Simple Molecular Substances

This lesson explains the structure and properties of simple molecular substances (also called simple molecules). You need to understand the difference between the strong covalent bonds within molecules and the weak intermolecular forces between molecules, and how this explains their properties. This is a key topic in the Edexcel GCSE Chemistry (1CH0) specification.

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What Are Simple Molecular Substances?

Simple molecular substances are made up of small molecules. Each molecule contains a fixed number of atoms held together by strong covalent bonds.

Examples include:

Substance	Formula	State at Room Temperature
Hydrogen	H₂	Gas
Oxygen	O₂	Gas
Nitrogen	N₂	Gas
Water	H₂O	Liquid
Carbon dioxide	CO₂	Gas
Methane	CH₄	Gas
Ammonia	NH₃	Gas
Hydrogen chloride	HCl	Gas

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Two Types of Force

It is essential to understand that there are two types of force at work in simple molecular substances:

1. Covalent bonds (intramolecular forces) — the strong shared-electron-pair bonds within each molecule, holding the atoms together.
2. Intermolecular forces — the weak forces of attraction between neighbouring molecules.

When a simple molecular substance melts or boils, it is the weak intermolecular forces that are overcome — not the covalent bonds. The molecules themselves remain intact.

Exam Tip: This distinction is one of the most commonly tested points. Many students lose marks by saying that "covalent bonds break" when a substance melts. The covalent bonds do not break — only the weak intermolecular forces between molecules are overcome.

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Low Melting and Boiling Points

Simple molecular substances have low melting and boiling points.

Why?

• The intermolecular forces between molecules are weak.
• Only a small amount of energy is needed to overcome these weak forces.
• Therefore, relatively low temperatures are sufficient to melt or boil these substances.

This is why many simple molecular substances are gases or liquids at room temperature.

Substance	Melting Point (°C)	Boiling Point (°C)
Hydrogen (H₂)	-259	-253
Oxygen (O₂)	-218	-183
Water (H₂O)	0	100
Methane (CH₄)	-182	-161
Carbon dioxide (CO₂)	-78 (sublimes)	-78 (sublimes)

Exam Tip: Notice that water has a relatively high boiling point for a simple molecule (100 °C). This is because water has particularly strong intermolecular forces (hydrogen bonds) between its molecules. You don't need to know the details of hydrogen bonding for GCSE, but be aware that water is unusual.

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Larger Molecules Have Higher Boiling Points

As the size of the molecule increases, the intermolecular forces become stronger.

• Larger molecules have more electrons.
• More electrons mean stronger intermolecular forces.
• This means more energy is needed to separate the molecules.
• Therefore, larger simple molecules tend to have higher melting and boiling points.

For example, comparing alkanes:

Alkane	Formula	Number of Carbons	Boiling Point (°C)
Methane	CH₄	1	-161
Ethane	C₂H₆	2	-89
Propane	C₃H₈	3	-42
Butane	C₄H₁₀	4	-1
Pentane	C₅H₁₂	5	36

As the chain gets longer, the boiling point increases.

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Do Not Conduct Electricity

Simple molecular substances do not conduct electricity (in any state).

Why?

• They contain no free electrons that can move.
• They contain no ions that can carry charge.
• The molecules are electrically neutral.

Even when melted or dissolved, simple molecular substances remain as intact molecules with no mobile charge carriers.

Exam Tip: When explaining why a simple molecular substance doesn't conduct, state both: "no free electrons" and "no ions." This covers all the marking points.

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Solubility

• Many simple molecular substances are insoluble in water (e.g. oils, waxes, many organic compounds).
• However, some simple molecular substances do dissolve in water (e.g. sugar, hydrogen chloride, ammonia).
• Some dissolve because they react with water (e.g. HCl dissolves to form hydrochloric acid).

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Summary Table

Property	Simple Molecular Substance	Reason
Melting/boiling points	Low	Weak intermolecular forces — little energy needed to overcome them
Electrical conductivity	Does not conduct	No free electrons or ions
State at room temp	Often gas or liquid	Weak intermolecular forces easily overcome
Effect of molecular size	Larger = higher mp/bp	Stronger intermolecular forces

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Common Misconception

WRONG: "Water has a low boiling point because covalent bonds are weak."

RIGHT: "Water has a relatively low boiling point because the intermolecular forces between water molecules are weak compared to the forces in ionic or giant covalent structures. The covalent bonds within the water molecule (O–H bonds) are strong but are not broken during boiling."

Exam Tip: Always be precise about which forces you are describing. In simple molecular substances: covalent bonds = strong (within the molecule), intermolecular forces = weak (between molecules). Boiling/melting overcomes the intermolecular forces only.

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Worked Example

Question: Bromine (Br₂) is a liquid at room temperature with a boiling point of 59 °C. It does not conduct electricity. Explain these properties in terms of its structure and bonding.

Answer:

• Bromine is a simple molecular substance consisting of Br₂ molecules.
• Within each molecule, the two bromine atoms are held together by a strong covalent bond (a shared pair of electrons).
• Between the Br₂ molecules, there are weak intermolecular forces.
• Only a small amount of energy is needed to overcome these weak intermolecular forces, so the boiling point is low (59 °C).
• Bromine does not conduct electricity because its molecules are neutral — there are no free electrons and no ions to carry charge.

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Key Points

• Simple molecular substances contain small molecules with strong covalent bonds within them.
• The forces between molecules (intermolecular forces) are weak.
• When melting or boiling, only the intermolecular forces are overcome — the covalent bonds are not broken.
• This gives simple molecular substances low melting and boiling points.
• They do not conduct electricity because there are no free electrons or ions.
• Larger molecules have stronger intermolecular forces and therefore higher melting and boiling points.

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Practice Questions

1. Explain why methane (CH₄) is a gas at room temperature.
2. Explain why oxygen (O₂) does not conduct electricity.
3. A student says: "Water boils at 100 °C because the covalent bonds in water are broken." Explain what is wrong with this statement.
4. Explain why pentane (C₅H₁₂) has a higher boiling point than methane (CH₄), even though both are simple molecules.
5. Describe the bonding and structure in a sample of carbon dioxide gas.
6. Iodine is a solid at room temperature. It sublimes (turns directly to gas) at 184 °C. Explain why this temperature is relatively low for a solid.

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Worked Example: The Two-Force Distinction in Ice and Water

Question (6 marks): When ice melts at 0 °C and water boils at 100 °C, which bonds or forces are broken?

Answer: