Small Molecules and Polymers

This lesson covers the properties of small molecules (simple molecular substances) and introduces polymers, as required by the AQA GCSE Chemistry specification (4.2.2). You need to understand why small molecules have low melting and boiling points, why they do not conduct electricity, and how polymers differ in their properties due to their giant molecular structure.

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Simple Molecular Substances

Simple molecular substances are made up of small molecules containing a fixed number of atoms held together by strong covalent bonds. However, between the molecules there are only weak intermolecular forces (forces of attraction between molecules). It is these intermolecular forces — not the covalent bonds — that determine the physical properties of the substance.

Key Distinction: Bonds vs Forces

	Covalent Bonds	Intermolecular Forces
Location	Within the molecule (between atoms)	Between molecules
Strength	Strong	Weak
What happens when substance melts/boils?	Bonds do NOT break	Forces ARE overcome
Energy required	Large	Small

This distinction is critical for the exam. When a simple molecular substance melts or boils, the weak intermolecular forces between molecules are overcome, but the strong covalent bonds within the molecules are not broken. The molecules themselves remain intact.

Exam Tip: This is one of the most commonly misunderstood topics in GCSE Chemistry. When explaining the low boiling point of water, do NOT say "the covalent bonds break." The covalent bonds within H2O molecules do not break — it is the weak intermolecular forces between H2O molecules that are overcome. You MUST make this distinction for full marks.

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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 only the weak intermolecular forces between molecules need to be overcome. A small amount of energy is needed to separate the molecules from each other.

As the size of the molecule increases, the intermolecular forces become stronger, so the melting and boiling points increase.

Substance	Formula	Melting Point (degrees C)	Boiling Point (degrees C)	State at Room Temp
Hydrogen	H2	-259	-253	Gas
Oxygen	O2	-218	-183	Gas
Water	H2O	0	100	Liquid
Chlorine	Cl2	-101	-34	Gas
Methane	CH4	-182	-164	Gas
Bromine	Br2	-7	59	Liquid
Iodine	I2	114	184	Solid

Exam Tip: Notice how iodine (I2) is a solid at room temperature even though it is a simple molecular substance. This is because iodine molecules are large, so the intermolecular forces between them are stronger than for smaller molecules. Larger molecules = stronger intermolecular forces = higher melting/boiling points.

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2. Do Not Conduct Electricity

Simple molecular substances do not conduct electricity in any state (solid, liquid, or gas). This is because they have no free charged particles — no ions or delocalised electrons. The molecules are neutral and do not carry charge.

3. Solubility

Some simple molecular substances dissolve in water (e.g. sugar, ethanol), but many are insoluble. Simple molecular substances often dissolve in non-polar solvents (e.g. hexane). Whether a substance dissolves depends on the interactions between the solute and solvent molecules.

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Intermolecular Forces in More Detail

There are different types of intermolecular forces, but for GCSE you need to understand the general concept that they are weak compared to covalent bonds.

graph TD
    A["Forces in Simple Molecular Substances"] --> B["Covalent Bonds<br/>(within molecules)"]
    A --> C["Intermolecular Forces<br/>(between molecules)"]
    B --> D["STRONG<br/>Hold atoms together<br/>within a molecule"]
    C --> E["WEAK<br/>Hold molecules<br/>together in a substance"]
    D --> F["Do NOT break when<br/>substance melts or boils"]
    E --> G["ARE overcome when<br/>substance melts or boils"]

    style B fill:#e74c3c,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#3498db,color:#fff

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Explaining Boiling Points — A Worked Example

Question: Explain why water (H2O) has a higher boiling point than methane (CH4).

Answer:

1. Both water and methane are simple molecular substances with strong covalent bonds within the molecules.
2. When these substances boil, the intermolecular forces between the molecules are overcome — the covalent bonds do not break.
3. Water has stronger intermolecular forces between its molecules than methane.
4. Therefore, more energy is needed to overcome the intermolecular forces in water, so water has a higher boiling point.

Exam Tip: When comparing boiling points of simple molecular substances, always compare the strength of the intermolecular forces, NOT the covalent bonds. The covalent bonds are irrelevant because they do not break during boiling.

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Polymers

Polymers are very large molecules made up of many thousands of small repeating units called monomers joined together by covalent bonds. Examples include poly(ethene), poly(propene), nylon, and DNA.

Structure of Polymers

• Polymers are giant molecules but they are NOT giant covalent structures (like diamond). They are long chain molecules.
• The atoms within the polymer chain are held together by strong covalent bonds.
• Between polymer chains there are intermolecular forces.
• Because polymer chains are very long, the intermolecular forces between them are relatively strong (compared to small molecules).

Properties of Polymers

Property	Explanation
Solid at room temperature	The intermolecular forces between long chains are strong enough to hold the chains together.
Higher melting points than small molecules	Longer chains = more intermolecular forces = more energy needed to separate them.
Do not conduct electricity	No free charged particles (no ions or delocalised electrons).
Generally insoluble in water	The intermolecular forces between chains are usually too strong for water to overcome.
Can be flexible or rigid	Depends on the type of polymer and how the chains are arranged.

Polymer Properties Depend on Structure

The properties of a polymer depend on: