Simple Molecular Substances

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 1.47 — Why simple molecular substances have low melting and boiling points
• 1.48 — Why melting and boiling points of simple molecular substances rise with relative molecular mass
• 1.51 — Covalent compounds do not usually conduct electricity

Specification section 1(e) Chemical formulae, equations and calculations — also taught in The Mole, Reacting Masses and Percentage Yield, Empirical and Molecular Formulae, Concentration and Gas Volumes.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A SIMPLE MOLECULAR substance consists of small, separate molecules. Water, methane, carbon dioxide, oxygen, iodine and ammonia are all of this kind. Inside each molecule the atoms are held by strong covalent bonds; between one molecule and the next there are also forces, called INTERMOLECULAR FORCES, and these are very much weaker.

Everything on this page follows from that one asymmetry, so it is worth stating in its sharpest form. The bonds INSIDE a molecule are strong. The forces BETWEEN molecules are weak. And melting or boiling a molecular substance separates the MOLECULES from one another — it does not break a single covalent bond.

Think of it like a box of paperclips rather than a chain

Each paperclip is a stiff loop of steel that you would need pliers to pull apart. Tip a boxful onto a table and they scatter with no effort at all, because although each clip is strong, nothing much holds one clip to the next. Melting a molecular substance is tipping out the box: the molecules move apart and every one of them survives intact. That is why methane is a gas at room temperature while the carbon–hydrogen bonds inside it are among the strongest in organic chemistry — the two facts are about completely different things, and confusing them is the commonest error in this part of the specification.

Why the melting point is low — the chain that earns the marks

1. The substance consists of separate small molecules.
2. The forces of attraction BETWEEN the molecules are WEAK.
3. Only these weak intermolecular forces need to be overcome to melt or boil the substance.
4. Little energy is therefore required.
5. So the melting and boiling points are low, and many such substances are liquids or gases at room temperature.
6. The strong COVALENT BONDS inside each molecule are NOT broken — say so explicitly, because it is a marking point.

That last step is not optional politeness. Mark schemes for this question routinely award a mark specifically for stating that the covalent bonds are not broken, because it is the point that distinguishes an answer that understands the structure from one that has memorised 'weak forces, low melting point'.

Statement 1.48 asks about a trend, and it has a satisfying explanation. Compare the halogens: fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid — and their relative molecular masses rise in exactly that order. The same pattern appears in the alkanes, where methane and ethane are gases, petrol-range molecules are liquids, and the waxy long ones are solids.

Why bigger molecules melt and boil higher

1. As the relative molecular mass increases, the molecules are larger and contain more electrons.
2. The intermolecular forces between larger molecules are STRONGER.
3. More energy is therefore needed to separate the molecules from one another.
4. So the melting and boiling points RISE as relative molecular mass increases.
5. Note carefully what has NOT changed: the covalent bonds inside the molecules are no stronger. It is only the forces between molecules that have grown.

Now conductivity, statement 1.51, and the test is the same one used for ionic compounds. To conduct, a substance needs CHARGED PARTICLES FREE TO MOVE. A simple molecular substance has neither: its molecules are neutral overall, because the electrons have been shared rather than transferred, so there are no ions — and there are no free electrons either, because every outer electron is either in a bond or in a lone pair on a particular atom.

No charged particles at all, therefore no conduction, in any state. Melting a molecular substance does not help, which is a useful contrast with ionic compounds: melting an ionic solid frees ions that were already there, whereas melting a molecular solid frees molecules that were never charged in the first place.

Predicting properties for a substance you have never met

Substance X consists of small molecules with a relative molecular mass of 30. Substance Y consists of small molecules with a relative molecular mass of 250. Predict which has the higher boiling point, and state whether either conducts electricity.

1. Both are simple molecular, so both have weak forces between their molecules and low boiling points compared with ionic solids.
2. Y has the greater relative molecular mass, so its molecules are larger and the intermolecular forces between them are stronger.
3. More energy is needed to separate Y's molecules, so Y has the HIGHER boiling point.
4. Neither conducts, in any state: the molecules are neutral, so there are no ions, and all the electrons are held in bonds or lone pairs, so none is free.
5. Notice that nothing about the identity of X or Y was needed — the structure alone settled both questions.

Answer: Y has the higher boiling point; neither conducts in any state.

One honest exception is worth knowing, because a learner will meet it. Some molecular substances DO conduct when dissolved in water — hydrogen chloride is a gas of neutral molecules, and yet hydrochloric acid conducts well. That is because water causes those particular molecules to split into ions, which is a chemical change rather than a property of the molecular structure itself. This is why the specification says covalent compounds do not USUALLY conduct rather than never.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Simple molecular substance

A substance consisting of small separate molecules, with strong covalent bonds inside each molecule and weak intermolecular forces between them.

Learn this definition · Intermolecular forces

The forces of attraction between one molecule and another. They are much weaker than covalent bonds, and they are what is overcome when a molecular substance melts or boils.

The low melting point, as an examiner wants it written

1. The substance consists of separate small molecules.
2. The intermolecular forces BETWEEN the molecules are weak.
3. Only these weak forces need to be overcome to melt or boil it.
4. Little energy is therefore required, so the melting and boiling points are low.
5. State explicitly that the covalent bonds INSIDE the molecules are not broken — this is a marking point.

Statement 1.48 — the trend with relative molecular mass

1. A greater relative molecular mass means larger molecules with more electrons.
2. The intermolecular forces between larger molecules are stronger.
3. More energy is needed to separate them.
4. So the melting and boiling points rise as relative molecular mass increases.
5. The covalent bonds inside the molecules are unchanged — only the forces between them have grown.

Why simple molecular substances do not conduct

• Conduction requires CHARGED PARTICLES FREE TO MOVE
• The molecules are NEUTRAL, because electrons are shared rather than transferred — so there are no ions
• There are no FREE ELECTRONS either: every outer electron is in a bond or a lone pair
• So they do not conduct in ANY state, solid, liquid or gas
• EXCEPTION: some, such as hydrogen chloride, split into ions when dissolved in water — which is a chemical change, and why the specification says 'not usually'

Covalent bonds against intermolecular forces

A. COVALENT BONDS are inside a molecule, are very strong, and are NOT broken when the substance melts or boils.

B. INTERMOLECULAR FORCES are between molecules, are weak, and are the only thing overcome on melting or boiling.

Model answer [4 marks]

Explain why carbon dioxide is a gas at room temperature, even though the bonds within each molecule are strong. [4]

Carbon dioxide is a simple molecular substance, consisting of separate small molecules. The forces of attraction between these molecules are weak, and only these weak intermolecular forces have to be overcome for the substance to melt or boil. Very little energy is therefore required, so its boiling point is below room temperature and it is a gas. The strong covalent bonds within each carbon dioxide molecule are not broken at all when it boils.

Model answer [3 marks]

Explain why iodine is a solid at room temperature while chlorine is a gas. Both are simple molecular. [3]

Iodine molecules have a much greater relative molecular mass than chlorine molecules, so they are larger and contain more electrons. The intermolecular forces between the larger iodine molecules are therefore stronger than those between chlorine molecules. More energy is needed to separate iodine's molecules from one another, so iodine has a higher melting and boiling point and is a solid at room temperature.

Not this: Water boils at 100 °C, so the bonds holding a water molecule together must be weak.

This: Boiling separates whole MOLECULES from one another and breaks no covalent bonds at all — steam is still made of intact H₂O molecules. The bonds inside a water molecule need well over two thousand degrees to break.

Mark-losing trap. Melting a molecular substance breaks NO covalent bonds. Say so — it is a marking point.

Mark-losing trap. Larger molecules have stronger forces BETWEEN them; the bonds inside are no stronger.

Mark-losing trap. No ions and no free electrons means no conduction in ANY state, including molten.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is overcome when a simple molecular substance such as carbon dioxide boils?
2. Grade 7 · Explain [3 marks] — Explain why simple molecular substances do not conduct electricity.
3. Grade 8 · Explain [3 marks] — Bromine is a liquid at room temperature and iodine is a solid. Both are simple molecular. Explain the difference.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why methane has a very low boiling point and does not conduct electricity in any state.
5. 9+ · Analyse [6 marks] — A student notes that hydrogen chloride is a simple molecular gas of neutral molecules, but that hydrochloric acid conducts electricity well. They conclude that simple molecular substances can conduct after all, so the rule is wrong. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who established that a gas is mostly empty space, and the one who worked out that a mixture of gases behaves as separate crowds. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Robert Boyle — the one who established that a gas is mostly empty space

This page rests on molecules being separate and far apart in a gas, and Boyle is the person who first established that experimentally. Trapping air in the short arm of a J-shaped tube and pouring in mercury, he found the volume falls as the pressure rises — which is only possible if a gas is mostly empty space with something small distributed through it. His air-pump work made the same point from the other end: a candle goes out, a bell falls silent, and yet the vessel is not truly nothing. He is also the right person for the caution on this page, since he insisted on publishing procedures in full so that others could check them.

• “What happens to a gas when you squeeze it, and what does that tell you?”
• “How can something be compressed if it is solid all the way through?”
• “What did the air pump show you about what air is?”
• “Why did you publish the experiments that failed?”
• “How far did you think matter could be divided?”

John Dalton — the one who worked out that a mixture of gases behaves as separate crowds

Dalton's route to the atomic theory ran through exactly the idea this page needs — that the particles of a gas are separate and largely independent. He found that each gas in a mixture exerts its own pressure as though the others were not there, which makes no sense for a continuous fluid and perfect sense for a crowd of separate particles that barely interact. That independence is the same fact as the weak intermolecular forces on this page, met from the other direction, and it is why he concluded that different gases must be made of particles of differing weights.

• “Why does each gas in a mixture exert its own pressure?”
• “What did that tell you about how gas particles interact?”
• “How did you get from the weather to atoms?”
• “Do the particles of a gas attract each other at all?”
• “Why must different gases have particles of different weights?”

Carry on through the course

• Previous lesson: Covalent Bonding
• Next lesson: Giant Covalent Structures
• The Three States of Matter — What the particles are doing in a solid, a liquid and a gas — and what really changes when ice melts.
• Pure Substances, Mixtures and How to Separate Them — Why a pure substance melts at one temperature, and which separation technique to reach for when.
• Inside the Atom — Protons, neutrons and electrons — where they are, what they weigh, what they carry, and how isotopes give an element a decimal mass.
• Electronic Configurations and the Periodic Table — Why the table has the shape it has: the group number IS the outer-shell electron count, and everything follows from that.
• Solubility and Solubility Curves — Why a solution can be full, why hot water holds more, and how one curve tells you exactly how many crystals you will get back.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification