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C3.4 · Explain physical properties using particles and inter- and intramolecular forces

Learn to explain physical properties using particles and inter- and intramolecular forces through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Structure and Properties of Matter

Ontario Grade 12 Chemistry — study topic C3.4

A drop of water can remain on a surface as a rounded bead, while some liquids spread out. Water also boils at a much higher temperature than many molecules of similar size. These visible properties have a particle-level explanation: particles attract one another, and the strength and type of those attractions matter. In this lesson, a particle means an atom, ion, or molecule. A molecule is a group of atoms joined by bonds. We will connect observations to particle models and then use chemical symbols to describe those models.

What you will learn

Describe how particle arrangement and movement help explain physical properties.
Distinguish attractions between particles from bonds within a particle.
Use particle attractions to explain differences in boiling point, volatility, viscosity, and solubility.
Compare substances by considering particle size, shape, and polarity.

1. From observable properties to particle models

A physical property can be observed or measured without changing a substance into a different substance. Examples include boiling point, melting point, viscosity, and solubility. Viscosity describes how much a liquid resists flowing. Solubility describes how much of one substance can dissolve in another under stated conditions.

Before using forces to explain these properties, recall two ideas from earlier chemistry. Matter is made of particles, and particles are always moving. In a solid, particles stay close to fixed positions but can vibrate. In a liquid, particles stay close together but can move past one another. In a gas, particles are much farther apart and move freely.

Particles can attract other particles. These attractions help keep particles close together. Separating particles usually requires energy. When a liquid boils, its particles separate enough to form a gas. The attractions between particles are overcome; the molecules themselves do not need to be broken apart.

A force within a molecule is called an intramolecular force. A covalent bond, such as an oxygen–hydrogen bond in water, is intramolecular. A force between separate particles is called an intermolecular force. The attractions between separate water molecules are intermolecular. These terms describe different interactions and should not be mixed up.

Physical changes alter particle arrangement or separation, not the identity of the molecules.
Intramolecular bonds hold atoms together within a molecule.
Intermolecular forces attract separate molecules.

2. Main attractions between molecules

All molecules experience London dispersion forces. These are attractions that occur because electrons are in motion and can be unevenly distributed for a short time. Dispersion forces are present in both polar and non-polar molecules. A non-polar molecule has no lasting separation of electrical charge across the molecule.

Dispersion forces tend to become stronger as a molecule has more electrons and a larger, more spread-out electron cloud. Shape also matters. For molecules with similar size and type, a long shape can allow more contact between neighbouring molecules than a compact, branched shape. Greater contact can strengthen the overall attraction.

Some molecules are polar. A polar molecule has an uneven distribution of charge, so one region is slightly positive and another is slightly negative. Attractions between the oppositely charged regions of polar molecules are called dipole–dipole forces. Polarity depends on the bonds and the overall shape of the molecule; having polar bonds alone does not always make the whole molecule polar.

Hydrogen bonding is a particularly strong type of attraction between molecules. It occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine and is attracted to nitrogen, oxygen, or fluorine on a nearby molecule. In water, the hydrogen of one molecule is attracted to the oxygen of another. The O–H covalent bond inside a water molecule is still an intramolecular bond, not a hydrogen bond.

A molecule may experience more than one kind of intermolecular force. For example, a polar molecule also has dispersion forces. When comparing substances, consider all relevant factors, including particle size, shape, and polarity, rather than relying on one label.

Dispersion forces occur between all molecules.
Dipole–dipole forces occur between polar molecules.
Hydrogen bonding requires hydrogen bonded to nitrogen, oxygen, or fluorine and attraction to one of those atoms on a nearby molecule.

3. Connecting attractions to physical properties

A liquid with stronger attractions between its particles generally needs more energy to separate those particles into a gas. It therefore tends to have a higher boiling point than a comparable substance with weaker attractions. A higher boiling point often means lower volatility. Volatility describes how readily a liquid forms a gas.

The same reasoning helps explain viscosity. In a liquid with stronger attractions, particles may be less able to slide past one another easily, so the liquid can be more viscous. Viscosity can also depend on particle size and shape. The force comparison gives a useful explanation, but it does not let us ignore those other factors.

Melting involves particles leaving their ordered arrangement in a solid and becoming able to move as a liquid. Attractions affect how much energy is needed, but melting points can be harder to predict from a simple force ranking. How particles pack in the solid also matters.

Solubility depends on how the particles of the solute and solvent interact. A solute is the substance being dissolved; a solvent is the substance doing the dissolving. Dissolving requires separating some solute particles and making room between solvent particles. New attractions form between solute and solvent particles. A substance tends to dissolve well when these new attractions can make up for the attractions that were separated.

This is why substances with similar kinds of polarity often mix more readily. Water is polar, so it can attract other polar molecules and ions. A non-polar substance usually has stronger interactions with other non-polar substances than with water. This is a useful pattern, not a guarantee that every pair will mix in every amount.

Stronger particle attractions often correspond to higher boiling points and lower volatility.
Viscosity depends on attractions as well as particle size and shape.
Solubility depends on the balance between attractions separated and attractions formed.

4. Compare carefully: bonds within versus forces between

The structure of a molecule affects its intermolecular attractions. Its covalent bonds determine which atoms are joined and help establish the molecule's shape and polarity. Those features then affect how separate molecules attract one another. This is the link between intramolecular structure and many physical properties.

During boiling, intermolecular attractions are overcome as molecules move farther apart. The covalent bonds within each molecule remain. If a covalent bond were broken and atoms rearranged into different molecules, that would be a chemical change rather than an ordinary change of state.

When explaining a physical property, state what is observed first. Then identify the relevant particles and attractions. Finally, connect the strength or type of attraction to the property. For a comparison, name the factors that make the comparison fair, such as similar molecular size, and mention other factors that could affect the result.

Molecular structure affects polarity and shape, which influence intermolecular attractions.
A change of state does not normally break the molecule's covalent bonds.
A sound comparison names both the attraction and the property it helps explain.

Worked example

Why does ethanol boil at a much higher temperature than dimethyl ether?

Ethanol and dimethyl ether both have the molecular formula

ext{C}_2 ext{H}_6 ext{O}

, but their boiling points at standard atmospheric pressure are about

78.4\ ^\circ\text{C}

and

-24.8\ ^\circ\text{C}

, respectively. Use their structures and intermolecular forces to explain the difference.

Compare the structures
Both substances have the same number and kinds of atoms, so their molecular sizes are similar. However, the atoms are connected differently. Ethanol contains an O–H bond. Dimethyl ether has an oxygen atom between two carbon-containing groups, but it has no O–H bond. $\text{CH}_3\text{CH}_2\text{OH}$ and $\text{CH}_3\text{OCH}_3$
Identify the attractions
Both substances have dispersion forces. Both are polar and have dipole–dipole attractions. Ethanol molecules can also form hydrogen bonds with one another because ethanol has hydrogen bonded to oxygen. Dimethyl ether has no hydrogen bonded to nitrogen, oxygen, or fluorine, so dimethyl ether molecules do not form hydrogen bonds with one another under this rule.
Connect attractions to boiling
Ethanol has an additional strong attraction between its molecules. More energy is needed to separate ethanol molecules enough to form a gas. This explains why ethanol has the higher boiling point, even though the two substances have the same molecular formula.

Answer: Ethanol forms hydrogen bonds between its molecules, as well as dispersion and dipole–dipole attractions. Dimethyl ether has dispersion and dipole–dipole attractions but cannot hydrogen-bond to another dimethyl ether molecule. The stronger overall attractions in ethanol require more energy to overcome, giving ethanol the higher boiling point.

Check: The molecules have similar size, so the presence of hydrogen bonding is a key difference in this comparison. The boiling process separates molecules; it does not break ethanol's O–H covalent bonds.

Common mistakes and how to avoid them

Calling a covalent bond between atoms in a molecule an intermolecular force.

Correction: A bond within one molecule is intramolecular. An attraction between separate molecules is intermolecular.

Saying that boiling breaks the covalent bonds in each molecule.

Correction: Boiling separates molecules by overcoming intermolecular attractions. The molecules usually remain intact.

Assuming every molecule with an oxygen atom forms hydrogen bonds with itself.

Correction: For this course-level rule, the molecule must have hydrogen bonded to nitrogen, oxygen, or fluorine to hydrogen-bond to another molecule of its own kind.

Explaining solubility only by saying that polar substances dissolve polar substances.

Correction: Consider the attractions that must be separated and the new attractions that form between solute and solvent particles.

Lesson summary

Particles attract one another, and these attractions help explain physical properties.
Intramolecular bonds join atoms within a particle; intermolecular forces attract separate particles.
Dispersion forces occur in all molecules; polar molecules also have dipole–dipole attractions, and some molecules can hydrogen-bond.
Stronger attractions often raise boiling point and reduce volatility, while solubility depends on interactions between solute and solvent particles.

Check your understanding

Question 1

Which statement best explains why a liquid with stronger intermolecular attractions often has a higher boiling point?

Its molecules have to gain more energy to separate far enough to form a gas.
Its covalent bonds must all be broken before the liquid can boil.
Its molecules stop moving until the boiling point is reached.
Its particles become larger as the liquid is heated.

Show answer and explanation

Its molecules have to gain more energy to separate far enough to form a gas.

Boiling requires molecules to separate from one another. Stronger attractions between particles generally require more energy to overcome; the molecules do not need to have their covalent bonds broken.

Question 2

Which sample can form hydrogen bonds between its own molecules?

Methane, $ext{CH}_4$
Ethanol, $ext{CH}_3 ext{CH}_2 ext{OH}$
Dimethyl ether, $ext{CH}_3 ext{OCH}_3$
Carbon dioxide, $ext{CO}_2$

Show answer and explanation

Ethanol,

ext{CH}_3 ext{CH}_2 ext{OH}

Ethanol has hydrogen directly bonded to oxygen. The other listed molecules do not have hydrogen bonded to nitrogen, oxygen, or fluorine.

Key terms

Physical property: A feature that can be observed or measured without changing a substance into a different substance.
Intramolecular force: A bond or force within a particle, such as a covalent bond within a molecule.
Intermolecular force: An attraction between separate molecules.
Polarity: An uneven distribution of electrical charge across a molecule.
Volatility: How readily a liquid forms a gas.
Viscosity: A measure of how much a liquid resists flowing.
Solute and solvent: The solute is the substance being dissolved; the solvent is the substance that dissolves it.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation C3.4. 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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