Properties of Ionic Compounds — CBSE Class 10 Science
Ionic compounds are made when metals transfer electrons to non-metals. Ions thus formed are held together by strong electrostatic attraction, called an ionic bond.
Let’s understand the important properties of ionic compounds with detailed explanations and fun examples!
1. High Melting and Boiling Points
Explanation
- Ionic compounds have strong forces between their positive and negative ions. These are called electrostatic forces.
- Breaking these forces needs a lot of energy.
- So, ionic compounds must be heated to very high temperatures to melt (melting point) or boil (boiling point).
Important: Melting and boiling points are much higher for ionic compounds compared to most covalent compounds (like sugar or water).
Examples
- Sodium chloride (NaCl):
- Melts at about 800°C.
- Boils at 1413°C.
- Magnesium oxide (MgO):
- Melts at about 2852°C.
- Boils at 3600°C.
- Potassium chloride (KCl):
- Melts at 770°C.
- Boils at 1420°C.
These temperatures are much higher than those of substances like ice (melting point 0°C) or wax (melting point ~60°C).
2. Solubility in Water
Explanation
- Ionic compounds dissolve easily in water.
- This is because water is a polar solvent. It surrounds the positive and negative ions and pulls them apart.
- However, ionic compounds do not dissolve in non-polar solvents (like petrol, kerosene, oil).
Important: Solubility in water is a signature property of ionic compounds.
Examples
- Sodium chloride (NaCl): Table salt dissolves quickly in water.
- Potassium nitrate (KNO₃): Used in fertilizers, dissolves well.
- Copper(II) sulfate (CuSO₄): When dissolved, gives a blue solution.
Exceptions:
A few ionic compounds, like barium sulfate (BaSO₄) and lead(II) chloride (PbCl₂), are almost insoluble in water.
3. Conduct Electricity in Molten or Dissolved State (Not in Solid State)
Explanation
- In solid state, ionic compounds have ions locked in place. The ions can’t move, so solids do not conduct electricity.
- In molten state (when melted) or dissolved in water (aqueous), the ions are free to move.
- Moving ions can carry an electric current! That’s why these compounds conduct electricity as a liquid or in solution.
Important: Only liquids or solutions of ionic compounds allow electricity to pass through, not solids.
Examples
- Sodium chloride (NaCl):
- Solid salt does NOT conduct electricity.
- Molten or dissolved salt does!
- Copper(II) sulfate (CuSO₄): Blue solution conducts electricity.
- Aluminium oxide (Al₂O₃):
- Used in extraction of aluminium via electrolysis (needs molten Al₂O₃).
Activity: Testing Conductivity of Ionic Compounds
Aim: To show that ionic compounds conduct electricity in solution but not in solid.
Materials:
- 1 small beaker
- Sodium chloride (table salt)
- Distilled water
- 2 graphite or iron nails (as electrodes)
- Electrical wires
- Battery and lamp/bulb
Steps:
- Pour distilled water into the beaker.
- Add a small spoon of table salt. Stir till it dissolves.
- Attach the two electrodes (nails or graphite rods) to the wires and immerse them in the solution, not touching each other.
- Connect wires to a battery and lamp as shown:
(battery) — (wire) — (1st electrode in beaker) — (solution) — (2nd electrode in beaker) — (wire) — (lamp) — (battery)
- Observe if the lamp glows.
Observation:
- If the lamp glows, the solution conducts electricity.
- Now, try the same with dry salt (solid). Lamp does NOT glow.
Conclusion:
- Ionic compounds conduct electricity only when molten or dissolved (ions are free to move).
Summary Table
| Property | Solid State | Molten State | Aqueous Solution |
|---|
| Conducts electricity | No | Yes | Yes |
| Melting/boiling point | High | High | N/A |
| Solubility in water | N/A | N/A | Mostly soluble |
Key Points Recap
- Ionic compounds have high melting and boiling points due to strong ionic bonds.
- Soluble in water: easily dissolve, but not in oil or petrol.
- Conduct electricity only when molten or dissolved, because ions need freedom to move.
Fun Fact:
When you put salt on ice (in winter), it dissolves. The salty water then has a lower freezing point, so the ice melts!
Scenario-Based Questions & Answers
1. Scenario: You spill salt on the kitchen counter and want to clean it with kerosene.
- Question: Will kerosene dissolve the salt? Why or why not?
- Answer: No, salt (NaCl) will not dissolve in kerosene, as kerosene is a non-polar solvent. Ionic compounds dissolve only in polar solvents like water.
2. Scenario: In winter, the temperature drops below zero, and you notice salt used on icy roads.
- Question: Why is salt effective for melting ice on roads?
- Answer: Salt dissolves in water from the melting ice and forms an ionic solution. This lowers the freezing point, causing more ice to melt at lower temperatures.
3. Scenario: You are asked to use sodium chloride in an experiment to conduct electricity. The salt is in solid form.
- Question: Will solid sodium chloride conduct electricity? Why?
- Answer: No, solid sodium chloride will not conduct electricity, because its ions are fixed and cannot move. Only molten or dissolved salt can conduct electricity.
4. Scenario: You need to extract a metal from its ore using electricity.
- Question: Why must the ore be molten or dissolved, not solid, for electrolysis?
- Answer: The ore must be molten or dissolved, so that the ions are free to move and carry electric current during electrolysis.
5. Scenario: You find a white substance that melts at 180°C and dissolves in petrol but not in water.
- Question: Could this be an ionic compound? Why or why not?
- Answer: No, it is likely not an ionic compound. Ionic compounds have higher melting points and dissolve in water, not petrol.
Ionic compounds are strong, soluble (in water), and good conductors—but only when their ions are free! Always remember these fun facts for your exams!
