Why Substances Conduct — the Four Structures Compared

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.

• 1.55 — Why covalent compounds do not conduct electricity (bold C statement — Paper 2 only)
• 1.56 — Why ionic compounds conduct only when molten or in solution (bold C statement — Paper 2 only)
• 1.57 — The terms anion and cation (bold C statement — Paper 2 only)

Specification section 1(g) Covalent bonding — also taught in Electrolysis.

1 · Understand it

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

Four structures have now been met — ionic, simple molecular, giant covalent and metallic — and each has been asked separately whether it conducts. This page puts them side by side, because the answers are not four facts to memorise. They are one question asked four times.

THE QUESTION IS ALWAYS THE SAME: does the substance contain CHARGED PARTICLES, and are they FREE TO MOVE? Both are required. A substance with charged particles that cannot move does not conduct, and neither does one whose particles can move freely but carry no charge.

Think of it like asking whether a message can get through

For a message to travel you need someone carrying it and a road they can use. A messenger locked in a room delivers nothing; an open road with nobody on it delivers nothing either. Both conditions, every time. That is why the four structures give four different answers to one question: a metal has messengers and roads at all times; a molecular substance has roads and no messengers, in any state; and an ionic solid has messengers locked in rooms until you melt it and open the doors. The failure is different in each case, and naming WHICH condition fails is what the marks are for.

The four structures, one test

1. METALLIC — delocalised electrons are charged AND free to move, in the solid and when molten. CONDUCTS ALWAYS.
2. IONIC — ions are charged but fixed in the lattice when solid, so nothing moves. Melting or dissolving frees them. CONDUCTS ONLY WHEN MOLTEN OR DISSOLVED.
3. SIMPLE MOLECULAR — molecules are neutral, and every electron is in a bond or a lone pair, so there are no charged particles at all. DOES NOT CONDUCT as a pure substance (a few, such as hydrogen chloride, react with water to form ions, and that solution does).
4. GIANT COVALENT — atoms are neutral and all electrons are used in bonding, so again no charged particles. DOES NOT CONDUCT — except graphite.
5. GRAPHITE is the exception, because each carbon uses only three of its four electrons in bonding and the fourth is delocalised. CONDUCTS.

It is worth noticing the two very different reasons for a 'no'. A simple molecular substance fails because there is nothing charged — that failure is permanent and nothing you do to the substance will change it. A solid ionic compound fails because its charges cannot move — that failure is temporary and melting fixes it. An answer that says only 'it does not conduct' has not distinguished them, and an answer that says WHICH condition fails always has.

That comparison also gives the most useful identification test in the topic. Take an unknown solid with a high melting point: it must be ionic, giant covalent or metallic, since a molecular substance could never melt high. Now test it as a solid and again when molten. Conducts in both — metallic. Conducts only when molten — ionic. Conducts in neither — giant covalent.

Finally, statement 1.57 gives the two words the next lesson needs. A CATION is a POSITIVE ion and an ANION is a NEGATIVE ion. The names come from where each one goes during electrolysis: a cation travels to the CATHODE, which is the negative electrode, and an anion travels to the ANODE, which is the positive electrode. Both words were coined by Faraday, from Greek roots meaning to go down and to go up.

Keeping the four words straight

1. CATION — positive ion. It is attracted to the negative electrode, because opposite charges attract.
2. CATHODE — the NEGATIVE electrode. Cations go there.
3. ANION — negative ion. It is attracted to the positive electrode.
4. ANODE — the POSITIVE electrode. Anions go there.
5. The memory hook worth having: a caT-ion is posiT-ive, and the ions go to the electrode of the OPPOSITE name — cations to the cathode, anions to the anode.

Identifying an unknown substance from conduction alone

Three white solids all melt above 700 °C. Solid A conducts when solid and when molten. Solid B conducts only when molten. Solid C conducts in neither state. Identify the structure of each.

1. All three melt high, so none is simple molecular — weak intermolecular forces could not require that much energy.
2. A conducts as a solid, which requires charged particles able to move within a rigid lattice. Only delocalised electrons can do that, so A is METALLIC.
3. B conducts only when molten, which means it has charged particles that were locked in place and are freed by melting. Those are ions, so B is IONIC.
4. C conducts in neither state, so it has no charged particles at all. It is GIANT COVALENT.
5. The one trap: if C had been graphite it would have conducted, so a giant covalent substance that conducts is not a contradiction — it is graphite.

Answer: A metallic, B ionic, C giant covalent — decided entirely by which state it conducts in.

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.

The four structures against one test

Statement 1.57 — the four electrolysis words

Cation

A POSITIVE ion. Attracted to the cathode.

Anion

A NEGATIVE ion. Attracted to the anode.

Cathode

The NEGATIVE electrode. Cations go there.

Anode

The POSITIVE electrode. Anions go there.

Identifying a structure from conductivity

1. Check the melting point. Low means simple molecular, and you are finished.
2. If it is high, test the solid and then test it molten.
3. Conducts in BOTH states → METALLIC.
4. Conducts only when MOLTEN → IONIC.
5. Conducts in NEITHER → GIANT COVALENT, unless it is graphite.

