Physical properties from structure
| English | Chinese | Pinyin |
|---|---|---|
| conductivity | 电导率 | diàn dǎo lǜ |
| solubility | 溶解度 | róng jiě dù |
| molten | 熔融 | róng róng |
| soluble | 可溶 | kě róng |
| insoluble | 不溶 | bù róng |
Structure decides everything
- The structure decides a substance's physical properties.
- Melting point, conductivity 电导率 and solubility 溶解度 all follow from the bonding.
- You can even work backwards to identify an unknown.
Structure-property lab
Link a material property to the structure that causes it.
Simple molecular substances have low melting points because the forces between molecules are weak.
The covalent bonds within molecules are strong.
Melting and boiling points
- High when strong forces (ionic, covalent or metallic) must be broken.
- Low when only weak intermolecular forces break (simple molecular).
Quartz is a giant covalent structure of silicon and oxygen
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.
Match each structure to its properties.
Properties come from structure: strong giant lattices melt high; simple molecules melt low.
Conductivity and solubility
- Conductivity needs charged particles that can move:
- ionic solids conduct only when molten 熔融 or dissolved (ions free);
- metals conduct (delocalised electrons); graphite conducts too.
- Solubility: ionic solids are usually soluble 可溶 in water; molecular and giant-covalent solids are usually insoluble 不溶.
An ionic solid conducts electricity:
The ions are fixed in the solid lattice; only when molten or dissolved can they move and carry charge.
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.
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
- 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