Electrolytes and Non-Electrolytes
Senior 2 · Chemistry
Electrolytes and Non-Electrolytes
Lesson 1 of 3
25 min
You have been warned since primary school never to touch a switch with wet hands. Yet pure water hardly conducts electricity at all. The danger comes from what is dissolved in the water: salts from your skin, from the soil and from the tap. Substances that let a solution carry an electric current are called electrolytes, and they matter far beyond safety. They run your nerves and muscles, power car batteries and make electroplating possible.
An electric current is a flow of charge. In a metal wire the charge is carried by electrons moving through the metal, and the metal itself is not changed. In a solution or a molten (melted) compound, the charge is carried by ions, charged particles that move through the liquid. Positive ions move towards the negative electrode and negative ions towards the positive electrode, and a chemical change happens at each electrode.
Sodium chloride is made of Na+ and Cl− ions. In the solid the ions are held in fixed positions in a lattice. They can vibrate but cannot move from place to place, so solid sodium chloride does not conduct. When the salt is melted or dissolved in water, the lattice breaks up and the ions become free to move, so it conducts. Dissolving an ionic compound and separating its ions is called dissociation:
Most covalent substances, such as sugar and ethanol, dissolve as whole molecules and are non-electrolytes. A few covalent molecules react with water to produce ions. Hydrogen chloride is a covalent gas containing no ions at all, yet its solution, hydrochloric acid, conducts very well:
HCl(g) + H2O(l) → H3O+(aq) + Cl−(aq)
Ammonia does something similar, but only a small fraction of its molecules react: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq).
Pure water ionises only very slightly, about two molecules in every billion, so pure (distilled) water is an extremely poor conductor. Tap water, river water and rainwater conduct noticeably better because they contain dissolved salts.
A simple tester is a battery, a small bulb and two carbon rods (taken from old dry cells) used as electrodes, all connected in series. The electrodes dip into the liquid being tested and are rinsed with distilled water between tests. Habimana's class obtained these results:
| Substance tested | Bulb | Conclusion |
|---|---|---|
| Solid sodium chloride (crystals touching both rods) | off | ions not free to move |
| Sodium chloride solution | bright | electrolyte |
| Dilute hydrochloric acid | bright | electrolyte |
| Vinegar (ethanoic acid) | dim | electrolyte, few ions |
| Sugar solution | off | non-electrolyte |
| Ethanol | off | non-electrolyte |
| Distilled water | off | almost no ions |
| Tap water | very dim or off | a few dissolved ions |
Reasoning. The bulb lights only when the liquid contains ions that can move. Solid salt and salt solution contain the same ions; the difference is whether the ions are free to move. A dim bulb means the current is small because there are few ions, which is the subject of the next lesson.
Warning
Safety
Use only a low-voltage battery (a few volts) for conductivity tests, never mains electricity. Molten salts are tested only by a teacher, behind a safety screen, because they are several hundred degrees hot.
Tip
Ions that can move = conduction
For every "does it conduct?" question, ask two things: does it contain ions (or form them in water)? Are those ions free to move (molten or dissolved, not solid)? Only "yes" to both means it conducts as an electrolyte. Metals and graphite conduct by electrons, so they are conductors, not electrolytes.
Define an electrolyte.
Solution: A substance that conducts electricity when molten or in aqueous solution, and is decomposed by the current, because it contains ions that are free to move.
Explain why solid potassium chloride does not conduct electricity, but its solution does.
Solution: In the solid, the K+ and Cl− ions are held in fixed positions in the lattice and cannot move. In solution the ions separate and move freely, so they can carry charge.
Classify each substance as an electrolyte, a non-electrolyte or a conductor that is not an electrolyte: copper wire, glucose solution, dilute sulfuric acid, molten lead(II) bromide.
Solution: Copper wire: conductor, not an electrolyte (conducts by electrons). Glucose solution: non-electrolyte. Dilute sulfuric acid: electrolyte. Molten lead(II) bromide: electrolyte.
Hydrogen chloride gas is covalent, so why does its solution in water conduct?
Solution: It reacts with water to form ions: HCl + H2O → H3O+ + Cl−. These mobile ions carry the current.
Write the dissociation equation for magnesium chloride dissolving in water.
Solution: MgCl2(s) → Mg2+(aq) + 2Cl−(aq). Charge check: left 0; right +2 + 2(−1) = 0.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
