Why shouldn’t you touch a switch with wet hands?
Pure water barely conducts electricity, but tap water and sweat do, which is why wet hands and electrical outlets don’t mix. Put two electrodes in salt water and connect a bulb, and it lights; switch to sugar water, and it stays dark. In 2.4.1 you saw that solid salt doesn’t conduct, but molten or dissolved salt does. The key is whether there are ions free to move. This lesson classifies substances by whether they produce ions.
Electrolytes and nonelectrolytes
In US textbooks, an electrolyte电解质 is a substance that dissolves in water to give a solution that conducts electricity, such as NaCl, NaOH, HCl, or CH₃COOH. A nonelectrolyte非电解质 dissolves without producing ions, so its solution does not conduct; sugar and ethanol are examples.
Two words describe how the ions form:
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Dissociation电离: an ionic compound’s ions separate as it dissolves.
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Ionization电离: a molecular compound reacts with water to form ions that were not there before. (AP Chemistry often writes H⁺(aq) as H₃O⁺(aq).)
Strong and weak electrolytes
- A strong electrolyte强电解质 exists in solution almost entirely as ions: strong acids (HCl, HNO₃, H₂SO₄), strong bases (NaOH, KOH, Ba(OH)₂), and nearly all soluble salts. Use a single arrow.
- A weak electrolyte弱电解质 ionizes only partly: weak acids (CH₃COOH, H₂CO₃), weak bases (NH₃), and water itself. Use a double arrow:
Strength is about how completely a substance ionizes, not how much of it dissolves. BaSO₄ and CaCO₃ barely dissolve, but the little that does dissolve is completely ionized, so they count as strong electrolytes.
How Chinese textbooks define them
Chinese textbooks define 电解质 (electrolytes) more strictly:
- An electrolyte is a compound that conducts in water solution or when molten; a nonelectrolyte is a compound that conducts in neither state.
- Elements and mixtures are neither. Copper conducts, but it is an element. NaCl solution conducts, but it is a mixture, and the electrolyte is the NaCl in it.
- What matters is whether the substance itself ionizes. NH₃ dissolves to form NH₃·H₂O, which partly ionizes into NH₄⁺ and OH⁻. Chinese textbooks therefore call NH₃·H₂O a weak electrolyte but NH₃ itself a nonelectrolyte, while US textbooks simply call NH₃ a weak electrolyte. CO₂ and SO₂ are nonelectrolytes in Chinese textbooks for the same reason.
- Chinese uses one word, 电离, for both dissociation and ionization, and Chinese textbooks write ionization equations without state symbols.
Pick a substance and a state to see whether the bulb lights, what particles are inside, and how each tradition classifies it:
Example 1: classify the substances
Classify each substance as a strong electrolyte, a weak electrolyte, or a nonelectrolyte, and note any that Chinese textbooks would treat differently: KNO₃, CO₂, sucrose, acetic acid, BaSO₄, copper.
- The test. Ask what the substance gives in water (or when molten): all ions (strong electrolyte), a few ions and mostly molecules (weak electrolyte), or only molecules (nonelectrolyte).
- KNO₃. A soluble salt that dissociates completely: strong electrolyte.
- Acetic acid. A weak acid that ionizes only slightly: weak electrolyte.
- BaSO₄. Barely soluble, but what dissolves is all ions, and it conducts when molten: strong electrolyte.
- Sucrose. Dissolves as whole molecules: nonelectrolyte.
- CO₂. Its solution conducts very slightly, because some CO₂ reacts with water to form carbonic acid, H₂CO₃, a weak acid. Chinese textbooks call CO₂ itself a nonelectrolyte and H₂CO₃ the weak electrolyte.
- Copper. It conducts by free electrons, not ions, and it is an element, so it is not an electrolyte in either tradition.
- A common mistake. Calling BaSO₄ weak because it barely dissolves. Strong or weak depends on whether the dissolved part is fully ionized, not on how much dissolves.
Example 2: write the equations
Write the dissociation or ionization equation for each substance in water, and give the number of moles of ions from 1 mol of it: Ba(OH)₂, Al₂(SO₄)₃, NaHCO₃, CH₃COOH.
