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B3.5 · Compare physical properties of ionic and molecular compounds

Learn to compare physical properties of ionic and molecular compounds through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Matter, Chemical Trends, and Chemical Bonding

Ontario Grade 11 Chemistry — B3.5

A crystal of table salt is hard and brittle, while sugar crystals are usually easier to crush. Both are solids, but they are made of different kinds of particles. Comparing their properties helps us connect what we can observe to how particles are arranged. In this lesson, you will compare ionic and molecular compounds using course-level models. The patterns are useful, but they are not guarantees for every compound.

What you will learn

Identify the basic particle model for ionic and molecular compounds.
Compare common physical properties of ionic and molecular compounds.
Use particle arrangement to explain patterns in melting, hardness, and electrical conductivity.
Describe solubility patterns without treating them as rules that have no exceptions.

1. Start with the particle models

A physical property can be observed or measured without changing a substance into a different substance. Examples include state, melting point, hardness, solubility, and electrical conductivity. Conductivity is a material’s ability to carry electric charge.

First recall two course-level terms. An atom is a basic unit of an element. A molecule is a group of atoms held together by covalent bonds. A covalent bond is a strong attraction that holds atoms together by sharing electrons. In this lesson, a molecular compound is made of separate molecules.

An ion is an atom or group of atoms with an electric charge. An ionic compound contains positive and negative ions. These ions are arranged in a repeating three-dimensional pattern called a crystal lattice. The compound as a whole has no net charge, even though its ions are charged.

A molecular solid has separate molecules packed near one another. The covalent bonds hold atoms together inside each molecule. Attractions between neighbouring molecules hold the solid together. These attractions are generally weaker than the forces holding an ionic lattice together. This difference helps explain several common property patterns.

Ionic compounds are made of positive and negative ions in a lattice.
Molecular compounds are made of separate molecules.
The particles and their arrangement help explain physical properties.

2. Compare melting, state, and hardness

Many ionic compounds are crystalline solids at room temperature. Their ions are held in a repeating lattice, so a great deal of heating is usually needed to separate the ions enough for the solid to melt. As a result, ionic compounds commonly have high melting points.

Many molecular compounds have lower melting points than ionic compounds. When a molecular solid melts, the molecules move apart from one another; the covalent bonds within each molecule usually remain intact. The attractions between separate molecules are generally weaker than the attractions throughout an ionic lattice.

Do not conclude that every molecular compound is a liquid or gas at room temperature. Some are solids. Molecular compounds also differ in how strongly their molecules attract one another, so their melting points vary. The comparison describes a common pattern, not an absolute rule.

Ionic crystals are often hard, but they can be brittle. Brittle means that a solid can crack or break when struck. If layers in an ionic crystal shift, ions with the same charge can line up near one another. Their repulsion can help split the crystal. Molecular solids vary in hardness and may be soft, but their behaviour depends on the substance.

Ionic compounds commonly have high melting points and form hard, brittle crystals.
Molecular compounds often have lower melting points, but their states and hardness vary.
Use words such as commonly and often; avoid claiming that every compound follows the pattern.

3. Compare electrical conductivity and solubility

A solid ionic compound usually does not conduct electricity. Its ions are charged, but they are held in fixed positions in the lattice and cannot move through the solid. When an ionic compound is melted or dissolved in water, its ions can move. The liquid or solution can then conduct electricity.

A molecular compound usually does not conduct electricity as a pure substance. Its molecules are neutral overall, and they do not provide freely moving charged particles in the way an ionic solution does. Do not assume that dissolving any molecular compound makes a conducting solution.

Solubility describes how much of a substance can dissolve in a particular solvent. Water dissolves many ionic compounds and many molecular compounds, but not all. For example, table salt and sugar can both dissolve in water. Their different particle types do not mean that only one kind can dissolve. A substance’s behaviour depends on the substance and the solvent, so use course reference information when a particular solubility decision is needed.

These are physical comparisons: dissolving, melting, and measuring conductivity do not by themselves identify a new substance. For this expectation, focus on the property pattern and its particle-level explanation rather than on reaction rules.

Ionic solids usually do not conduct, but molten ionic compounds and ionic solutions can conduct because ions can move.
Molecular compounds are generally poor electrical conductors as pure substances.
Both ionic and molecular compounds can dissolve in water; solubility is not a simple either-or test.

4. Make a fair comparison

When comparing two substances, name the property and the conditions. For conductivity, say whether the sample is solid, molten, or dissolved in water. For solubility, name the solvent. For state and melting point, specify the temperature or compare the melting-point pattern.

Then connect the observation to the particle model. Ask whether charged particles are present and able to move. Ask whether the solid is a lattice of ions or a collection of separate molecules. This gives a clear explanation without claiming that one observation proves every detail about a substance.