The two ways a substance can fail to conduct

A. NO CHARGED PARTICLES — molecular and giant covalent. The failure is permanent, and melting changes nothing at all.

B. CHARGES THAT CANNOT MOVE — solid ionic. The failure is temporary, and melting or dissolving fixes it completely.

The two different ways a substance can fail to conduct

• NO CHARGED PARTICLES — simple molecular and giant covalent substances. The failure is permanent; melting changes nothing
• CHARGED PARTICLES THAT CANNOT MOVE — solid ionic compounds. The failure is temporary; melting or dissolving fixes it
• Say WHICH condition fails — an answer that just says 'it does not conduct' has not explained anything
• GRAPHITE is the exception among covalent substances, with one delocalised electron per carbon

Model answer [3 marks]

Explain why a covalent compound such as ethanol does not conduct electricity in any state. [3]

Conduction requires charged particles that are free to move. In a covalent compound the electrons are shared between atoms rather than transferred, so the molecules are neutral overall and there are no ions. Every outer electron is held either in a covalent bond or as a lone pair on a particular atom, so there are no delocalised electrons either. With no charged particles present at all, the compound cannot conduct in the solid, liquid or gaseous state.

Model answer [4 marks]

An unknown white solid melts at 801 °C. It does not conduct when solid but conducts well when molten. Identify its structure and justify your answer. [4]

The structure is giant ionic. The high melting point shows it cannot be simple molecular, because only weak intermolecular forces would need to be overcome in that case. The fact that it conducts when molten shows that it contains charged particles, which must be ions, since a giant covalent substance has none. The fact that it does not conduct when solid shows that those ions are held in fixed positions in the lattice and are only free to move once the solid has melted, which is exactly the behaviour of a giant ionic compound.

Model answer [3 marks]

Explain why a metal conducts electricity in the solid state but a solid ionic compound does not. [3]

In a metal the delocalised electrons are charged particles that are free to move throughout the structure, even though the positive ions are held in fixed positions, so a current can flow through the solid. In a solid ionic compound the only charged particles are the ions themselves, and these are held in fixed positions within the giant lattice, so nothing is able to move and no current flows.

Not this: Substances that do not conduct all fail for the same reason.

This: There are two entirely different failures. A molecular substance has NO CHARGED PARTICLES, and no treatment will change that. A solid ionic compound has plenty of charged particles that simply CANNOT MOVE, and melting it fixes the problem completely.

Mark-losing trap. CATion is POSitive and goes to the cathode. Anion is negative and goes to the anode.

Mark-losing trap. The cathode is the NEGATIVE electrode. This is the reverse of what the name suggests to most people.

Mark-losing trap. Name WHICH condition fails — no charged particles, or charged particles that cannot move.

Mark-losing trap. GRAPHITE conducts and is covalent. It is the one exception you must be able to state.

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 a cation?
2. Grade 7 · Explain [3 marks] — Explain why a simple molecular substance cannot be made to conduct electricity by melting it.
3. Grade 8 · Deduce [3 marks] — A solid melts at 1400 °C, conducts electricity when solid, and still conducts when molten. What type of structure does it have?
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why a solid ionic compound does not conduct while a covalent compound does not conduct either, and why these are different failures.
5. 9+ · Analyse [6 marks] — A student is given four unlabelled solids and told one is each of the four structure types. They propose identifying all four using melting point alone. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who invented the words this whole topic uses, and the one who showed that classifying well is itself a discovery. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Michael Faraday — the one who invented the words this whole topic uses

Anion, cation, anode, cathode, electrode, electrolyte and electrolysis are all Faraday's coinages, devised with the philosopher William Whewell in the 1830s because the existing vocabulary carried assumptions he thought were wrong. The older terms implied that the current pulled compounds apart by attraction from a distance; Faraday wanted words that described only what was observed — that something travels to one place or the other. Ion means simply 'that which goes'. He is the right person to ask how much difference a word makes to what you can think, and why he was so careful to choose ones that assumed nothing.

• “Why did you need to invent new words for this?”
• “What was wrong with the terms people used before?”
• “Why does 'ion' mean 'that which goes'?”
• “How did you decide which electrode was which?”
• “Does naming something change how you think about it?”

Dmitri Mendeleev — the one who showed that classifying well is itself a discovery

This page is a classification: four structures, one test, and every property falling out of where a substance sits. Mendeleev is the great exemplar of that method — he did not discover a new element, he arranged the known ones so well that the arrangement itself began making predictions, leaving gaps for elements not yet found and stating their properties in advance. He is the right figure to ask why a good classification is more than tidiness, and what it feels like to trust a scheme enough to bet on something nobody has seen.

• “Why is a good classification worth more than a list?”
• “How did your table start predicting rather than just recording?”
• “What made you leave gaps rather than close them up?”
• “How do you know when a classification is the right one?”
• “Were you ever tempted to force an element into the wrong place?”

Carry on through the course

• Previous lesson: Metallic Bonding
• Next lesson: Electrolysis
• 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