Physical properties from structure
Properties from structure
- The structure decides a substance's physical properties.
- Melting point, conductivity and solubility all follow from the bonding.
- You can even work backwards to identify an unknown.
Melting and boiling points
- High when strong forces (ionic, covalent or metallic) must be broken.
- Low when only weak intermolecular forces break (simple molecular).
Practice
A substance has a high melting point when:
Breaking strong bonds needs lots of energy, giving a high melting/boiling point; weak forces give a low one.
Conductivity and solubility
- Conductivity needs charged particles that can move:
- ionic solids conduct only when molten or dissolved (ions free);
- metals conduct (delocalised electrons); graphite conducts too.
- Solubility: ionic solids are usually soluble in water; molecular and giant-covalent solids are usually insoluble.
Practice
An ionic solid conducts electricity:
The ions are fixed in the solid lattice; only when molten or dissolved can they move and carry charge.
Practice
Graphite conducts electricity because:
Graphite has delocalised electrons between its layers, so it conducts even though it is a non-metal.
Identifying an unknown
- From a substance's melting point, conductivity and solubility, you can deduce its structure and bonding.
Practice
A solid with a very high melting point that does not conduct and is insoluble is most likely:
Very high mp + no conductivity (not graphite) + insoluble points to a giant covalent network like diamond or SiO₂.
You've got it
Key idea
- high mp/bp = strong forces (ionic/covalent/metallic); low = weak intermolecular (simple molecular)
- conduct: ionic only when molten/dissolved; metals + graphite always
- ionic usually soluble; molecular & giant covalent insoluble
- use mp + conductivity + solubility to identify the structure