Mistake Master
Properties of solids
Diamond and dry ice are both solids, but one is the hardest natural material and the other vanishes into gas on your desk. The difference is entirely in what particles they are made of and how those particles are held.
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Four kinds of solid.
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A solid's properties come from its particles and the forces between them. There are four main types. Ionic solids are lattices of ions (high-melting, hard, brittle, conduct only when molten or dissolved).
Molecular solids are molecules held by intermolecular forces (soft, low-melting, poor conductors — think ice or dry ice). Covalent network solids are bonded throughout by covalent bonds (extremely hard and high-melting — diamond, quartz).
Metallic solids are cations in a delocalized electron sea (conduct, malleable, lustrous). Reading the particle-level structure tells you the melting point, hardness, and electrical behavior to expect.
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Classifying a solid.
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Name the particles and the force, then read off the properties.
- Identify the particles. Ions? Molecules? A network of covalently bonded atoms? Metal atoms? This is the key first move.
- Name the force holding them. Ionic attraction, intermolecular forces, covalent bonds throughout, or the metallic electron sea.
- Predict melting point and hardness. Network and ionic solids melt high and are hard; molecular solids melt low and are soft; metals vary but are malleable.
- Predict conductivity. Metals conduct as solids; ionic solids conduct only when molten or dissolved; molecular and network solids generally do not conduct.
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The pieces you'll meet.
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Match each solid type to its telltale properties.
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Worked example: diamond versus dry ice.
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Question. Diamond is extremely hard and melts above 3500 °C; dry ice (solid CO₂) sublimes at −78 °C. Explain from their structures.
Diamond. It is a covalent network solid: every carbon is covalently bonded to four others, throughout the crystal. Melting means breaking countless strong covalent bonds, so it is enormously hard and high-melting.
Dry ice. It is a molecular solid: discrete CO₂ molecules held only by weak London dispersion forces. Very little energy separates them, so it sublimes far below room temperature.
Key point. Same element carbon in different roles: bonded throughout (diamond) versus discrete molecules (CO₂). The particle-level structure, not the formula, sets the properties.
§5
Mistakes that cost real points.
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"A molecular solid is held together by the bonds within its molecules."
A molecular solid is held together by the intermolecular forces between its molecules, which are weak — that is why molecular solids melt low. The strong bonds inside each molecule stay intact when the solid melts.
Fix. For a molecular solid, the melting point reflects the IMFs between molecules, not the covalent bonds inside them.
"Every solid with strong internal bonds must melt high."
It depends on whether those bonds must be broken to melt. A covalent network solid melts high because bonds run throughout. A molecular solid has strong internal bonds too, but melting only overcomes the weak forces between molecules, so it melts low.
Fix. Ask what has to be broken to melt: a network of bonds (high melting) or just intermolecular forces (low melting).
"Ionic solids conduct electricity because they contain charges."
Solid ionic compounds do not conduct, because the ions are locked in place. They conduct only when molten or dissolved, when the ions become free to move. Having charges is not enough; the charges must be mobile.
Fix. For conduction, ask whether the charge carriers can move. Ionic solids need melting or dissolving to free their ions.
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Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.