Atomic Foundations of Matter
Ionic and covalent properties
Compare the properties of ionic and covalent compounds.
Activity 9.4 tests five substances: camphor, sodium chloride, copper sulfate, sugar and naphthalene. Does each dissolve in water, kerosene and petrol? Does its solution conduct electricity (tested with a 9 V battery, two electrodes and a bulb)? Careful: petrol and kerosene catch fire easily.
What you find:
| Ionic (sodium chloride, copper sulfate) | Covalent (camphor, naphthalene) | |
|---|---|---|
| Dissolves in water? | Generally yes | Mostly no |
| Dissolves in kerosene or petrol? | No | Yes |
| Conducts as a solid? | No | No |
| Conducts in water or when melted? | Yes | No |
| Melting and boiling points | Generally high | Usually low |
"Why does a salt solution conduct, but solid salt doesn't?" Electricity needs charged particles that can move. In a salt crystal, the ions are locked in place. Dissolve it in water (or melt it), and the ions are free to move, so the bulb lights. Pure distilled water hardly conducts at all.
Sugar is the interesting one: it dissolves in water, but makes no ions, so its solution does not conduct. Camphor and naphthalene don't conduct either. Covalent compounds have no free ions.
"Why do ionic compounds melt only when very hot?" Each ion is held tightly by its six neighbours of opposite charge, so it takes a lot of heat to pull them apart.
A common mix-up: covalent bonds are not weak. The bonds inside a covalent molecule are strong. It is the pull between separate molecules that is weak, which is why covalent compounds usually melt easily.
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