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Ba(OH)₂ (strong base, dissociates completely). 1 mol Ba²⁺ and 2 mol OH⁻, so 3 mol of ions:
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Al₂(SO₄)₃ (soluble salt, dissociates completely). The subscripts 2 and 3 become coefficients: 2 mol Al³⁺ and 3 mol SO₄²⁻, so 5 mol of ions:
Charge check: 2(+3) + 3(−2) = 0.
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NaHCO₃ (soluble salt). HCO₃⁻ comes from the weak acid H₂CO₃ and stays together as one polyatomic ion, so 2 mol of ions:
Compare NaHSO₄: HSO₄⁻ comes from a strong acid, and in Chinese textbooks it is split into Na⁺, H⁺, and SO₄²⁻.
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CH₃COOH (weak acid, ionizes partly). Only a small fraction ionizes, so use a double arrow; 1 mol gives far less than 2 mol of ions:
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A common mistake. Writing 2Al³⁺ as Al₂³⁺, or 3SO₄²⁻ as (SO₄)₃²⁻. The ions separate; their numbers go in front as coefficients.
- KOH
A strong base that dissociates completely: KOH → K⁺ + OH⁻.
- HNO₃
A strong acid. It is a molecular compound, but it ionizes completely in water.
- CH₃COOH
A weak acid; only a small fraction ionizes.
- NH₃
NH₃ reacts slightly with water to give NH₄⁺ and OH⁻: weak electrolyte in US textbooks. Chinese textbooks call NH₃ itself a nonelectrolyte.
- C₂H₅OH (ethanol)
Ethanol dissolves as molecules, and the solution does not conduct.
- Sucrose
Sugar dissolves as whole molecules: nonelectrolyte.
- Iron
Iron conducts with free electrons, but it is an element, not an electrolyte.
- HCl
A molecular gas that ionizes completely in water: strong electrolyte.
- CaCO₃
Barely soluble, but the dissolved part is all ions. Electrolyte strength does not depend on solubility.
Common mistakes
Mistake 1: “Anything that conducts is an electrolyte.” Copper and graphite conduct with electrons, not ions, and they are elements. An electrolyte produces mobile ions.
Mistake 2: “Insoluble compounds are weak electrolytes.” Strength measures how completely the dissolved part ionizes, not how much dissolves. BaSO₄, AgCl, and CaCO₃ are barely soluble but are strong electrolytes; acetic acid mixes with water in any amount but is a weak electrolyte.
Mistake 3: “Only ionic compounds can be electrolytes.” HCl, HNO₃, and H₂SO₄ are molecular compounds, but they ionize completely in water, so they are strong electrolytes. Pure liquid HCl, with no water, does not conduct.
Quick check
Which substance is an electrolyte?
BaSO₄ is ionic; the little that dissolves is completely dissociated, and it conducts when molten. Copper is an element, and sucrose and ethanol are nonelectrolytes.
Which 0.1 M solution conducts electricity the least?
Acetic acid is a weak electrolyte, so only a small fraction ionizes and there are far fewer mobile ions. The other three are strong electrolytes.
Summary
- An electrolyte dissolves to give a conducting solution; a nonelectrolyte dissolves as molecules. Ionic compounds dissociate, and some molecular compounds, such as acids, ionize.
- Strong electrolytes (strong acids, strong bases, soluble salts) ionize completely; weak electrolytes (weak acids, weak bases, water) ionize partly and use ⇌. Strength is not the same as solubility.
- Chinese textbooks count only compounds and ask whether the substance itself ionizes in water or when molten, so NH₃ and CO₂ are nonelectrolytes there and elements and mixtures are neither.
On to AP (for later)
AP Chemistry often tests electrolytes with particle diagrams: NaCl(aq) is drawn as separate Na⁺ and Cl⁻ ions, while acetic acid is drawn as mostly whole CH₃COOH molecules with only a few ions. In Unit 8, strong and weak acids are strong and weak electrolytes put to work, and Ka measures exactly how weak an acid is. AP also uses changes in conductivity as lab evidence, for example to follow the ion concentration during a titration.