Properties of covalent compounds, compared with ionic ones
Covalent compounds are made of neutral molecules with weak forces between them. So they are often gases or liquids, melt easily, do not conduct and dissolve in organic solvents.
Ionic and covalent compounds compared
| Property | Ionic (such as NaCl) | Covalent (such as CCl₄, naphthalene) |
|---|---|---|
| Particles | Ions (cations and anions) | Neutral molecules |
| Forces | Strong electrostatic forces | Weak intermolecular forces |
| Physical state | Crystalline solids | Gases (O₂, N₂, CO₂), liquids (water, CCl₄) or solids (iodine) |
| Melting and boiling points | High: NaCl melts at 1074 K | Low: naphthalene melts at 353 K; CCl₄ boils at 350 K |
| Electrical conductivity | Conduct when molten or dissolved | Do not conduct: no ions or free electrons |
| Solubility | Soluble in water; insoluble in organic solvents | Usually insoluble in water; soluble in alcohol, benzene, ether, chloroform |
Why covalent compounds behave this way
Inside a molecule, the covalent bonds are strong. But between molecules the forces are weak. These are called intermolecular forces (inter means between). Little heat is needed to pull the molecules apart, so melting and boiling points are low. The molecules carry no charge, so nothing can carry a current. Ethyl alcohol gives no ions, not even H⁺, when mixed with water, so its solution does not conduct.
Activity: iodine in alcohol and in water
- 1Take about 5 mL of ethyl alcohol in a test tube. Add a few crystals of iodine and shake well.The alcohol turns dark brown: iodine dissolves
- 2Add the same amount of iodine to 5 mL of water in another test tube and shake.Very little dissolves: the water turns only pale yellow, and crystals remain
- 3★ConclusionIodine, a covalent substance, dissolves in an organic solvent but hardly in water
- 4UseIodine dissolved in alcohol is tincture of iodine, an antiseptic for cuts
★ key step
How to draw the alcohol conductivity test
- Draw a beaker of ethyl alcohol with two electrodes dipping into it.
- Join the electrodes to a 3 V battery, a switch and a bulb in one loop.
- Draw the bulb without rays, to show it does not glow.
- Label: beaker, ethyl alcohol, electrodes, bulb (not glowing), switch, battery.
Ionic or covalent? Sort them
| Substance | Bond | How you can tell |
|---|---|---|
| Potassium chloride, KCl | Ionic | A metal and a non-metal |
| Calcium oxide, CaO | Ionic | A metal and a non-metal |
| Sodium oxide, Na₂O | Ionic | A metal and a non-metal |
| Chlorine, Cl₂ | Covalent | Two non-metal atoms |
| Carbon dioxide, CO₂ | Covalent | Two non-metals |
| Ammonia, NH₃ | Covalent | Two non-metals |
The tincture of iodine in a first-aid box is iodine dissolved in alcohol, because iodine hardly dissolves in water. Naphthalene balls kept with woollen clothes slowly turn to vapour, a sign of weak forces between covalent molecules.
2 marks: “Naphthalene melts at 80 °C, but sodium chloride melts at 801 °C. Explain the difference in terms of the forces present.” Naphthalene is covalent: its molecules are held by weak intermolecular forces, which a little heat overcomes. NaCl is ionic: strong electrostatic forces between its ions need much more heat.
2 marks: “A bulb in a circuit does not glow when its electrodes dip into ethyl alcohol. Give the reason.” Ethyl alcohol is a covalent compound made of neutral molecules. It gives no ions, even in water, so there are no charged particles to carry the current.
3 marks: “Give three differences between ionic and covalent compounds.” Ionic: made of ions, strong forces, high melting points, conduct when molten or dissolved, soluble in water. Covalent: neutral molecules, weak intermolecular forces, low melting points, do not conduct, soluble in organic solvents.
Ionic: high melting, conducts when molten or wet. Covalent: low melting, often a liquid or gas, does not conduct.