Properties of ionic compounds
Ionic compounds are hard crystalline solids with high melting points. They dissolve in water and conduct electricity when molten or dissolved, but not as solids.
Properties and their reasons
| Property | Reason |
|---|---|
| Crystalline solids, hard and brittle | Ions are packed in a regular crystal lattice and held by strong forces |
| High melting and boiling points | A lot of heat is needed to overcome the strong attraction between ions |
| Conduct electricity when molten or dissolved in water | The ions are free to move and carry charge from one electrode to the other |
| Do not conduct as solids | The ions are held in fixed positions in the lattice |
| Usually soluble in water | Water weakens the attraction between the ions and pulls them apart |
| Insoluble in organic solvents such as alcohol, ether and carbon tetrachloride | These liquids cannot weaken the forces between the ions |
Activity: does salt solution conduct electricity?
- 1Dissolve 1 tablespoon of common salt in 100 mL of water.
- 2Take two graphite rods (from an old dry cell) as electrodes. Join them through a bulb and a switch to a 3 V battery.
- 3First dip the electrodes in plain water in a 200 mL beaker and switch on.The bulb does not glow, or glows very faintly: water alone has very few ions
- 4Now use the salt solution instead.The bulb glows brightly
- 5★ConclusionIn water, NaCl gives free Na⁺ and Cl⁻ ions, and these carry the current
★ key step
Activity: salt in water and in ethyl alcohol
- 1Take two boiling tubes. Put about 10 mL of water in one and about 10 mL of ethyl alcohol in the other.
- 2Add about 4 g of powdered common salt to each and shake both well.
- 3Observe.Most of the salt dissolves in water. In ethyl alcohol it stays at the bottom, almost unchanged
- 4★ConclusionIonic compounds dissolve in water but hardly at all in organic liquids such as alcohol. Kerosene gives the same result
★ key step
How to draw the conductivity set-up
- Draw a beaker about two-thirds full of salt solution.
- Draw two graphite rods dipping into the solution, not touching each other.
- Join them with wires to a 3 V battery, a bulb and a switch, all in one loop.
- Draw short rays around the bulb to show it glowing.
- Label: beaker, sodium chloride solution, electrodes, bulb, switch, battery.
Why water and heat help
Water is a solvent: a liquid that dissolves other substances. It weakens the attraction between Na⁺ and Cl⁻ ions, so they separate and move freely. Melting does the same job with heat: it breaks up the lattice so the ions can move. A few ionic compounds, such as barium sulphate, silver chloride and calcium fluoride, do not dissolve in water. Their ions attract each other too strongly.
Wet or sweaty hands conduct electricity, because salts from sweat dissolve into free ions, so never touch switches with wet hands. Salt does not melt on a hot tawa because its ions hold together so strongly.
2 marks: “Solid common salt does not conduct electricity, but its solution in water does. Explain.” In the solid, Na⁺ and Cl⁻ ions are fixed in the crystal lattice and cannot move. In water, the attraction weakens, the ions move freely and carry the current.
2 marks: “Why do ionic compounds have high melting and boiling points? Give the melting point of sodium chloride.” Strong electrostatic forces hold the ions in a lattice, so a lot of heat is needed to break it up. NaCl melts at 1074 K (801 °C).
3 marks: “Describe an activity to show that a solution of common salt conducts electricity. What does it tell you about the particles in salt?” Dissolve salt in water. Dip two graphite electrodes, joined through a bulb and switch to a 3 V battery. The bulb glows (little or no glow with plain water). So salt gives free ions, Na⁺ and Cl⁻, in water, which carry the current.
Ions locked, no current. Ions free (molten or in water), current flows.