Electrolytes: Strong, Weak, and Nonelectrolytes

An electrolyte is a substance that produces ions when dissolved in water, so that the solution conducts electricity; strong electrolytes dissociate completely, weak electrolytes only partly, and nonelectrolytes not at all. In brief: NaCl is a strong electrolyte, acetic acid is a weak electrolyte (about 1 % ionized at 0.10 M) and sugar is a nonelectrolyte.

Whether a compound produces ions in water determines how its solutions conduct electricity, how it reacts, and how it affects colligative properties such as boiling and freezing points. In the human body, electrolytes such as sodium, potassium and calcium ions are necessary for nerve and muscle function. This article explains the three classes of electrolytes, helps you to classify a compound and shows how to count the ions in a solution.

What are strong, weak and nonelectrolytes?

• Strong electrolytes dissociate (ionic compounds) or ionize (strong acids) essentially completely. Their solutions are good conductors. In an equation they are written with a single arrow: NaCl(s) → Na⁺(aq) + Cl⁻(aq).
• Weak electrolytes ionize only to a small degree, so most of the dissolved substance remains as molecules. The equation uses a double arrow: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq).
• Nonelectrolytes dissolve as intact molecules and produce no ions. Their solutions do not conduct electricity, and the dissolving is written, for example, as C₁₂H₂₂O₁₁(s) → C₁₂H₂₂O₁₁(aq).

A light bulb in a conductivity tester shows the difference: it shines brightly in a strong electrolyte solution, dimly in a weak electrolyte solution and not at all in a nonelectrolyte solution. Note that pure water is also a very weak electrolyte, with only about 10⁻⁷ mol/L of H⁺ at 25 °C.

Examples of each class

How do you identify the class of a compound?

1. Soluble ionic compounds (most salts of group 1 metals and ammonium, nitrates, most chlorides) are strong electrolytes.
2. The six common strong acids are HCl, HBr, HI, HNO₃, H₂SO₄ and HClO₄. Every other common acid, including HF, is weak.
3. Strong bases are the hydroxides of group 1 and of Ca, Sr and Ba. Ammonia and amines are weak bases.
4. Most molecular compounds, such as sugars and alcohols, are nonelectrolytes, unless they are acids or bases.

Insoluble salts, such as AgCl, are also poor conductors in water, but this is because very little of the salt dissolves, not because it fails to dissociate. The small amount that does dissolve is fully dissociated.

How do you count the ions in a solution?

For a strong electrolyte, multiply the formula concentration by the number of ions per formula unit.

In practice, the effect is smaller at high concentrations, because ions in real solutions attract one another and form ion pairs.

How much does a weak electrolyte ionize?

The extent of ionization is described by the percent ionization, which is the concentration of ionized acid divided by the initial concentration, multiplied by 100.

Worked example: acetic acid

Estimate the percent ionization of 0.10 M acetic acid, for which K_a = 1.8 × 10⁻⁵.

1. For a weak acid, K_a ≈ x² / 0.10, where x = [H⁺].
2. x = √(1.8 × 10⁻⁵ × 0.10) = √(1.8 × 10⁻⁶) = 1.3 × 10⁻³ M.
3. Percent ionization = (1.3 × 10⁻³ / 0.10) × 100 = 1.3 %.

So 98.7 % of the acid is present as intact molecules. At a lower concentration the percent ionization is higher, which is a consequence of Le Châtelier’s principle.

Why do electrolytes matter in the body?

Body fluids contain Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻ and phosphate. Nerve impulses and muscle contractions depend on the flow of these ions across membranes. Heavy sweating, vomiting or diarrhea can disturb the balance, so oral rehydration solutions contain salts and glucose. Sports drinks usually provide sodium and potassium in smaller amounts.

Who discovered electrolytes?

Michael Faraday introduced the words electrolyte, ion, anode and cathode in the 1830s, in work with William Whewell. He believed ions were produced by the current. Svante Arrhenius showed in 1884 that ions exist in the solution even without a current, and this theory of electrolytic dissociation was recognized by the Nobel Prize in 1903. Peter Debye and Erich Hückel explained in 1923 why strong electrolytes behave as if they are less than fully dissociated at high concentrations.

Which mistakes are common?

• Equating “strong” with “concentrated”: strength refers to the degree of ionization, and concentration to the amount of solute.
• Treating HF as a strong acid: it is a weak acid, despite being a halogen acid.
• Forgetting that polyatomic ions stay intact: sulfate stays as SO₄²⁻.
• Calling a solution conducting because it is acidic: a weak acid may conduct poorly.
• Writing a single arrow for a weak electrolyte: use the double arrow for equilibrium.