Properties of Solids
AP Chemistry· 12 min read
1. Core Classification of Solid Types★★☆☆☆⏱ 10 min
All solid substances tested on the AP exam fall into four distinct categories, defined entirely by the dominant force holding their constituent particles together. The category of a solid can be predicted almost entirely from its chemical formula, no experimental data required.
Crystalline vs Amorphous Solids
Crystalline solids have a perfectly repeating long-range lattice, while amorphous solids have only short-range order with no consistent repeating structure.
Solid Class | Constituent Particles | Dominant Binding Force |
|---|---|---|
Ionic | Cations + Anions | Electrostatic attraction |
Molecular | Discrete neutral molecules | Intermolecular forces |
Covalent Network | Neutral nonmetal atoms | Continuous covalent bonds |
Metallic | Metal cations | Delocalized metallic bonding |
Classify each of the following pure substances as one of the four solid types at standard conditions: KBr, C(diamond), H2O(s), Zn
- 1
- KBr is made of a metal cation (K+) and nonmetal anion (Br-), so it is an ionic solid
- 2
- Diamond is an extended lattice of covalently bonded carbon atoms, so it is a covalent network solid
- 3
- Ice is made of discrete H2O molecules held together by hydrogen bonds, so it is a molecular solid
- 4
- Zinc is a pure transition metal with delocalized valence electrons, so it is a metallic solid
Test your classification skills
What type of solid is SiO2 (quartz)?
Ionic
Molecular
Covalent Network
Metallic
Reveal answer
Covalent Network —SiO2 forms a continuous extended lattice of Si-O covalent bonds, not discrete molecules.
Exam tip:
AP graders will not award points for classification alone: you must explicitly reference the bonding force to justify your answer.
2. Bonding Strength and Melting Point Trends★★★☆☆⏱ 12 min
The melting point of a solid is directly proportional to the strength of the force holding its constituent particles together. Breaking stronger forces requires more thermal energy, leading to a higher melting temperature.
Rank the following solids from lowest to highest melting point: Sucrose (C12H22O11), MgO, Na, Diamond
- 1
- Sucrose is a molecular solid held together by weak intermolecular forces, so it has the lowest melting point ~186°C
- 2
- Sodium is a metallic solid with low charge density Na+ ions, so it melts at ~98°C? No, wait: sodium melts at 98°C which is lower than sucrose? Correct order: Na < Sucrose < MgO < Diamond
- 3
- MgO is an ionic solid with +2 and -2 charged ions, leading to very strong electrostatic attraction, melting point ~2800°C
- 4
- Diamond has continuous strong covalent bonds, so it has the highest melting point >3500°C
3. Electrical Conductivity of Solids★★★☆☆⏱ 8 min
A solid will only conduct electricity if it contains free, mobile charge carriers that can move through the lattice when a voltage is applied. No mobile charge carriers = zero conductivity.
Explain why solid NaCl does not conduct electricity, but molten NaCl at 900°C conducts very well
- 1
- In solid NaCl, Na+ and Cl- ions are locked in a rigid repeating lattice with no freedom of movement
- 2
- No mobile charge carriers exist in the solid state, so conductivity is zero
- 3
- When melted, the ionic lattice breaks apart, and individual Na+ and Cl- ions are free to move through the liquid phase
- 4
- These free moving ions act as charge carriers, allowing the molten sample to conduct electricity efficiently
Which of the following solids will conduct electricity at standard conditions?
Solid KCl
Solid Copper
Solid Sucrose
Solid Diamond
Reveal answer
Solid Copper —Metallic copper has delocalized free electrons that act as mobile charge carriers in the solid state.
4. Alloys and Special Solid Structures★★★★☆⏱ 7 min
Alloys are homogeneous mixtures of two or more elements, where at least one is a metal. They are classified as substitutional (atoms of the second metal replace host metal atoms in the lattice) or interstitial (small nonmetal atoms fit in gaps between host metal atoms).
Classify brass (mixture of Cu and Zn, similar atomic radii) and steel (mixture of Fe and small C atoms) as substitutional or interstitial alloys
- 1
- Zinc and copper have nearly identical atomic radii, so Zn atoms replace Cu atoms directly in the copper lattice: brass is a substitutional alloy
- 2
- Carbon atoms are far smaller than iron atoms, so C atoms fit in the gaps between Fe atoms in the iron lattice: steel is an interstitial alloy
5. Common Pitfalls
Wrong move:
Claiming molecular solids have low melting points because weak covalent bonds are broken during melting
Why:
Melting molecular solids only breaks weak intermolecular forces between discrete molecules, not the strong intramolecular covalent bonds inside individual molecules
Correct move:
Explicitly reference weak intermolecular forces (LDF, dipole-dipole, H-bonding) as the cause of low melting points for molecular solids
Wrong move:
Stating ionic solids conduct electricity in their solid crystalline form
Why:
Ions are locked in a rigid lattice with no freedom of movement, so no mobile charge carriers exist
Correct move:
Note ionic solids only conduct when molten or fully dissolved in a polar solvent, when ions are free to move independently
Wrong move:
Confusing covalent network solids with large molecular solids like C60 fullerene
Why:
Fullerene is made of discrete C60 molecules held together by weak LDFs, not a continuous extended covalent lattice
Correct move:
Check if the structure is a single unbroken lattice or made of separate discrete molecules to distinguish the two classes
Wrong move:
Ignoring graphite as a special covalent network solid variant
Why:
Graphite has a 2D layered structure with weak interlayer LDFs, so it cleaves easily even though its melting point is extremely high
Correct move:
Explicitly note the layered structure of graphite when discussing its unusual softness and conductivity
Wrong move:
Ranking metallic melting points only by group number
Why:
Metallic bonding strength depends on the charge density of the metal cation and number of delocalized electrons, not just group placement
Correct move:
Reference Coulomb's law for metallic bonding to justify relative melting point trends for different metals
6. Quick Reference Cheatsheet
Solid Type | Dominant Force | Typical Melting Point | Solid State Conductivity | Common Example |
|---|---|---|---|---|
Ionic | Electrostatic ion attraction | High (> 500°C) | Zero | NaCl, MgO |
Molecular | Intermolecular forces | Low (< 300°C) | Zero | Ice, Sucrose, Dry Ice |
Covalent Network | Continuous covalent bonds | Extremely High (> 2000°C) | Zero (except doped Si) | Diamond, Quartz, Graphite |
Metallic | Delocalized metallic bonding | Medium to High | Excellent | Copper, Iron, Sodium |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 · MCQ
Solid type property ranking
- 2022 · FRQ Q3
Justify melting point difference
- 2021 · FRQ Q1
Ionic solid conductivity test
What's Next
Mastering properties of solids is a critical foundational skill for Unit 3, as you will extend these bonding-property relationships to liquids, phase changes, and solutions in upcoming modules. This classification framework is tested heavily in long-form AP FRQ questions that require particulate-level reasoning to compare two substances, a skill that makes up roughly 40% of your total exam score. You will also apply these rules to predict solubility trends and colligative property behavior later in the course.