GCSE · Chemistry · AQA · Spec 8462

Properties of small molecules

The bonds inside each molecule are strong, yet many molecular substances melt or boil surprisingly easily.

What you need to know

  • Small-molecule substances are usually gases or liquids with relatively low melting and boiling points.
  • Forces between molecules are weak compared with the covalent bonds within molecules.
  • Melting and boiling overcome intermolecular forces, not covalent bonds.
  • Larger molecules have stronger intermolecular forces and higher melting and boiling points; molecular substances do not conduct electricity because their molecules have no overall charge.

The big picture

Substances made from small molecules are usually gases or liquids and have relatively low melting and boiling points. The reason is that the forces between their molecules are weak compared with the covalent bonds within each molecule. Melting and boiling overcome these intermolecular forces, not the covalent bonds. Larger molecules have stronger intermolecular forces and therefore higher melting and boiling points, while molecular substances do not conduct electricity because their molecules have no overall electric charge.

TONIGHT'S REVISION

Properties of small molecules

Strong bonds inside each molecule, weak forces between molecules — that distinction explains the properties.

Why are melting and boiling points low?

?

Reason it through

Why do substances made from small molecules usually have relatively low melting and boiling points?

Link 1 of 4

First link · your turn

First, where are the weak forces in a molecular substance?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Inside a molecule vs between molecules

Covalent bondsvsIntermolecular forces

This is the distinction that unlocks the whole topic.

Focus

Where are they?

Covalent bonds

Within each molecule

Intermolecular forces

Between separate molecules

The insight

The two forces act in different places, so do not treat them as the same thing.

Relative strength

Covalent bonds

Strong

Intermolecular forces

Weak by comparison

During melting

Covalent bonds

Not overcome

Intermolecular forces

Overcome

During boiling

Covalent bonds

Not overcome

Intermolecular forces

Overcome

What happens when it melts or boils?

Track which forces are overcome as the substance is heated.

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Intermolecular forcesMelting pointBoiling point
Smaller molecules
Larger molecules

Each cell hides a short answer and the reason behind it. Predict before you tap.

What actually breaks?

Commit to an answer before revealing the explanation.

A small-molecule substance boils. Which statement best describes what has been overcome?

Explain the properties from the structure

Problem

A substance consists of small molecules. Explain why it has a relatively low boiling point and does not conduct electricity.

Key points

1Weak forces act between the molecules.
2Covalent bonds within the molecules are much stronger.
3Melting and boiling overcome intermolecular forces, not covalent bonds.
4Larger molecules have stronger intermolecular forces and higher melting and boiling points.
5Molecules have no overall electric charge, so these substances do not conduct electricity.

Worked example

Problem

Substances A and B both consist of small molecules, but B has larger molecules. Predict which has the higher boiling point and explain why.

🧠

Memory hook

Think inside versus between: strong bonds inside molecules, weak forces between molecules.

⚠ Watch out

Melting or boiling does not break the covalent bonds inside the molecules.

✓

Check yourself

Why can a substance have strong covalent bonds but still have a relatively low boiling point?

Flashcards

(6)
What physical states are substances made from small molecules usually found in?
They are usually gases or liquids and have relatively low melting and boiling points.
What forces act between small molecules?
Weak intermolecular forces act between the molecules.
What is overcome when a molecular substance melts or boils?
The intermolecular forces between molecules are overcome, not the covalent bonds within the molecules.
How does increasing molecule size affect intermolecular forces?
The intermolecular forces become stronger as the molecules become larger.
How does increasing molecule size affect melting and boiling points?
Larger molecules have higher melting and boiling points because their intermolecular forces are stronger.
Why do substances made from small molecules not conduct electricity?
Their molecules do not have an overall electric charge.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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