Molecular and ionic substances
Dissolve two white solids in water — copper sulphate and glucose — and only one solution will light a bulb wired between two electrodes. The difference has nothing to do with how well each dissolves and everything to do with what kind of particles the solid was built from in the first place. Copper sulphate is packed with charged ions from the start; glucose is packed with neutral molecules. That single structural fact, rooted in the type of chemical bonding, decides whether a substance can carry current, whether it decomposes at an electrode, and whether chemists call it an electrolyte at all. This article sorts substances into ionic (electrovalent) and molecular (covalent), links each class to conduction behaviour in the solid, molten and dissolved states, and flags the important exception — covalent compounds like HCl that only turn into electrolytes once water gets involved.
A substance conducts electricity and undergoes electrolysis only if it contains free-moving ions, so ionic (electrovalent) compounds — built from charged particles in a lattice — become electrolytes when molten or dissolved, while molecular (covalent) compounds — built from neutral shared-electron molecules — stay non-conductors unless they specifically react with water to generate ions.
The core idea
Two ways to build a compound, two very different outcomes for conduction. Ionic (electrovalent) substances form when a metal atom transfers electrons to a non-metal atom, leaving behind a cation and an anion. These oppositely charged ions do not float apart — they lock into a rigid, repeating three-dimensional crystal lattice, held together by strong electrostatic attraction between the positive and negative charges. Sodium chloride (Na⁺Cl⁻), copper sulphate (Cu²⁺SO₄²⁻) and lead bromide (Pb²⁺(Br⁻)₂) are all built this way. The ions genuinely are charged all the time — even in the solid — but in the solid lattice they are pinned in fixed positions by the surrounding electrostatic forces on every side, so nothing can drift toward an electrode. Molecular (covalent) substances form when non-metal atoms share pairs of electrons rather than transferring them outright. The result is a discrete, electrically neutral molecule — sugar (C₁₂H₂₂O₁₁), glucose (C₆H₁₂O₆), alcohol and most organic compounds fall into this class. There is no separated positive and negative charge anywhere in the structure to begin with, so there is nothing for an electric field to pull toward an electrode, whether the substance is solid, liquid, or dispersed in water.
What has to be true for a substance to conduct. Conduction by a chemical substance (as opposed to a metal, which conducts by free electrons) always needs mobile, charged particles — ions that are free to travel through the material toward the oppositely charged electrode. Ionic solids fail this test because their ions, though charged, are locked in place. Supply enough heat to melt the solid, or enough solvent (water) to break the lattice apart, and the ions become mobile: melting and dissolving are simply two different ways of breaking the same electrostatic lattice. Once mobile, the ions migrate under an applied potential difference and get discharged at the electrodes — this directed ion movement and electrode discharge is exactly what electrolysis is. Molecular substances have no such lattice to break in the first place, so melting or dissolving a purely covalent compound just spreads neutral molecules further apart — it does not create ions from nothing. Sugar dissolved in water is still sugar molecules, dispersed but uncharged, so the solution stays a non-conductor. The one place this simple split needs a footnote is reactive covalent molecules. A small number of covalent compounds — hydrogen chloride and acetic acid are the classic ICSE examples — are molecular in their pure form but chemically react with water on dissolving, and that reaction itself manufactures ions:
where: HCl is covalent hydrogen chloride gas, H₂O is the solvent it reacts with, and H₃O⁺ / Cl⁻ are the ions the reaction produces. Dry HCl gas has no ions and does not conduct or turn dry litmus paper red; the moment it dissolves in water, the reaction above generates free ions, and the resulting hydrochloric acid is a strong electrolyte. The ionisation happens because of the reaction with water, not because dissolving automatically frees pre-existing ions the way it does for an ionic solid.
Why it works
This behaviour is purely a consequence of bonding type, so the reasoning is qualitative rather than a numeric formula — but the logic follows a fixed chain every time you apply it.
Step 1. Identify the bonding. A compound of a metal with a non-metal (or a salt, acid, or base built from a metal-non-metal combination) is almost always ionic — electrons have transferred, so charged ions already exist. A compound built entirely from non-metals sharing electrons (organic compounds, most simple gases, sugars) is molecular — no charge separation exists at the particle level.
Step 2. Ask whether the charged particles, if any, can move. In an ionic solid the ions exist but are trapped in the lattice by strong electrostatic forces acting from every direction, so despite being charged they cannot travel to an electrode — solid ionic compounds do not conduct.
Step 3. Ask what breaks the lattice. Supplying heat (melting) or a polar solvent like water (dissolving) overcomes the lattice's electrostatic attraction and releases the ions to move freely. Either route works for an ionic compound, because the ions were always there — melting and dissolving just free them.
