Investigation 6: Bonding in Solids
A solid's melting point, solubility, and conductivity all come from how its particles are held together, so measuring those three properties lets you identify the bonding type without any chemical test.
Four white powders can look identical and still be completely different substances, because what you see as a solid is really a signature left by its bonding. Ionic solids hold ions in a rigid lattice through strong electrostatic attraction, so they need a lot of energy to melt and tend to have high melting points. Covalent network solids like quartz have atoms linked by continuous covalent bonds throughout the whole structure, so they melt at even higher temperatures and almost never dissolve. Molecular solids are held together by weaker intermolecular forces between individual molecules, so they melt at low temperatures and dissolve or not depending on whether they can hydrogen-bond or otherwise interact with the solvent.
Conductivity is the property that separates ionic from everything else. Melted or dissolved ionic compounds conduct because they break apart into freely moving charged ions. Molecular solids, even when dissolved, keep their atoms bonded together as neutral units, so their solutions carry no current. This is why testing all three properties together, rather than just one, pins down the bonding type with confidence.
This investigation is the hands-on partner to the bonding and intermolecular force content earlier in the course: here you are not memorizing which compounds are ionic, you are using measurable evidence to prove it. It also previews the error-analysis practices used elsewhere in the lab unit, since melting point ranges and conductivity readings always carry some uncertainty that has to be reported honestly.
Key terms
- Ionic solid
- A solid held together by electrostatic attraction between oppositely charged ions in a rigid lattice.
- Covalent network solid
- A solid like quartz in which atoms are joined by continuous covalent bonds throughout the whole structure.
- Molecular solid
- A solid made of individual molecules held together by weaker intermolecular forces such as hydrogen bonding.
- Conductivity
- The ability of a melted or dissolved substance to carry electric current, which requires freely moving charged particles.
Watch it explained
- How it worksHow do melting point, solubility, and conductivity together identify the bonding in a solid?
- Worked exampleWalk me through classifying an unknown white solid that melts at 800 degrees C and conducts when molten.
- Common mistakeWhy isn't a solid that dissolves in water automatically ionic?
- Going furtherWhat test distinguishes a covalent network solid from an ionic one with a similar melting point?
Check yourself
End of topic.