Understanding Electrolytes and Ion Movement Revision Notes | GCSE AQA Higher Chemistry

1. Why Some Liquids Conduct Electricity

Picture a solid salt crystal sitting on your desk. The sodium and chloride ions are locked in place, unable to move. Now imagine heating that salt until it melts, or dissolving it in water. Suddenly, those same ions are free to swim around.

This freedom to move is the key to electrical conduction. When ions can move, they can carry electrical charge from one place to another. This is completely different from metals, where electrons do the moving.

2. The Journey of Ions During Electrolysis

When you connect electrodes to an electrolyte and switch on the power, something fascinating happens. The ions don't just sit there - they start moving with purpose, like iron filings drawn to a magnet.

Here's the pattern that never changes: positive ions head towards the negative electrode, and negative ions head towards the positive electrode. Think of it as opposites attracting on a microscopic scale.

Remember this memory trick: AN OX, RED CAT. ANode = OXidation (lose electrons), REDuction at CAThode (gain electrons). The cathode is where positive ions pick up electrons to become neutral atoms.

3. What Actually Happens at the Electrodes

The real key scientific idea happens when ions reach the electrodes. They don't just touch and bounce off - they undergo chemical changes that produce new substances. This is where ions transform back into elements.

Worked example: Electrolysis of Molten Lead Bromide

Predict the products when molten lead bromide (PbBr₂) undergoes electrolysis.

1. PbBr₂ → Pb²⁺ + 2Br⁻ — Lead bromide contains lead ions and bromide ions
2. Pb²⁺ moves to cathode (negative electrode) 2Br⁻ moves to anode (positive electrode) — Opposite charges attract
3. Pb²⁺ + 2e⁻ → Pb — Lead ions gain electrons (reduction)
4. 2Br⁻ → Br₂ + 2e⁻ — Bromide ions lose electrons (oxidation)

Answer: Lead metal forms at the cathode, bromine gas forms at the anode

4. Aqueous vs Molten Electrolysis

Here's where it gets tricky. When you dissolve an ionic compound in water instead of melting it, you're not just adding the compound's ions - you're also adding water molecules that can break apart into H⁺ and OH⁻ ions.

This creates competition at the electrodes. Multiple types of ions are present, but only some will be discharged. The winners depend on the reactivity series and concentration.

Worked example: Electrolysis of Sodium Chloride Solution

Explain why hydrogen gas forms at the cathode when aqueous NaCl undergoes electrolysis, not sodium metal.

1. From NaCl: Na⁺ and Cl⁻ From water: H⁺ and OH⁻ — Aqueous solutions contain ions from both the compound and water
2. Na⁺ + e⁻ → Na (very reactive) 2H+ + 2e- -> H2 — Both positive ions compete for electrons
3. Sodium is above hydrogen in reactivity series — More reactive metals stay as ions in solution
4. H⁺ ions are discharged, forming H₂ gas — Less reactive ion wins the competition

Answer: Hydrogen gas forms because H⁺ ions are less reactive than Na⁺ ions

5. Exam Success Strategy

Electrolysis questions follow predictable patterns. Examiners love to test whether you can predict products and explain ion movement. The key is systematic thinking, not guesswork.

Common exam mistake: Writing 'Cl' instead of 'Cl₂' for chlorine gas. Halogens exist as diatomic molecules, so write Cl2, Br2, and I2 rather than single atoms when those elements are formed.

Exam precision note: In Understanding Electrolytes and Ion Movement - The process of electrolysis - Electrolysis, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

During electrolysis, positively charged ions move to the negative electrode, called the cathode, and negatively charged ions move to the positive electrode, called the anode. The ions are then discharged at the electrodes to produce elements or simpler substances. This movement of ions through the molten or dissolved ionic substance is what allows the electrolyte to conduct electricity.