Step 4. For a molecular compound, repeat the same question and get a different answer. There is no lattice of ions to break, because there were never any ions — melting or dissolving a molecular solid just separates or disperses neutral molecules, which still carry no charge and still cannot conduct.
Step 5. Check for the water-reaction exception. Before concluding "molecular, therefore never conducts," check whether the molecular compound chemically reacts with water to generate ions (as HCl and acetic acid do). If it does, the compound becomes an electrolyte only in aqueous solution; if it does not (as with glucose or sugar), it remains a non-electrolyte in every state.
Worked example
Question: A student is given three unlabelled white solids — solid sodium chloride, solid glucose, and solid lead bromide — plus a battery, wires, an ammeter, and a Bunsen burner. Describe, with reasons, what the student would observe on (a) testing each solid as-is with the electrodes, and (b) melting lead bromide and testing the melt.
Step 1. Test each solid as-is. Sodium chloride and lead bromide are ionic compounds (metal-non-metal combinations); glucose is molecular. In all three solids, whatever charged or neutral particles exist are held rigidly in place — for the two ionic solids, the Na⁺/Cl⁻ and Pb²⁺/Br⁻ ions are locked in their lattices; glucose has no ions at all. So the ammeter reads zero current for all three solids: no ionic solid conducts, and no molecular solid conducts either, for different underlying reasons.
Step 2. Melt the lead bromide. Heating supplies enough energy to overcome the strong electrostatic forces holding Pb²⁺ and Br⁻ ions in the crystal lattice. The lattice collapses into a liquid in which the ions are now free to move.
Step 3. Test the molten lead bromide. With electrodes dipped into the melt and a potential difference applied, the mobile Pb²⁺ ions migrate toward the cathode and Br⁻ ions migrate toward the anode. The ammeter now registers a current, and at the electrodes the ions are discharged — Pb²⁺ gains electrons to deposit as molten lead metal at the cathode, and Br⁻ loses electrons to form bromine at the anode. This is electrolysis: conduction accompanied by chemical decomposition.
Answer: all three solids give zero current when solid; molten lead bromide conducts and is decomposed by electrolysis, because melting frees its pre-existing Pb²⁺ and Br⁻ ions, while glucose — molecular, with no ions to free — would give no current even if melted.
Common traps
- Assuming "dissolves in water" automatically means "will conduct" — solubility and ionic character are different properties; plenty of molecular substances dissolve completely without releasing a single ion.
- Forgetting that reactive covalent gases like HCl need water to ionise — the dry gas itself does not conduct or affect dry litmus.
- Believing ions are "created" only during electrolysis — for a true electrolyte the ions already exist in the molten or dissolved state before any current flows; applying a potential difference just makes the existing ions move in a directed way and get discharged at the electrodes.
- Confusing "ionic bond" (electron transfer, giving separately charged ions) with "covalent bond" (electron sharing, giving one neutral molecule) when explaining why a substance does or does not conduct.
Ask the tutor: Ask the AI tutor to walk through why dry HCl gas fails the litmus test but hydrochloric acid turns it red, or to quiz you on classifying more compounds as ionic versus molecular.
Why does melting an ionic solid let it conduct, if the ions were already charged in the solid?
The ions in a solid ionic lattice are genuinely charged the whole time, but they are held in fixed positions by strong electrostatic attraction from every neighbouring ion, so none of them can travel toward an electrode. Melting supplies enough thermal energy to overcome those forces and break the rigid lattice apart. Once the structure collapses into a liquid, the same Na⁺ and Cl⁻ ions (for example) are still there, but now free to drift, so they can migrate to the electrodes and carry current. Nothing new is created by melting — mobility is unlocked, not charge.
Why doesn't dissolving glucose in water make it conduct, if dissolving copper sulphate does?
It comes down to what each solid is made of before it ever touches water. Copper sulphate is ionic: it is already built from Cu²⁺ and SO₄²⁻ ions locked in a lattice, and water molecules simply pull those existing ions apart and free them into solution. Glucose is molecular: its solid is built from neutral C₆H₁₂O₆ molecules with no separated charges anywhere inside them. Dissolving glucose just spreads those same neutral molecules through the water — there is no lattice of ions to release, because there never were any ions to begin with. No ions means no charge carriers means no conduction.
Are all acids ionic compounds, since their solutions conduct electricity?
No — this is exactly the HCl-type exception. Many acids, including hydrochloric and acetic acid, are covalent (molecular) compounds in their pure form; there are no ions in pure liquid or gaseous HCl. What makes their solutions conduct is a genuine chemical reaction with water on dissolving, which manufactures H₃O⁺ and the corresponding anion. So the acid is molecular by bonding type but becomes an electrolyte in water because of a reaction, not because it was secretly ionic all along — a distinction worth stating explicitly if a question asks you to classify the bonding.