What is an electrolyte?
Electric current in a metal wire is carried by tiny electrons. In a liquid, the carriers are different: they are ions. An ion is an atom (or a group of atoms) with an electric charge.
An electrolyte is a substance that makes ions when it is dissolved in water or melted. Because it has free ions, the liquid can carry current.
A non-electrolyte stays as whole neutral molecules. It has no ions, so the liquid cannot carry current.
- Electrolytes: salt (NaCl), hydrochloric acid (HCl), sodium hydroxide (NaOH), vinegar.
- Non-electrolytes: sugar, alcohol, glucose, pure (distilled) water.
Electrolytic dissociation: how ions appear
When salt goes into water, the water molecules pull the ions apart. This splitting into ions is called electrolytic dissociation.
NaCl → Na⁺ + Cl⁻
HCl → H⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
The total positive charge always equals the total negative charge, so the solution as a whole stays neutral. A solid salt crystal has the same ions, but they are locked in place, so a dry crystal does not conduct. A melted salt does conduct, because the ions are free again.
Why does the solution carry current?
Connect two rods (electrodes) to a battery and dip them in the solution. The rod joined to the battery's minus end is the cathode (−). The other is the anode (+).
Positive ions (cations) move to the cathode. Negative ions (anions) move to the anode. Moving charge is current, so the bulb glows. More free ions means a bigger current and a brighter bulb.
Strong and weak electrolytes
A strong electrolyte splits almost completely into ions. Examples: HCl, H₂SO₄, HNO₃, NaOH, KOH and nearly all soluble salts. The bulb glows brightly.
A weak electrolyte splits only a little. Most of it stays as whole molecules. Examples: acetic acid (vinegar), carbonic acid, ammonia solution. The bulb glows dimly. For weak electrolytes the splitting is reversible, shown with a two-way arrow: CH₃COOH ⇌ CH₃COO⁻ + H⁺.
Strong or weak is about how much it splits, not about how dangerous it is, and not about how much is dissolved.
Degree of dissociation (α)
The degree of dissociation, written α, tells what share of the dissolved units has split into ions.
α = number of units that split ÷ total number of units dissolved
If 5 out of every 100 acetic acid molecules split, α = 5/100 = 0.05, or 5%. For a strong electrolyte α is close to 1 (100%). For a weak one α is small. Diluting a weak electrolyte makes α larger, because the ions have more room and the water pulls more units apart.
Key formulas and definitions
- α = (units split into ions) ÷ (total units dissolved)
- α as a percentage = α × 100
- NaCl → Na⁺ + Cl⁻ (strong, one arrow)
- CH₃COOH ⇌ CH₃COO⁻ + H⁺ (weak, two-way arrow)
- Total + charge = total − charge in the solution
Worked examples
1. Sort into electrolyte or non-electrolyte: glucose, NaCl solution, HCl solution, alcohol.
Electrolytes: NaCl solution and HCl solution (they make ions). Non-electrolytes: glucose and alcohol (they stay as molecules).
2. Write the dissociation of NaOH in water.
NaOH → Na⁺ + OH⁻. NaOH is a strong electrolyte, so one arrow is used.
3. Out of 200 acetic acid molecules, 8 split into ions. Find α.
α = 8 / 200 = 0.04, which is 4%. This is a weak electrolyte.
4. Why does solid NaCl not conduct but molten NaCl does?
In the solid the Na⁺ and Cl⁻ ions are locked in a grid and cannot move. When melted, the ions become free and can carry current.
5. Two beakers have the same amount of dissolved substance. Bulb A is bright, bulb B is dim. What can you say?
Beaker A has more free ions, so its substance is a stronger electrolyte (higher α). Beaker B holds a weak electrolyte.
Common mistakes
- Thinking electrons carry the current in the solution. In a solution, ions carry it.
- Thinking a dry salt crystal conducts. The ions are locked; they must be free (dissolved or melted).
- Mixing up strong with concentrated. Strong means α is high. Concentrated means a lot is dissolved.
- Believing that electricity makes the ions. The ions are already there, or form when the substance dissolves. The battery only moves them.