Chemistry: Atoms First 2e · Solutions and Colloids

Electrolytes

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Pure water conducts electricity only very weakly. Dissolve a pinch of table salt in it, however, and the solution suddenly conducts well enough to light a bulb — because the salt breaks apart into ions that can move through the liquid and carry charge. Substances that produce ions when dissolved are electrolytes; those that dissolve without producing ions are nonelectrolytes. Electrolytes are further divided into strong electrolytes, which dissociate completely (nearly all soluble ionic compounds, strong acids, strong bases), and weak electrolytes, which dissociate only partially (weak acids and weak bases like acetic acid and ammonia). This topic explains how to classify substances, how to write equations, and how to calculate the concentration of each in solution — the numbers behind every "" claim on a sports-drink label.

Why this matters

Body chemistry: Sodium, potassium, calcium, chloride, and bicarbonate ions dissolved in blood and cells are electrolytes; nerve signaling, muscle contraction, and fluid balance all depend on their concentrations.
Medical fluids: IV saline (0.9% NaCl by mass, about 0.15 M), oral rehydration solutions, and dialysis fluids are formulated to replace specific ions — the chemistry of electrolytes in action.
Sports drinks: Labels advertise "electrolytes" because sweating loses Na⁺, K⁺, and Cl⁻; replacing them helps maintain hydration and muscle function.
Batteries and industry: Lead-acid batteries, fuel cells, and electroplating baths all require electrolytes to carry charge between electrodes.
Exams: Classifying strong vs. weak vs. nonelectrolytes, writing dissociation equations, and computing ion concentrations are reliable test questions.

The college version

Core Concepts

Electrolytes, nonelectrolytes, and the conductivity test

An electrolyte is a substance whose aqueous solution conducts electricity; a produces a solution that does not. The reason is the presence of mobile ions. In a simple conductivity apparatus, a bright bulb means many ions (), a dim bulb means few ions (), and no glow means no ions (nonelectrolyte). Sugar (C₁₂H₂₂O₁₁) and ethanol (C₂H₅OH) are nonelectrolytes: they dissolve as neutral molecules. NaCl and HCl are strong electrolytes; acetic acid (CH₃COOH) and ammonia (NH₃) are weak electrolytes.

Strong electrolytes: complete dissociation

A strong electrolyte dissociates essentially 100% into ions in water. Three families qualify:

Soluble ionic compounds: NaCl(s) → Na⁺(aq) + Cl^-(aq)
Strong acids: HCl(aq) → H⁺(aq) + Cl^-(aq) (the seven strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₃, HClO₄)
Strong bases: NaOH(aq) → Na⁺(aq) + OH^-(aq) (group 1 hydroxides and Ba(OH)₂, Sr(OH)₂)

The single arrow → means the forward reaction goes to completion: no intact solute molecules remain.

Weak electrolytes: partial dissociation

A weak electrolyte dissociates only partially; most of the solute stays as intact molecules, and an equilibrium exists between molecules and ions. The double arrow ⇌ marks this equilibrium:

$CH 3 COOH (aq) ⇌ CH 3 COO - (aq) + H + (aq)$

At equilibrium only a small fraction (typically a few percent) of acetic acid molecules have ionized — which is why vinegar conducts weakly and tastes acidic but does not conduct like salt water.

Ion concentrations from formula units

Because each formula unit of a strong electrolyte produces a fixed number of ions, ion concentrations follow directly from stoichiometry:

0.10 M NaCl → [Na⁺] = 0.10 M, [Cl^-] = 0.10 M
0.10 M CaCl₂ → [Ca²⁺] = 0.10 M, [Cl^-] = 0.20 M
0.10 M Al₂(SO₄)₃ → [Al³⁺] = 0.20 M, [SO₄^2-] = 0.30 M

In general, the concentration of ion X equals the molarity of the compound times the number of X ions per formula unit.

Common Confusions

Do Not Confuse	With	Difference
Electrolyte	Nonelectrolyte	Electrolytes produce ions and conduct (NaCl); nonelectrolytes dissolve as molecules and do not (sugar)
Strong electrolyte	Weak electrolyte	Strong dissociates ~100% (single arrow, full ion concentrations); weak dissociates partially (double arrow, equilibrium)
Dissolved	Ionized	A substance can dissolve without ionizing (sugar); ionizing is what makes a solution conduct
Strong acid	Concentrated acid	"Strong" means complete ionization, not high concentration — 0.001 M HCl is still a strong electrolyte
Ionic compound	Molecular compound	Soluble ionic compounds are strong electrolytes; most molecular compounds are nonelectrolytes (except acids)
→	⇌	Single arrow = complete reaction; double arrow = equilibrium with both molecules and ions present
Concentration of compound	Concentration of each ion	0.10 M CaCl₂ is 0.10 M in Ca²⁺ but 0.20 M in Cl⁻ — always multiply by the ion ratio

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a battery as a tiny waterfall for electricity: the water only flows if there are little "charge boats" to carry it. Electrolytes are chemicals that split into charged boats (ions) when they get wet, so electricity can flow through the water. Salt splits into lots of boats (strong electrolyte), vinegar splits into only a few (weak electrolyte), and sugar does not split at all (nonelectrolyte) — that's why a sugar-water bulb stays dark.

