Simple Molecular Substances

This lesson covers the properties of simple molecular substances as required by the AQA GCSE Combined Science Trilogy specification (8464), section 4.2.2. You need to understand why these substances have low melting and boiling points, why they do not conduct electricity, and distinguish between intermolecular forces and covalent bonds.

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What Are Simple Molecular Substances?

Simple molecular substances are made up of small molecules in which atoms are joined by strong covalent bonds. However, between the molecules there are only weak intermolecular forces (also called van der Waals forces or weak intermolecular forces of attraction).

Examples of simple molecular substances include:

Substance	Formula	State at Room Temperature
Hydrogen	H₂	Gas
Oxygen	O₂	Gas
Nitrogen	N₂	Gas
Water	H₂O	Liquid
Methane	CH₄	Gas
Carbon dioxide	CO₂	Gas
Ethanol	C₂H₅OH	Liquid
Iodine	I₂	Solid

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Two Types of Force

It is essential to distinguish between the two types of force in simple molecular substances:

1. Covalent Bonds (Intramolecular Forces)

• These are the strong bonds within each molecule.
• They hold the atoms together in the molecule.
• A large amount of energy is needed to break them.
• Covalent bonds are not broken when a simple molecular substance melts or boils.

2. Intermolecular Forces

• These are the weak forces of attraction between molecules.
• They are much weaker than covalent bonds.
• Only a small amount of energy is needed to overcome them.
• It is these forces that are overcome when a simple molecular substance melts or boils.

graph TD
    A["Simple Molecular<br/>Substance"] --> B["WITHIN molecules:<br/>Strong covalent bonds"]
    A --> C["BETWEEN molecules:<br/>Weak intermolecular forces"]
    B --> D["NOT broken when<br/>melting or boiling"]
    C --> E["OVERCOME when<br/>melting or boiling"]

    style A fill:#2c3e50,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#e74c3c,color:#fff

Exam Tip (AQA 8464): A very common exam question asks you to explain the low boiling point of a simple molecular substance. The key phrase is: "The weak intermolecular forces between molecules require only a small amount of energy to overcome." Do NOT say "the covalent bonds are weak" — the covalent bonds are strong; it is the intermolecular forces that are weak.

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Properties of Simple Molecular Substances

1. Low Melting and Boiling Points

Simple molecular substances have low melting and boiling points because:

• The intermolecular forces between molecules are weak.
• Only a small amount of energy is needed to overcome these weak forces.
• The strong covalent bonds within the molecules are not broken.

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

2. Effect of Molecular Size on Boiling Point

As molecules get larger, the intermolecular forces between them become stronger. This means larger molecules have higher boiling points.

Substance	Formula	Relative Molecular Mass	Boiling Point (°C)
Methane	CH₄	16	−162
Ethane	C₂H₆	30	−89
Propane	C₃H₈	44	−42
Butane	C₄H₁₀	58	−1
Pentane	C₅H₁₂	72	36
Octane	C₈H₁₈	114	126

Exam Tip: When comparing boiling points of simple molecular substances, always link the molecule size to the strength of intermolecular forces. Say "larger molecules have stronger intermolecular forces, so more energy is needed to overcome them."

3. Do Not Conduct Electricity

Simple molecular substances do not conduct electricity because:

• They have no free electrons — all electrons are involved in covalent bonds or exist as lone pairs.
• They have no ions that are free to move and carry a charge.
• The molecules are neutral overall.

This applies in all states — solid, liquid and gas.

4. Solubility

• Many simple molecular substances are insoluble in water (e.g. methane, oils).
• Some are soluble in water (e.g. ethanol, sugar) due to interactions between the molecules and water.
• Simple molecular substances tend to be more soluble in non-polar solvents.

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Comparing Intramolecular and Intermolecular Forces

Feature	Intramolecular (Covalent Bonds)	Intermolecular Forces
Location	Within molecules	Between molecules
Strength	Strong	Weak
Energy to break	Large	Small
Broken when melting/boiling?	No	Yes
Example	O–H bond in water	Forces between H₂O molecules

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Worked Example: Why is water a liquid at room temperature?

Water (H₂O) has a relatively low molecular mass but is a liquid at room temperature. This is because water molecules have relatively strong intermolecular forces compared to other molecules of similar size. However, water still has a much lower boiling point (100°C) than ionic compounds like sodium chloride (801°C), because the intermolecular forces between water molecules are still much weaker than the electrostatic forces between ions.

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Structure-Property Map: Simple Molecular Substances

graph LR
    A["Structure:<br/>Small discrete<br/>molecules"] --> B["Weak intermolecular<br/>forces between<br/>molecules"]
    B --> C["Low melting and<br/>boiling points"]
    B --> D["Often gases or<br/>liquids at RT"]
    A --> E["No free electrons<br/>or mobile ions"]
    E --> F["Do not conduct<br/>electricity"]
    A --> G["Larger molecules =<br/>stronger intermolecular<br/>forces"]
    G --> H["Higher boiling<br/>points"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#3498db,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#3498db,color:#fff
    style H fill:#27ae60,color:#fff

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Common Mistakes