To predict a substance’s physical properties, first identify its particles and the forces between them. The strongest attractions, and whether charged particles can move, usually determine the observable behavior.

A systematic way to predict properties

For this topic, always start with four questions:

• What are the particles present: atoms, molecules, ions, or electrons?

• What is the structure: simple molecular, simple atomic, giant ionic, giant covalent, or metallic?

• What type of bonding or attraction holds the structure together?

• Are there any mobile charged particles that can carry charge?

A substance’s properties depend on what must be overcome when it melts or boils, and on whether charge can move through the substance or solution.

The term lattice is often used when describing giant structures.

Predicting melting and boiling temperatures

Simple molecular substances

Simple molecular substances contain molecules. The atoms inside each molecule are joined by strong covalent bonds, but the attractions between molecules are intermolecular forces.

When a simple molecular substance melts or boils:

• the intermolecular forces are overcome

• the covalent bonds within molecules are not broken

Because intermolecular forces are usually much weaker than ionic, metallic, or covalent bonds in giant structures, simple molecular substances usually have low melting and boiling temperatures.

The exact temperature depends on the strength of the intermolecular forces:

• stronger London forces give higher boiling temperatures

• hydrogen bonding gives higher melting and boiling temperatures than similar molecules without it

Simple atomic substances

Some substances exist as individual atoms, such as the noble gases. They are held together only by weak London forces.

This means they have:

• very low melting and boiling temperatures

• little energy needed to separate the atoms

Giant ionic lattices

Ionic compounds contain ions arranged in a giant ionic lattice.

A labeled sodium chloride (NaCl) crystal structure showing alternating $Na^+$ and $Cl^-$ ions in a repeating 3D lattice. The regular arrangement highlights how each ion is surrounded by oppositely charged ions, giving strong electrostatic attractions throughout the structure. This helps explain why ionic solids typically have high melting and boiling temperatures. Source

Oppositely charged ions attract strongly in all directions.

This gives ionic substances:

• high melting temperatures

• high boiling temperatures

A stronger ionic attraction gives a higher melting or boiling temperature. Stronger attraction is associated with:

• higher ionic charge

• smaller ionic radius

Giant covalent structures

Giant covalent substances contain atoms joined by covalent bonds throughout a continuous network.

To melt or boil them, many strong covalent bonds must be broken. As a result, they usually have:

• very high melting temperatures

• very high boiling temperatures

Metallic structures

Metals contain positive metal ions and delocalized electrons.

The strong electrostatic attraction between them is metallic bonding.

This usually gives metals:

• high melting temperatures

• high boiling temperatures

The exact values vary because metallic bond strength differs from metal to metal.

Predicting electrical conductivity

A substance conducts electricity only if it contains charged particles that are free to move.

Metals

Metals conduct in both the solid and liquid states because they have delocalized electrons that can move through the structure.

Ionic substances

Ionic compounds do not conduct when solid because the ions are fixed in position in the lattice.

They do conduct when:

• molten, because the ions can move

• dissolved in water, because the ions separate and become mobile

When explaining this, always link conductivity to mobile ions, not just to the presence of ions.

Simple molecular and simple atomic substances

These substances do not conduct electricity because they have:

• no ions

• no delocalized electrons

• no other mobile charged particles

Giant covalent substances

Most giant covalent substances do not conduct because their electrons are held in covalent bonds.

However, some are exceptions if they contain delocalized electrons that can move. This means you must check the structure carefully rather than assuming all giant covalent substances behave identically.

Predicting water solubility

To predict solubility in water, consider how well water can interact with the particles in the substance. Water is a polar solvent, so it dissolves substances best when strong attractions can form between water molecules and the solute particles.

Ionic substances

Many ionic compounds dissolve in water because water molecules surround the ions and stabilize them. This can overcome the attractions within the ionic lattice.

So ionic substances are often:

• soluble in water

• more likely to dissolve if water can attract the ions strongly enough

However, not all ionic compounds are soluble, so the prediction should be made carefully.

Simple molecular substances

Water solubility depends on the intermolecular forces the molecules can form with water.

• Molecules that can form hydrogen bonds with water are often soluble.

• Molecules that are strongly nonpolar are usually poorly soluble.

A simple molecular substance with only London forces between its molecules is usually insoluble or only slightly soluble in water.

Giant covalent substances

Giant covalent substances are generally insoluble in water because the network is very large and strong, and water cannot easily separate the atoms from the structure.

Metals

Metals do not dissolve in water as simple solutes because metallic bonding holds the structure together. Some may react with water, but that is different from dissolving.

Building a full prediction

A strong answer links structure to property in a clear chain of reasoning:

• identify the particles present

• identify the structure

• state the important bonding or intermolecular forces

• explain how this affects melting or boiling

• state whether there are mobile charged particles

• explain whether water can form strong enough attractions to dissolve the substance

Use precise wording. For example:

• say strong electrostatic attraction between oppositely charged ions

• say weak intermolecular forces between molecules

• say mobile ions or delocalized electrons

• avoid vague statements such as it has strong bonds, unless you name which bonds and where they act

In exam questions, the best predictions always connect particles present, structure, and type of attraction directly to the property being asked about.