Worked example

Example 1: Classifying substances

Classify each as strong electrolyte, weak electrolyte, or nonelectrolyte: (a) KBr, (b) CH₃COOH, (c) C₆H₁₂O₆ (glucose), (d) HNO₃, (e) NH₃.

(a) KBr: a soluble ionic compound → strong electrolyte.
(b) CH₃COOH: a weak acid → weak electrolyte.
(c) Glucose: a covalent molecular compound that dissolves as molecules → nonelectrolyte.
(d) HNO₃: one of the seven strong acids → strong electrolyte.
(e) NH₃: a weak base → weak electrolyte.

Shortcut: ionic compounds and strong acids/bases are strong; weak acids/bases are weak; molecular nonelectrolytes (sugars, alcohols) are neither.

Example 2: Ion concentrations from a strong electrolyte

Calculate the concentration of each ion in 0.25 M Al₂(SO₄)₃ solution, assuming complete dissociation.

Step 1 — Write the dissociation equation:

$Al 2 (SO 4) 3 (s) \to 2 Al 3+ (aq) + 3 SO 4 2- (aq)$

Step 2 — Multiply compound molarity by the ion ratio (formula before substitution):

$[Al 3+] = 0.25 M \times 2 mol Al 3+ 1 mol Al 2 (SO 4) 3 = 0.50 M$

$[SO 4 2-] = 0.25 M \times 3 mol SO 4 2- 1 mol Al 2 (SO 4) 3 = 0.75 M$

Dimensional check: the formula-unit moles cancel, leaving mol/L for each ion. Total ion concentration is the sum: 0.50 + 0.75 = 1.25 M of particles.

Example 3: Preparing an electrolyte solution

How many grams of CaCl₂ (molar mass 110.98 g/mol) are needed to prepare 250.0 mL of 0.100 M CaCl₂? What is [Cl^-] in the final solution?

Step 1 — Moles of solute needed:

$n = M \times V = 0.100 mol L \times 0.2500 L = 0.0250 mol$

Step 2 — Mass of CaCl₂:

$m = n \times M = 0.0250 mol \times 110.98 g mol -1 = 2.77 g$

Step 3 — Chloride concentration (2 Cl⁻ per formula unit):

$[Cl -] = 0.100 M \times 2 mol Cl - 1 mol CaCl 2 = 0.200 M$

Dimensional check: L cancels in step 1, mol cancels in step 2, and formula-unit moles cancel in step 3. Note [Ca²⁺] = 0.100 M while [Cl^-] = 0.200 M — ions do not share the compound's concentration when the formula has unequal numbers.

Key takeaways

Electrolyte = dissolves into ions and conducts; nonelectrolyte = dissolves as molecules and does not conduct.
Strong electrolytes dissociate ~100%: soluble ionic compounds, the seven strong acids, strong bases (group 1 hydroxides).
Weak electrolytes dissociate partially and set up an equilibrium ( ⇌ ): weak acids (acetic acid), weak bases (ammonia).
Use → for complete dissociation, ⇌ for partial.
Ion concentration = compound molarity × ions per formula unit (0.10 M CaCl₂ gives 0.20 M Cl⁻).
Memorize the strong-acid list: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₃, HClO₄.
"Soluble ionic compound" and "strong electrolyte" go together; "weak acid" and "weak electrolyte" go together.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Define electrolyte and nonelectrolyte, and give one example of each.
Show answer
An electrolyte produces ions in solution and conducts electricity (NaCl); a nonelectrolyte dissolves as neutral molecules and does not conduct (sugar).
List the three classes of strong electrolytes and give an example of each.
Show answer
Soluble ionic compounds (NaCl), strong acids (HCl), strong bases (NaOH).
Write the dissociation equation for 0.10 M MgCl₂ and give [Mg²⁺] and [Cl^-].
Show answer
MgCl₂ → Mg²⁺ + 2Cl^-; [Mg²⁺] = 0.10 M, [Cl^-] = 0.20 M.
Why does vinegar (acetic acid in water) conduct electricity only weakly?
Show answer
Acetic acid is a weak electrolyte: only a small fraction of molecules ionize, so few ions are available to carry current — the bulb glows dimly.
How many grams of Na₂SO₄ (molar mass 142.04 g/mol) are needed to make 500.0 mL of 0.200 M Na₂SO₄? What is [ Na⁺]?
Show answer
n = 0.200 × 0.5000 = 0.100 mol; m = 0.100 × 142.04 = 14.2 g. [Na⁺] = 0.200 × 2 = 0.400 M.
Is 0.0001 M HCl a strong or weak electrolyte? Explain.
Show answer
Strong electrolyte. "Strong" refers to complete ionization, not concentration — even a very dilute strong acid ionizes essentially completely.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Electrolyte: Substance whose solution conducts electricity because it produces ions
Nonelectrolyte: Substance that dissolves as neutral molecules (sugar, ethanol)
Strong electrolyte: Dissociates ~100% into ions (soluble ionic compounds, strong acids/bases)
Weak electrolyte: Dissociates only partially (weak acids/bases)
Dissociation: The splitting of a compound into ions in solution
Ion: An atom or group with a net charge (Na⁺, Cl⁻, SO₄²⁻)
Concentration [X]: Moles of species X per liter of solution
Strong acid: One of seven acids that ionize completely (HCl, HNO₃, …)

Sources & references

openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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