Properties of ionic and covalent compounds

This page covers a single Form 4 content standard: the properties of ionic and covalent compounds. It is where the whole chapter pays off, because now you use the bonding you have learned to explain and predict physical properties. It is also a favourite for data questions, where you are given experimental results and asked to deduce the type of bonding. Learn the properties, learn the reasons, and learn to argue from evidence.

Why the two types differ

The differences all come back to structure. An ionic compound is a giant ionic lattice, a huge, regular arrangement of positive and negative ions held together by strong electrostatic forces acting in every direction. A simple covalent compound is made of separate molecules: the covalent bonds inside each molecule are strong, but the forces between the molecules are weak. Everything below follows from this one contrast.

Melting and boiling points

• Ionic compounds have high melting and boiling points. A great deal of energy is needed to overcome the strong electrostatic forces throughout the giant lattice.
• Simple covalent compounds have low melting and boiling points. Only the weak forces between molecules need to be overcome, the strong covalent bonds inside the molecules are not broken, so little energy is required. Many are liquids or gases at room temperature.

Electrical conductivity

This is the property examiners test most, because it discriminates so clearly:

• An ionic compound conducts electricity when molten or dissolved in water, because the ions are then free to move and carry charge. It does not conduct when solid, because the ions are held in fixed positions in the lattice and cannot move.
• A simple covalent compound does not conduct electricity in any state, solid, liquid or in solution, because it contains no ions or other freely moving charged particles.

Solubility

• Ionic compounds are usually soluble in water but insoluble in organic solvents.
• Simple covalent compounds are usually insoluble in water but soluble in organic solvents.

Worked example

Question. A student tests two substances. Substance P has a high melting point, dissolves in water, and conducts electricity when molten or in solution but not when solid. Substance Q has a low melting point, does not dissolve in water, and does not conduct electricity in any state. Deduce the type of bonding in P and in Q, giving a reason for each conclusion.

Step 1, Analyse P’s melting point. A high melting point means strong forces throughout the structure, which points to a giant ionic lattice.

Step 2, Analyse P’s conductivity. P conducts when molten or in solution but not when solid. This is the signature of an ionic compound: the ions are free to move when molten or dissolved, but fixed in the solid.

Step 3, Conclude for P. P is an ionic compound (it also dissolves in water, which fits).

Step 4, Analyse Q. A low melting point means only weak forces between molecules, and Q conducts in no state, showing there are no free-moving ions. Being insoluble in water fits too. So Q is a simple covalent compound.

Answer. P is ionic, high melting point (strong lattice forces) and it conducts only when molten or in solution (mobile ions). Q is covalent, low melting point (weak intermolecular forces) and it never conducts (no free-moving ions).

Practice question

Question. Substance R melts at a low temperature and does not conduct electricity even when it is molten. State whether R is an ionic or a covalent compound, and explain your answer using both pieces of evidence.

Answer. R is a simple covalent compound. The low melting point shows that only weak forces between molecules have to be overcome, which is typical of a molecular substance rather than a giant ionic lattice. The fact that it does not conduct electricity even when molten shows there are no freely moving ions present, if R were ionic, it would conduct once molten because its ions would be free to move. Both observations point to covalent bonding.

Exam tip

The two properties that decide the marks are melting point and electrical conductivity, and you must always give the reason, not just the fact. For conductivity, the winning phrase is “free to move”: an ionic compound conducts when molten or in solution because its ions are free to move, but not when solid because they are held in fixed positions; a covalent compound never conducts because there are no free-moving ions. For melting point, always link a high value to strong forces in the giant lattice and a low value to weak forces between molecules. When you deduce bonding from data, quote the specific evidence for each conclusion.

Where this fits

This is the final standard of the Chemical Bond chapter, and it ties the ionic bond and covalent bond back to the physical world. The reasoning here, structure explains properties, returns throughout Chemistry, so it is worth mastering now. Consolidate the comparison table with the revision notes and drill the data-deduction questions with the practice questions. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure every property you state is paired with its reason and that your conductivity answers always mention ions being free to move or held in place, which is exactly what secures full marks on this standard across SPM Chemistry.

Quick recap

• Ionic: giant lattice, high melting point, soluble in water, conducts when molten or in solution (not solid).
• Covalent (simple): separate molecules, low melting point, usually insoluble in water, conducts in no state.
• Conductivity depends on freely moving ions, free when molten or dissolved, fixed when solid, absent in covalent compounds.
• Melting point depends on the strength of the forces: strong lattice forces vs weak forces between molecules.
• To deduce bonding from data, use melting point and electrical conductivity together, with reasons.