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B3.2 · Compare physical properties within organic compound classes

Learn to compare physical properties within organic compound classes through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

How molecular size and structure help explain observable differences

Two clear liquids can look alike but behave differently when heated or mixed with water. These observable differences are physical properties: they can be measured without changing a substance into a different substance. In this lesson, you will compare compounds within a class, such as alcohols with other alcohols. You will connect the observations to particle attractions and molecular structure. The goal is to explain patterns, not to memorize a separate rule for every compound.

What you will learn

Identify physical properties that can be compared for compounds in the same organic class.
Explain how molecular size and structure affect boiling point and water solubility.
Use a fair comparison by keeping the compound class and conditions in mind.

1. Set up a fair comparison

A compound class is a group of organic compounds with the same key structural feature. For example, alcohols contain a hydroxyl group, written

\mathrm{-OH}

, while alkanes contain only carbon and hydrogen atoms joined by single bonds. A homologous series is a family within a class in which each next member differs by one

\mathrm{CH_2}

unit. Members of a homologous series share a general pattern but do not have identical properties.

A physical property is observed or measured without changing the chemical identity of a substance. Examples include boiling point, melting point, density, viscosity, and solubility. Solubility describes how much of one substance can dissolve in a specified amount of another at a stated temperature. Boiling point depends on pressure, so comparisons should use the same pressure. For the values in the example, the pressure is approximately

101.3\ \mathrm{kPa}

Before comparing, check that the compounds belong to the same class and that the property is measured under matching conditions. Then note what changes in their structures. A comparison within a class is different from comparing unrelated classes, because different functional groups can create different attractions between particles.

Use the same physical property and measurement conditions.
Check that the compounds are members of the same class.
Describe the structural difference before explaining the property pattern.

2. Particle attractions and boiling point

In a liquid, molecules are close together and attract one another. These attractions are called intermolecular forces. When a liquid boils, particles separate from the liquid and enter the gas. The molecules themselves are not being changed into different molecules. A higher boiling point generally means more energy must be supplied to separate the particles enough for boiling.

For compounds in the same class, increasing the number of carbon atoms usually increases molecular size and the area over which neighbouring molecules can attract. This often raises the boiling point along a straight-chain homologous series. The change is a pattern, not a reason to guess an exact value without data.

Molecular shape also matters. Among compounds with the same molecular formula and class, a more compact, branched shape often has a lower boiling point than a less branched shape. The shapes make different amounts of contact with neighbouring molecules. This comparison is most useful when the compounds are otherwise similar.

Alcohols have an additional attraction involving their

\mathrm{-OH}

groups. An attraction between an

\mathrm{-OH}

group on one molecule and another molecule is often called hydrogen bonding. It is stronger than the attractions usually considered between alkane molecules of similar size. Within a series of alcohols, however, adding carbon atoms also changes molecular size, so boiling points can still rise as the chain gets longer.

Boiling point reflects how readily liquid particles separate into a gas.
Within a straight-chain class, larger molecules often have higher boiling points.
Shape can affect boiling point even when compounds belong to the same class.

3. Solubility and other comparisons

Water molecules attract one another strongly. A substance dissolves well in water when its particles can interact favourably with water particles. The phrase “like dissolves like” is a useful first guide: substances with similar kinds of attraction are often more compatible. It is a guide, not a rule that predicts an exact solubility.

Within the alcohol class, the

\mathrm{-OH}

group can attract water molecules. Small alcohols, such as methanol and ethanol, mix with water in all proportions. As the carbon chain becomes longer, the part of the molecule made mostly of carbon and hydrogen becomes a larger fraction of the whole. Water solubility tends to decrease along the series. The

\mathrm{-OH}

group is still present; the overall balance of the molecule’s parts has changed.

Alkanes do not have an

\mathrm{-OH}

group and do not mix well with water. Within an alkane series, adding carbon atoms does not make the molecules water-compatible. This example shows why comparisons should stay within a class: an alcohol’s

\mathrm{-OH}

group gives it a feature that an alkane does not have.

Other physical properties can also be compared, but the trend may depend on the property and the exact structures. For example, viscosity describes resistance to flowing. A longer molecule may flow less easily than a smaller one under the same conditions, but temperature and molecular shape also matter. Do not claim that every property changes in one direction for every class. Use reliable data and identify the conditions.

Small alcohols mix readily with water; increasing carbon-chain length tends to reduce water solubility.
Alkanes are poorly soluble in water because their structure offers no alcohol-like $\mathrm{-OH}$ group.
State the conditions and avoid turning a trend into an absolute rule.

