Properties of Simple Molecular Substances

This lesson covers the properties of simple molecular substances as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand why substances like water, oxygen and methane have low melting and boiling points and why they do not conduct electricity. The key is the distinction between strong covalent bonds within molecules and weak forces between molecules.

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What Are Simple Molecular Substances?

A simple molecular substance (also called a simple molecule) is made up of a small number of atoms held together by strong covalent bonds.

Examples include:

Substance	Formula	Atoms per Molecule
Hydrogen	H₂	2
Water	H₂O	3
Oxygen	O₂	2
Carbon dioxide	CO₂	3
Methane	CH₄	5
Ammonia	NH₃	4

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Two Types of Force in Simple Molecular Substances

To explain the properties, you must distinguish between two types of force:

Force	Location	Strength
Covalent bonds	Within each molecule (intramolecular)	Strong — a lot of energy needed to break them
Intermolecular forces	Between neighbouring molecules	Weak — relatively little energy needed to overcome them

graph TD
    A["Simple Molecular<br/>Substance"] --> B["Strong covalent bonds<br/>WITHIN molecules<br/>(intramolecular)"]
    A --> C["Weak intermolecular<br/>forces BETWEEN<br/>molecules"]
    B --> D["NOT broken during<br/>melting or boiling"]
    C --> E["Overcome during<br/>melting and boiling"]

    style A fill:#2c3e50,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#1a1a2e,color:#fff
    style E fill:#1a1a2e,color:#fff

Exam Tip: A very common mistake is to say that covalent bonds are broken when a simple molecular substance melts or boils. They are NOT. It is the weak intermolecular forces between molecules that are overcome. The covalent bonds within molecules stay intact.

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Properties of Simple Molecular Substances

1. Low Melting and Boiling Points

Simple molecular substances have low melting and boiling points.

Explanation:

• When a simple molecular substance melts or boils, it is the weak intermolecular forces between molecules that are overcome — not the covalent bonds.
• Because the intermolecular forces are weak, only a small amount of energy is needed to overcome them.
• This is why many simple molecular substances are gases or liquids at room temperature.

Examples:

Substance	State at Room Temp	Melting Point (°C)	Boiling Point (°C)
Oxygen (O₂)	Gas	−219	−183
Water (H₂O)	Liquid	0	100
Methane (CH₄)	Gas	−182	−162
Ethanol (C₂H₅OH)	Liquid	−114	78

2. Larger Molecules Have Higher Boiling Points

As molecules get larger, the intermolecular forces between them become stronger. This means more energy is needed to overcome them, so the boiling point is higher.

Molecule	Relative Size	Boiling Point (°C)
H₂	Very small	−253
CH₄	Small	−162
C₄H₁₀ (butane)	Medium	−1
C₈H₁₈ (octane)	Larger	126

Exam Tip: When explaining trends in boiling points of simple molecules, refer to the strength of the intermolecular forces — not the covalent bonds. Larger molecules have stronger intermolecular forces.

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3. Do Not Conduct Electricity

Simple molecular substances do not conduct electricity in any state — solid, liquid or gas.

Explanation:

• There are no free electrons or ions to carry an electrical charge.
• The molecules are neutral overall.
• Even when melted, the substance consists of neutral molecules — not charged particles.

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4. Many Are Insoluble in Water (but Some Are Soluble)

• Many simple molecular substances (e.g. oils, fats, wax) are insoluble in water.
• However, some simple molecular substances (e.g. sugar, ethanol, hydrogen chloride) do dissolve in water because they can interact with polar water molecules.

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Comparing Bond Strength and Intermolecular Forces

It is essential to keep the two separate in exam answers:

Feature	Covalent Bonds	Intermolecular Forces
Where	Within molecules	Between molecules
Strength	Strong	Weak
Broken when?	Only in chemical reactions	When a substance changes state
Energy required	A lot	A little

Exam Tip: If a question asks you to explain a property of a simple molecular substance, make it clear whether you are talking about covalent bonds (within molecules) or intermolecular forces (between molecules). Mixing these up is the most common error.

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Summary

• Simple molecular substances contain small molecules held together by strong covalent bonds within each molecule.
• Between molecules there are only weak intermolecular forces.
• They have low melting and boiling points because only weak intermolecular forces need to be overcome during changes of state.
• They do not conduct electricity because there are no free electrons or ions.
• Larger molecules have stronger intermolecular forces and therefore higher boiling points.

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Worked Example — Why Does Water Boil at 100 °C but Oxygen at −183 °C?

Both water and oxygen are simple molecular substances with strong covalent bonds inside each molecule. Yet water boils at 100 °C while oxygen boils at −183 °C. Why the huge difference?

The answer is that boiling involves overcoming the intermolecular forces between molecules — not the covalent bonds inside them. Water molecules experience particularly strong intermolecular forces (hydrogen bonds, which are a special type of intermolecular attraction). Oxygen molecules only have weaker induced-dipole forces. So although both have strong internal covalent bonds, the forces between molecules are very different — and these are what set the boiling point.

Exam Tip: For GCSE Combined Science you are not required to name hydrogen bonds, but you must be clear that melting and boiling points depend on intermolecular forces, not covalent bonds.

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Worked Example — Explaining Trends in Alkane Boiling Points