A good comparison states both the shared feature and the difference when relevant. For example, both salt and sugar can be solids and can dissolve in water. Solid salt is made of ions in a lattice, while solid sugar is made of molecules. Their particles explain why their electrical behaviour differs.

State the conditions for properties that change with state or solvent.
Use particle type and particle movement to explain conductivity.
Separate a general trend from a rule that applies to every substance.

Common physical-property patterns

Property	Ionic compounds	Molecular compounds
Particles	Positive and negative ions in a lattice	Separate molecules
Melting point	Commonly high	Often lower, with variation
Solid form	Often crystalline, hard, and brittle	May be solid, liquid, or gas; hardness varies
Electrical conductivity	Solid: usually no; molten or aqueous: can conduct	Usually poor as a pure substance
Water solubility	Many dissolve; some do not	Many dissolve; some do not

Worked example

Compare salt and sugar

A learner compares solid sodium chloride, table salt, with solid sucrose, a type of sugar. Use the particle model to compare their expected electrical conductivity and explain what can be concluded about dissolving them in water.

Identify the particles
Sodium chloride is an ionic compound made of sodium ions and chloride ions in a lattice. Sucrose is a molecular compound made of separate molecules. Its formula is $\mathrm{C_{12}H_{22}O_{11}}$ .
Compare the solid samples
In solid sodium chloride, the ions cannot move through the lattice, so the solid does not conduct electricity. Sucrose molecules are neutral overall, so solid sucrose also does not conduct electricity under the usual course-level model.
Consider water as the solvent
Both substances can dissolve in water. Dissolving sodium chloride allows its ions to move through the solution, so the solution conducts electricity. Dissolving sucrose does not make its molecules into ions, so the solution is not expected to conduct in the same way.

Answer: Both pure solids are expected to be poor electrical conductors. Their solutions differ: dissolved sodium chloride has mobile ions and conducts, while dissolved sucrose remains molecular and does not conduct in the same way.

Check: The comparison accounts for particle type and mobility. It does not incorrectly claim that only ionic compounds dissolve in water.

Common mistakes and how to avoid them

All molecular compounds are gases or liquids at room temperature.

Correction: Some molecular compounds are solids. Molecular compounds vary in state and melting point.

A solid ionic compound conducts because it contains charged ions.

Correction: The ions in a solid lattice cannot move freely. Ionic compounds conduct when molten or dissolved if their ions are mobile.

Only ionic compounds dissolve in water.

Correction: Many molecular compounds, including sugar, dissolve in water. Solubility depends on the substance and solvent.

Every ionic compound has exactly the same physical properties.

Correction: The ionic model predicts useful general patterns, but the details vary between compounds.

Lesson summary

Ionic compounds contain positive and negative ions arranged in a lattice. Molecular compounds contain separate molecules.
Ionic compounds commonly have high melting points and are often hard, brittle solids. Molecular compounds often have lower melting points, but their states and hardness vary.
A solid ionic compound usually does not conduct because its ions cannot move. Molten ionic compounds and ionic solutions can conduct because ions can move.
Molecular compounds are generally poor conductors as pure substances. Both ionic and molecular compounds may or may not dissolve in water.

Check your understanding

Question 1

Why does solid sodium chloride usually not conduct electricity?

Its ions are fixed in the solid lattice and cannot move through the sample.
It contains no charged particles.
Its atoms are joined into separate neutral molecules.
It has a low melting point.

Show answer and explanation

Its ions are fixed in the solid lattice and cannot move through the sample.

Solid sodium chloride contains charged ions, but the ions are held in fixed positions. Conductivity requires charge to move.

Question 2

Which statement about water solubility is most accurate?

All ionic compounds dissolve, and no molecular compounds dissolve.
Only molecular compounds can dissolve in water.
Some ionic and some molecular compounds dissolve in water.
Dissolving always makes a substance conduct electricity.

Show answer and explanation

Some ionic and some molecular compounds dissolve in water.

Both kinds of compounds can dissolve in water, but not every compound dissolves. A dissolved molecular compound does not necessarily produce mobile ions.

Question 3

Which is a common comparison of melting points?

Ionic compounds commonly have higher melting points than molecular compounds.
Molecular compounds always have higher melting points.
All compounds have the same melting point if they are solids.
Ionic compounds cannot melt.

Show answer and explanation

Ionic compounds commonly have higher melting points than molecular compounds.

The ionic lattice commonly requires more heating to separate its ions than is needed to separate molecules in many molecular solids. This is a trend, not an exception-free rule.

Key terms

Physical property: A feature that can be observed or measured without changing a substance into a different substance.
Molecule: A group of atoms held together by covalent bonds.
Ion: An atom or group of atoms with an electric charge.
Crystal lattice: A repeating arrangement of particles in a crystal.
Conductivity: The ability of a material to carry electric charge.
Solubility: How much of a substance can dissolve in a particular solvent.
Brittle: Likely to crack or break when struck rather than bend easily.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation B3.5. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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