close contact but move past one another. Particles in a gas are far apart and move quickly and independently, with little effect from the forces between them.

Why do solids and liquids have similar molar volumes?

In both phases the particles are in close contact at all times, so a mole takes up about the same space. Liquid water is 18.1 mL/mol and ice is 19.7 mL/mol, while water vapor at 100 degrees Celsius and 1 atm is about 30,600 mL/mol.

Are phase diagrams on the AP Chemistry exam?

No. The course description says that understanding or interpreting phase diagrams will not be assessed. You do need to represent solids, liquids, and gases at the particle level and explain their properties using intermolecular forces.

What is the difference between a crystalline solid and an amorphous solid?

A crystalline solid has its particles in a regular three-dimensional arrangement, as in table salt. An amorphous solid, such as glass, has no regular arrangement. In both, the particles have limited motion and do not move past one another.

Why does a gas have no definite shape or volume?

Gas particles are in constant motion and the forces between them have little effect, so they spread out to fill whatever container holds them. Their spacing and how often they collide depend on temperature, pressure, and volume.

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