Solids, Liquids, and Gases
Represent motion, spacing and intact particle identity in solids, liquids and gases.
AP Chemistry Topic 3.3 · CED printed p. 62 / PDF p. 69. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Particle identity
- Interactions and molecular solids
Change arrangement and motion, not particle identity
In a crystalline solid, particles occupy a regular three-dimensional arrangement and vibrate around positions. In an amorphous solid, there is no long-range periodic order, but particles still lack overall translation relative to their neighbors on the relevant timescale. A frozen particle diagram is a representation of arrangement, not evidence that motion has stopped.
In a liquid, particles remain close and continually move and collide, changing neighbors. Attractions and temperature influence this behavior. Warming while the substance stays liquid increases average kinetic energy and promotes contact rearrangement; it does not automatically break the molecules’ covalent bonds. Particles remain close, and an exact spacing change needs density or thermal-expansion information. Solid and liquid molar volumes are often similar because both have close contacts; the particle size does not expand dramatically when melting. Water’s open ice arrangement is a useful exception: its solid is less dense than its liquid under ordinary conditions.
A gas occupies the available container
Gas particles travel through large separations with brief collisions. Weak interparticle effects in the ideal model let a gas adopt both the volume and shape of its container. Compressing a fixed number into a smaller volume reduces average spacing and increases collision frequency at fixed temperature. Heating at fixed volume increases average kinetic energy and collision impact, raising pressure. Specify what remains fixed before deciding how spacing or pressure changes.
For a molecular substance, show the same intact molecules in each phase; do not turn liquid H2O into separate H and O atoms in the gas panel. Use arrows for motion, a spacing scale and labels explaining that a two-dimensional snapshot cannot show every collision or three-dimensional neighbor. Phase-diagram interpretation is outside this AP requirement.
Let's work through it
Work it through
Sketch a fixed sample of a molecular substance as crystalline solid, liquid and gas; justify the drawings with volume and motion.
- Use identical molecule symbols and the same total number in each whole-sample sketch, preserving identity and amount.
- Solid: close regular arrangement, small vibration arrows; liquid: close irregular arrangement with changing-neighbor arrows. Their whole-sample volumes should be broadly similar for an ordinary substance.
- Gas: separate intact molecules much farther apart in a larger container, with random motion arrows.
- If depicting equal observation-window volumes instead of whole samples, fewer gas particles can be appropriate, but label that changed comparison rather than silently losing particles.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Does a solid’s ordered diagram mean its particles are motionless?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Does a solid’s ordered diagram mean its particles are motionless?
Worked answer and reason
No. They vibrate around positions without overall translational neighbor exchange.
A static representation omits time-dependent motion.
Retrieval 2
Why are liquid and solid molar volumes generally much more similar than either is to a gas molar volume?
Worked answer and reason
Both condensed phases have particles in close contact; gases have large separations.
Spacing, rather than an enlarged particle, changes bulk volume.
Catch the tempting mistake
- Gas particles are larger versions of liquid particles.
- Physical phase change preserves particle identity and roughly its intrinsic size; spacing changes.
- All solids must be crystalline.
- Amorphous solids lack long-range periodic order but still have restricted translational motion.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 3 problem 7 · ap application · particle
- Unit 3 problem 8 · ap synthesis · multiple
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For the teacher
- Have students declare whole-sample versus equal-volume-window comparisons.
- Ask for motion as well as spacing in every phase drawing.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 3.3.A
- Represent the differences between solid, liquid, and gas phases using a particulate-level model.
- 3.3.A.1
- Solids can be crystalline, where the particles are arranged in a regular three-dimensional structure, or they can be amorphous, where the particles do not have a regular, orderly arrangement. In both cases, the motion of the individual particles is limited, and the particles do not undergo overall translation with respect to each other. The structure of the solid is influenced by interparticle interactions and the ability of the particles to pack together.
- 3.3.A.2
- The constituent particles in liquids are in close contact with each other, and they are continually moving and colliding. The arrangement and movement of particles are influenced by the nature and strength of the forces (e.g., polarity, hydrogen bonding, and temperature) between the particles.
- 3.3.A.3
- The solid and liquid phases for a particular substance typically have similar molar volume because, in both phases, the constituent particles are in close contact at all times.
- 3.3.A.4
- In the gas phase, the particles are in constant motion. Their frequencies of collision and the average spacing between them are dependent on temperature, pressure, and volume. Because of this constant motion, and minimal effects of forces between particles, a gas has neither a definite volume nor a definite shape.
- Understanding/interpreting phase diagrams will not be assessed on the AP Exam.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
Glossary and deeper reading · 1 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Covalent bond
A covalent bond involves shared electron density between atoms. The sharing can be approximately equal or unequal depending on the atoms and their environment. For example, the bond in a hydrogen molecule shares electrons between two identical atoms.