How to explain the properties of ionic and covalent compounds

Once you know how ionic and covalent bonds form, the exam pushes one step further: it asks you to explain why the two kinds of compound behave so differently. Ionic compounds are hard, high-melting solids that conduct when molten; simple covalent compounds are soft, low-melting substances that never conduct. Every one of those differences comes back to structure, and if you can explain the structure, you can explain the property. This guide sets out the comparison and the reasoning behind it.

Two very different structures

An ionic compound is not made of molecules. It is a giant ionic lattice, a huge, regular three-dimensional arrangement of positive and negative ions, held together throughout by strong electrostatic forces of attraction. You can revise this at our giant ionic lattice glossary entry.

A simple covalent compound is made of separate small molecules. Inside each molecule the atoms are held by strong covalent bonds, but between one molecule and the next there are only weak intermolecular forces. This is a simple molecular structure, defined at simple molecular structure. Holding these two pictures in mind explains everything that follows.

Melting and boiling points

Ionic compounds have high melting and boiling points. To melt one, you must overcome the strong electrostatic forces holding the whole lattice together, and that takes a great deal of heat energy. Sodium chloride, for example, is a solid well above room temperature.

Simple covalent compounds have low melting and boiling points, and many are liquids or gases at room temperature, water, carbon dioxide, ammonia. Melting them does not break the covalent bonds; it only separates whole molecules from one another, and the weak intermolecular forces between molecules need little energy to overcome.

Here is the single most common trap: students say “covalent bonds are weak.” They are not, the covalent bonds inside a molecule are strong. What is weak is the force between molecules. Say “weak intermolecular forces,” never “weak covalent bonds,” and you protect an easy mark.

Electrical conductivity

This is the property examiners love, because it separates the two cleanly.

Ionic compounds conduct electricity when molten or dissolved in water, but not when solid. Electricity needs charged particles that are free to move. In the solid lattice the ions are locked in fixed positions, so it does not conduct. Once melted or dissolved, the ions break free and can move to carry the current. State all three cases, solid no, molten yes, aqueous yes, because questions often test the solid case specifically.

Simple covalent compounds do not conduct electricity in any state, because their molecules are neutral: there are no ions and no free-moving electrons to carry a charge.

Solubility

Ionic compounds are generally soluble in water but insoluble in organic solvents. Simple covalent compounds are generally the opposite, often insoluble in water but soluble in organic solvents such as ethanol. This is a useful confirming test: if a solid dissolves readily in water and its solution conducts, it is very likely ionic.

A side-by-side summary

• Structure: ionic = giant ionic lattice of ions; covalent = separate molecules.
• Melting/boiling point: ionic = high; covalent = low.
• State at room temperature: ionic = solid; covalent = often liquid or gas.
• Conducts electricity: ionic = only when molten or aqueous; covalent = never.
• Solubility: ionic = usually soluble in water; covalent = usually soluble in organic solvents.

Learning the table is not enough, you must be able to give the reason, because Paper 2 asks “explain,” not just “state.” You can revise how these properties sit within the topic on our chemical bond chapter page.

One nuance worth knowing

Not every covalent substance is soft and low-melting. A few form a giant covalent structure instead of small molecules, diamond, graphite and silicon dioxide (sand), where a continuous network of strong covalent bonds gives very high melting points. Graphite even conducts electricity, because it has free-moving delocalised electrons between its layers. SPM focuses mainly on simple molecular compounds, but knowing this exception stops you over-applying the “covalent means low-melting” rule. Many of these properties are things you can watch in the lab, and our experiments hub shows the practical work behind them.

Explaining properties from structure is exactly the kind of reasoning that separates a good answer from a full-mark one. If you would like a teacher to check that your explanations use the precise wording examiners want, “strong electrostatic forces,” “ions free to move,” “weak intermolecular forces”, our online one-to-one lessons with our experienced SPM Chemistry teachers give that feedback, from RM50 an hour with a paid one-hour trial.