Atomic Foundations of Matter · Lesson 11 of 14
Properties of the Ionic and the Covalent Compounds
“Solubility, conductivity and melting behaviour reveal important differences between ionic and covalent substances.”
• Compare solubility patterns of ionic and covalent compounds. • Explain why ionic solids do not conduct electricity. • Explain why ionic compounds can conduct when dissolved in water. • Explain why dissolved sugar does not conduct. • Predict conductivity of molten ionic compounds. • Compare typical melting and boiling behaviour.
Two substances can both look like white solids and still behave very differently. Common salt dissolves in water and its solution conducts electricity. Sugar also dissolves in water, but its solution does not conduct in the same way. Camphor and naphthalene behave differently again. These observations can be connected to the kinds of particles present and whether charged particles are free to move.
Properties of the Ionic and the Covalent Compounds
Activity: Comparing Solubility and Conductivity
The chapter proposes comparing samples such as camphor, sodium chloride, copper sulfate, sugar and naphthalene. Their solubility is tested in water, kerosene and petrol, and their electrical conductivity is tested in solid form and after dissolving in water where appropriate.
Use only a low-voltage battery for the conductivity setup and do not touch connected electrodes. Petrol and kerosene are flammable liquids and require careful supervised handling away from ignition sources.
Solubility Patterns
Ionic compounds such as sodium chloride and copper sulfate are generally soluble in water but insoluble in solvents such as kerosene and petrol. Many covalent compounds such as camphor and naphthalene are insoluble in water but dissolve in kerosene and petrol.
These are general patterns rather than a rule that every substance follows without exception. Sugar is a useful example from the chapter: it is a covalent compound that dissolves in water, yet its solution still does not provide ions for electrical conduction.
Electrical Conductivity
In a solid ionic compound, positive and negative ions occupy fixed positions in the crystal. Although charged particles are present, they cannot move freely through the solid, so the solid does not conduct electricity in this setup.
When an ionic compound such as sodium chloride or copper sulfate dissolves in water, its ions can move through the solution. These mobile charged particles can carry electric charge through the liquid, allowing the circuit to conduct.
A sugar solution contains dissolved particles, but those particles do not provide the mobile ions required for this form of conduction. Camphor and naphthalene similarly do not supply mobile ions.
Predicting the Molten State
When an ionic solid melts, the rigid arrangement breaks down and the ions become able to move. Using the same reasoning as for an aqueous ionic solution, the chapter asks you to predict that molten ionic compounds can conduct because charged particles are mobile.
Melting and Boiling Points
Ionic compounds generally have high melting and boiling points because strong attractions act between oppositely charged ions throughout the structure. Covalent compounds generally have lower melting and boiling points in the comparison used in this chapter.
| Property | Ionic compounds | Covalent compounds |
|---|---|---|
| Formation | Electron transfer produces ions | Electron sharing produces covalent bonds |
| Typical water solubility | Generally soluble | Often insoluble, with examples such as sugar showing exceptions |
| Kerosene and petrol | Generally insoluble | Many examples dissolve |
| Electrical conduction as solid | No, because ions are fixed | Generally no mobile ions |
| Electrical conduction in water | Often yes when ions are present and mobile | Generally no; sugar solution is a highlighted example |
| Molten conductivity | Expected when ions become mobile | Not explained through mobile ions |
| Melting and boiling points | Generally high | Usually lower |
Problem
A solid compound does not conduct electricity, but its aqueous solution conducts. What bonding type does this behaviour suggest in the chapter's comparison?
- 1.The solid contains charged particles but they are not free to move.
- 2.Dissolving allows the charged particles to move through water.
- 3.This behaviour matches the description of an ionic compound.
- 4.Therefore the compound is inferred to contain ionic bonding.
Presence of charge is not enough for conduction. The charged particles must also be free to move. That is why an ionic solid does not conduct while its solution or molten form can.
Quiz
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Which another statement is also a key takeaway from this lesson?
Which statement correctly applies to the lesson “Properties of the Ionic and the Covalent Compounds”?
Practice Problems
- Why does solid sodium chloride fail to conduct even though it contains ions?
- Why does sodium chloride solution conduct?
- Why can sugar dissolve in water without making the solution conduct?
- Predict whether a molten ionic compound conducts and justify your answer.
- Compare the typical melting behaviour of ionic and covalent compounds.
Key Takeaways
• Ionic and covalent compounds differ because their particles and bonding are different. • Ionic compounds generally have high melting and boiling points because of strong electrostatic attractions. • Ionic substances conduct electricity when their ions are free to move, such as in molten form or solution. • Covalent compounds generally have lower melting and boiling points and usually conduct electricity poorly.