Melting and boiling points
Conductivity is one test. Melting point is another, and it separates the two kinds of compound just as sharply.
| Compound | Melting point / °C | Boiling point / °C | Bonding |
|---|---|---|---|
| Sodium chloride | 801 | 1413 | Ionic |
| Potassium chloride | 776 | 1500 | Ionic |
| Calcium oxide | 2580 | 2850 | Ionic |
| Water | 0 | 100 | Covalent |
| Ethyl alcohol | −117 | 79 | Covalent |
| Ammonia | −78 | −33 | Covalent |
| Sulfur dioxide | −73 | −10 | Covalent |
| Oxygen | −218 | −183 | Covalent |
The three ionic compounds melt between 776 °C and 2580 °C. Every covalent compound in the table melts below 0 °C except water, which melts at exactly 0 °C. There is no overlap at all.
Why the gap is so wide
Melting means giving the particles enough energy to break free of one another, so the question is always: what has to be overcome?
Strong electrostatic attractions between ions throughout a lattice, each ion pulled on by six neighbours. Breaking that down needs a great deal of heat, so most ionic compounds are solids at room temperature.
Only the weak attractions between separate molecules. The covalent bonds inside each molecule are strong but do not have to break, so little heat is needed — and most covalent compounds are liquids or gases at room temperature.
Remember the lattice exception
This holds for covalent compounds made of separate molecules. Diamond and graphite are covalent too, but their atoms form one giant lattice with no separate molecules to pull apart, so their melting and boiling points are very high.