Properties of Ionic and Molecular Substances

The physical properties of a substance are decided by its bonding and structure. Whether something melts easily, conducts electricity or is a gas at room temperature all traces back to the type of particles it contains and how strongly they are held.

This lesson covers three types of substance and the properties they show:

• Ionic compounds — giant lattices of ions.
• Small molecules — a few atoms held by covalent bonds.
• Polymers — very large molecules made of repeating units.

Substance type	Structure	Typical melting/boiling point	Conducts electricity?
Ionic compound	Giant ionic lattice	High	Only when molten or dissolved
Small molecule	Separate small molecules	Low	No
Polymer	Very large molecules	Higher than small molecules; solid at room temp	No

The rule that ties it all together: to change state or conduct electricity, particles or charges must be freed, and how easily that happens depends on the bonding.

Each type is now taken in turn, explaining every property from the structure.

Ionic compounds have high melting and boiling points. This follows directly from their structure: a giant ionic lattice of oppositely charged ions held together by strong electrostatic forces of attraction acting in all directions.

To melt or boil an ionic compound, many of these strong forces must be overcome, which takes a large amount of energy, so the temperature required is high. Sodium chloride, for example, melts at 801 °C.

A full-mark explanation names the parts: strong electrostatic forces between oppositely charged ions act in all directions throughout the giant lattice, so a large amount of energy is needed to overcome them.

Because the forces act in every direction and throughout an enormous structure, there is no weak point to break easily. This is the opposite of a small molecule, where only weak forces need breaking. The strength and extent of ionic bonding is exactly why these compounds are solids with high melting points at room temperature.

Ionic compounds conduct electricity only when molten (melted) or dissolved in water — not when solid. The reason is about whether the charged ions can move.

• Solid: the ions are locked in fixed positions in the lattice. They cannot move, so no charge can flow and the compound does not conduct.
• Molten: melting frees the ions from the lattice. They are now free to move, so they can carry charge and the compound conducts.
• Dissolved (aqueous): dissolving in water also separates the ions and lets them move, so the solution conducts.

Conduction needs charged particles that are free to move. In an ionic compound those particles are the ions, and they are only free when molten or dissolved.

Worked reasoning. Why does solid sodium chloride not conduct, but molten sodium chloride does? In the solid, the Na⁺ and Cl⁻ ions are held in fixed positions in the lattice and cannot move. When melted, the ions break free and can move, carrying charge through the liquid. The charge carriers are the same ions in both cases; only their freedom to move changes.

Substances made of small molecules (such as O₂, H₂O, CO₂ and CH₄) are usually gases or liquids at room temperature, with low melting and boiling points. The explanation is the key idea of this section.

A small molecule has two kinds of force to consider:

• Strong covalent bonds hold the atoms together inside each molecule.
• Weak intermolecular forces act between separate molecules.

When a molecular substance melts or boils, it is the weak intermolecular forces that are overcome, not the strong covalent bonds. Because these forces are weak, only a little energy is needed, so melting and boiling points are low.

This is the single most important idea in the topic: melting a molecular substance breaks the forces between molecules, never the covalent bonds within them.

Intermolecular forces get stronger as the molecules get larger. That is why larger molecules have higher melting and boiling points: for example, longer hydrocarbon molecules are liquids or solids while the smallest are gases. More energy is needed to separate bigger molecules.

Small molecules do not conduct electricity because the molecules have no overall electric charge, so there are no free charged particles to carry a current.

Polymers are very large molecules made of many small units linked together in long chains by strong covalent bonds. Common examples are poly(ethene) and poly(propene).

The atoms within a polymer chain are joined by strong covalent bonds, just like in small molecules. The difference is the size of the molecule. Because polymer molecules are so large, the intermolecular forces between the chains are relatively strong — much stronger than between small molecules.

These relatively strong intermolecular forces need a fair amount of energy to overcome, so polymers are solids at room temperature, unlike the small molecules covered above which are often gases or liquids.

Feature	Small molecule	Polymer
Molecule size	Small (a few atoms)	Very large (long chains)
Bonds within the molecule	Strong covalent	Strong covalent
Forces between molecules	Weak	Relatively strong
State at room temperature	Often gas or liquid	Solid

You must be able to recognise a polymer from a diagram: look for a repeating unit shown in brackets with a small "n", meaning the unit repeats many times along a long chain.

Polymers do not conduct electricity, for the same reason as small molecules: the molecules have no overall charge.

1. Saying covalent bonds break when a molecular substance melts

Melting or boiling a small molecule overcomes the weak intermolecular forces, not the strong covalent bonds inside the molecule. This is the classic error and it loses the mark every time.

2. Claiming ionic compounds conduct when solid

Solid ionic compounds do not conduct, because the ions are fixed in the lattice. They conduct only when molten or dissolved, when the ions can move.

3. Explaining conductivity without mentioning free-moving ions

The reason ionic compounds conduct when molten or dissolved is that the ions are free to move and carry charge. Simply saying "they contain ions" is not enough; the ions must be able to move.

4. Forgetting that intermolecular forces increase with size

Bigger molecules have stronger intermolecular forces and therefore higher melting and boiling points. Do not assume all molecular substances have the same low boiling point.

5. Confusing polymers with small molecules

Polymers are solids at room temperature because their large molecules have relatively strong intermolecular forces. Do not describe a polymer as having weak intermolecular forces like a small molecule.