AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/3/3-2)
Unit 3 · Topic 3.2
3.2 Properties of Solids
The type of particles in a solid, and the forces between them, explain its properties. Ionic, covalent network, molecular and metallic solids each have a typical pattern of melting point, hardness and conductivity, and in liquids stronger intermolecular forces mean lower vapor pressure and higher boiling point.
Key terms
- ionic solid
- covalent network solid
- molecular solid
- metallic solid
- vapor pressure
- boiling point
Four kinds of solids
| Type | Particles and forces | Melting point | Conducts electricity? | Other properties | Examples |
|---|---|---|---|---|---|
| Ionic | cations and anions; strong Coulombic attractions | high | only when melted or dissolved | hard, brittle | NaCl, MgO |
| Covalent network | atoms linked by covalent bonds in a continuous network | very high | no (graphite is an exception) | very hard (graphite is soft) | diamond, SiO₂, SiC |
| Molecular | separate molecules held by IMFs | low | no | often soft | ice, I₂, sugar |
| Metallic | metal ions in a sea of delocalized electrons | varies, often high | yes, as solid and liquid | malleable, ductile | Cu, Fe, brass |
Explaining each type
Ionic solids: strong attractions between ions give high melting and boiling points and low vapor pressures. They're brittle because shifting one layer lines up like charges, and the repulsion cracks the crystal. They conduct only when ions can move, in the molten state or dissolved.
Covalent networks are found in some nonmetal and metalloid elements, like carbon as diamond or graphite, and in some compounds of two such elements, like SiO₂ (quartz) and SiC; metals don't form them. Melting one means breaking strong covalent bonds, so melting points are very high. 3-D networks like diamond are rigid and hard because bond angles are fixed. Graphite is made of 2-D sheets that slide past each other easily, so it's soft.
Molecular solids are separate molecules held together by relatively weak IMFs, so they melt at low temperatures. They don't conduct: every electron is tied up in a bond or a lone pair, so none can move through the solid. Some, like polymers, are made of very large molecules.
Metallic solids conduct electricity and heat thanks to mobile electrons, and they're malleable and ductile because layers of ions can rearrange without breaking the metallic bonding.
Vapor pressure and boiling point
In a closed container, some molecules of a liquid escape into the gas phase. The pressure of that gas at equilibrium is the vapor pressure. Weaker IMFs let more molecules escape, giving a higher vapor pressure. A liquid boils when its vapor pressure equals the pressure above it, so a liquid with weaker IMFs reaches that point at a lower temperature.
So weak IMFs mean high vapor pressure and low boiling point; strong IMFs mean low vapor pressure and high boiling point. At 20 °C, diethyl ether (dipole-dipole) has a vapor pressure of about 440 torr, ethanol (hydrogen bonding) about 44 torr and water (more hydrogen bonding per molecule) about 17.5 torr.
Boiling separates molecules completely, so boiling point tracks IMF strength closely. Melting only loosens and rearranges the attractions, so melting points follow the trend less reliably.
Large molecules
In polymers and biomolecules, attractions between different parts of the same molecule (and between neighboring molecules) set the molecule's shape, and the shape largely decides how it behaves. A protein that loses its hydrogen bonds unfolds and stops working.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Identifying unknown solids
Classify each solid. W: melts at 1610 °C, doesn't conduct as a solid or liquid, extremely hard. X: melts at 80 °C, doesn't conduct. Y: melts at 714 °C, conducts only when melted. Z: melts at 1085 °C, conducts as a solid, can be hammered into sheets.
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- Step 1: W: very high melting point, hard, never conducts. That's a covalent network solid (strong covalent bonds throughout, no mobile charges).
- Step 2: X: low melting point and no conductivity means weak IMFs between neutral molecules: molecular.
- Step 3: Y: high melting point, conducts only when melted, so ions that become mobile when the lattice breaks down: ionic.
- Step 4: Z: conducts as a solid and is malleable, so it has delocalized electrons: metallic.
Answer: W covalent network; X molecular; Y ionic; Z metallic
- Example 2
Ranking vapor pressures
Rank water (H₂O), ethanol (CH₃CH₂OH) and diethyl ether (CH₃CH₂OCH₂CH₃) from highest to lowest vapor pressure at the same temperature, and justify.
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- Step 1: Diethyl ether has no H on O, so it can't hydrogen bond with itself; it has only dipole-dipole and dispersion forces. Weakest overall IMFs.
- Step 2: Ethanol has one O–H group per molecule and hydrogen bonds.
- Step 3: Water has two O–H bonds and two lone pairs per molecule, so each molecule can form more hydrogen bonds, making water's IMFs the strongest of the three.
- Step 4: Weaker IMFs let more molecules escape, giving a higher vapor pressure.
Answer: Diethyl ether > ethanol > water
- Example 3
Why graphite conducts but diamond doesn't (classic trap)
Diamond and graphite are both pure carbon covalent network solids. Diamond is an electrical insulator, but graphite conducts along its layers. Explain.
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- Step 1: In diamond, every carbon is bonded to four others (sp³). All valence electrons are locked in localized C–C bonds, so none are free to move.
- Step 2: In graphite, each carbon bonds to three others in a flat sheet (sp²). The fourth valence electron of each carbon is part of a pi system that is delocalized across the whole sheet.
- Step 3: Those delocalized electrons can move along the layers, carrying current.
Answer: Graphite has delocalized pi electrons spread across each layer that can carry current; diamond's electrons are all held in localized bonds.
Common mistakes
- Saying ionic solids conduct as solids. The ions are locked in place until the solid melts or dissolves.
- Saying molecular solids melt by breaking covalent bonds. They melt by overcoming IMFs between molecules.
- Mixing up the vapor pressure trend: stronger IMFs mean lower vapor pressure, not higher.
- Calling SiO₂ a molecular solid like CO₂. SiO₂ is a covalent network; CO₂ is molecular.
On the exam
- Expect a table of properties to classify, or a question asking you to explain one property using a particle-level model. Name the particles and the forces between them, then connect to the property.
- For vapor pressure or boiling point questions, compare IMFs in both substances; a one-sided explanation usually isn't enough for full credit.
Connected topics
Videos
Check yourself
4 questions on 3.2 Properties of Solids. Pick an answer to see if you got it, and why.
| Solid | Melting point (°C) | Conducts as a solid? | Conducts when melted? | Dissolves in water? |
|---|---|---|---|---|
| W | 801 | No | Yes | Yes |
| X | 1713 | No | No | No |
| Y | 1085 | Yes | Yes | No |
| Z | 80 | No | No | No |
Hypothetical data. Properties of four pure solids measured by a student
Which solid is most likely a covalent network solid?
Which of the following best explains the low melting point of solid Z?
Solid W does not conduct electricity, but it conducts when melted. Which of the following best explains this?
At 25 °C, liquid P has a vapor pressure of 440 torr and liquid Q has a vapor pressure of 24 torr. Which of the following statements about P and Q is most likely true?
0 of 4 answered