4. Reading and reporting property data

A useful comparison names the compounds, states the property, gives the relevant data or trend, and offers a particle-level reason. Keep units with measured quantities. If values come from a data source, retain the precision justified by that source rather than adding extra decimal places.

For boiling points, make sure the pressure is the same. For solubility, note the solvent and temperature. A statement such as “compound A has a higher boiling point” is incomplete if the compounds or conditions are not identified. A stronger comparison explains which feature of their structures could account for the difference.

For isomers, compounds with the same molecular formula but different structures, compare the structures before explaining a property difference. Isomers can belong to the same class and still have different shapes. That is why membership in one class does not guarantee identical boiling points, melting points, or solubilities.

Include names, property, conditions, and a structural explanation.
Keep units and appropriate significant digits when reporting data.
Same class does not mean same physical properties.

Worked example

Compare two alcohols

At approximately

101.3\ \mathrm{kPa}

, ethanol has a boiling point of

78.4\ ^\circ\mathrm{C}

and propan-1-ol has a boiling point of

97.2\ ^\circ\mathrm{C}

. Both belong to the alcohol class. Which has the higher boiling point, and what structural feature helps explain the difference?

Confirm the comparison
Both substances are alcohols, and both boiling points are stated at the same pressure. This makes the comparison fair.
Compare the data
Subtract ethanol’s boiling point from propan-1-ol’s. The positive difference shows that propan-1-ol has the higher boiling point.
$97.2\ ^\circ\mathrm{C}-78.4\ ^\circ\mathrm{C}=18.8\ ^\circ\mathrm{C}$
Connect structure to particles
Both molecules have an $\mathrm{-OH}$ group, so both can attract one another through hydrogen bonding. Propan-1-ol has one more carbon atom than ethanol and a larger molecular structure. The additional size generally increases attractions between neighbouring molecules, so more energy is needed for the liquid to boil. This accounts for the direction of the difference; the data provide its measured size.

Answer: Propan-1-ol has the higher boiling point. The difference is

18.8\ ^\circ\mathrm{C}

at the stated pressure.

Check: The subtraction preserves the temperature unit, and the result is reported to one decimal place, matching the given data.

Common mistakes and how to avoid them

Assuming every compound in one class has the same boiling point.

Correction: Compounds in a class share a key structural feature, but size and shape can differ and affect physical properties.

Saying that a longer carbon chain always raises every physical property.

Correction: Trends depend on the property. For example, boiling point often rises along a straight-chain series, while water solubility in an alcohol series tends to fall.

Explaining a boiling point difference by saying that molecules break apart.

Correction: Boiling separates molecules from the liquid. It does not normally change the molecules into new substances.

Comparing data measured under different conditions as if the conditions were identical.

Correction: State or match conditions such as pressure for boiling point and temperature for solubility.

Lesson summary

Compare compounds within the same class and under the same conditions.
Boiling point is linked to how strongly liquid particles attract one another.
Within a homologous series, larger molecules often have higher boiling points.
Within an alcohol series, increasing carbon-chain length tends to lower water solubility.
Use data carefully and explain trends with molecular structure and particle attractions.

Check your understanding

Question 1

Two straight-chain alcohols differ by one

\mathrm{CH_2}

unit. Which statement is the best general prediction?

The larger alcohol will often have a higher boiling point and lower water solubility.
The larger alcohol must have a lower boiling point and higher water solubility.
Both alcohols must have identical physical properties because they are in the same class.
The smaller alcohol cannot attract water because it has fewer carbon atoms.

Show answer and explanation

The larger alcohol will often have a higher boiling point and lower water solubility.

A larger molecule in a straight-chain series often has stronger attractions between neighbouring molecules, raising boiling point. In an alcohol series, the larger carbon portion tends to reduce water solubility.

Question 2

Why should two boiling-point values be compared at the same pressure?

Because pressure affects the temperature at which a liquid boils.
Because pressure changes every alcohol into a different compound.
Because boiling point has no units unless pressure is stated.
Because molecules stop attracting one another at lower pressure.

Show answer and explanation

Because pressure affects the temperature at which a liquid boils.

Boiling point depends on pressure, so matching pressure makes the comparison meaningful.

Key terms

Physical property: A measurable or observable feature that does not change a substance into a different substance.
Compound class: A group of organic compounds that share a key structural feature.
Homologous series: A family within a compound class whose successive members differ by one $\mathrm{CH_2}$ unit.
Intermolecular forces: Attractions between separate molecules.
Solubility: How much of a substance dissolves in a specified amount of another substance under stated conditions.
Isomers: Compounds with the same molecular formula but different structures.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation B3.2. 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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