4.1 Solutions and Electrolytes
PRE-HEALTH PREP
Strong Electrolytes vs Weak Electrolytes vs Nonelectrolytes
General Chemistry 4.1
Electrolytes are substances that dissociate into ions when dissolved in water, and they are classified as strong, weak, or nonelectrolytes based on the degree of that dissociation. Strong electrolytes—including soluble ionic compounds, the 7 strong acids, and Group 1/Group 2 metal hydroxides—dissociate completely, while weak electrolytes such as weak acids and ammonia dissociate only partially, and nonelectrolytes produce no ions at all. The solubility rules for ionic compounds, which predict whether a salt dissolves based on its anion and specific cation exceptions, determine which ionic compounds qualify as strong electrolytes in aqueous solution.
Table of Contents
Solutions, Solvents, and Solutes in Aqueous Chemistry
Before studying electrolytes, you need to understand what a solution is. A solution consists of two components: a solvent and one or more solutes. The solvent is typically the liquid in which everything dissolves, while the solutes are the substances dissolved in it. Throughout this chapter, the solvent is exclusively water, and solutions made with water are called aqueous solutions.
Although most solvents you encounter will be liquids, solvents can technically be solids, liquids, or gases. The solute can also be a solid, a liquid, or a gas—and you will encounter examples of all three types in general chemistry.
Liquid and Gas Solutes: 2 Surprising Examples
When you think of dissolving something, you probably picture a solid like salt or sugar dissolving in water. However, liquids and gases dissolve in water too.
Example: Liquid Solute — Beer and Wine
A typical beer contains 5–8% ethanol, while wine contains roughly 12%. Since ethanol makes up the smaller fraction and water makes up the majority, water is the solvent and ethanol is the solute. When both the solvent and solute are liquids, whichever is present in larger quantity is designated the solvent.
Example: Gas Solute — Dissolved Oxygen
Fish breathe O₂ dissolved in water—not the oxygen atoms in H₂O. That dissolved O₂ enters from the atmosphere, which is why a sealed fish bowl eventually becomes lethal: the fish consume all the available oxygen with no way to replenish it.
Example: Gas Solute — Hydrochloric Acid
Pure HCl is actually a gas. The "hydrochloric acid" in a chemistry lab is an aqueous solution of HCl gas dissolved in water.
What Are Electrolytes in Chemistry?
Pure water is not electrically conductive. That might surprise you—after all, you have heard since childhood that swimming during a lightning storm is dangerous. The key distinction is that pure water does not conduct electricity, but water containing dissolved electrolytes does.
An electrolyte is any substance that dissociates into ions when it dissolves in water. Not everything that dissolves produces ions, but those that do are classified as electrolytes. The greater the degree of ion dissociation, the more electrically conductive the aqueous solution becomes.
Why Swimming Pools Conduct Electricity: Your swimming pool is loaded with electrolytes—dissolved salts from skin, sweat, and atmospheric carbon dioxide all contribute ions. These ions make the water highly conductive, which is exactly why lightning and swimming pools are a dangerous combination.
3 Types of Strong Electrolytes
Strong electrolytes dissociate into ions completely when dissolved in water, producing the maximum number of ions in solution. There are 3 types of strong electrolytes: soluble ionic compounds, strong acids, and strong bases.
Soluble Ionic Compounds
Ionic compounds are typically formed from a metal and a nonmetal, or a metal and a polyatomic ion. When an ionic compound is soluble in water, it dissociates completely into its constituent ions, making it a strong electrolyte in aqueous solution.
Example: Sodium Chloride (NaCl)
NaCl (table salt) is a soluble ionic compound. When it dissolves in water, it dissociates completely into Na⁺ and Cl⁻ ions, making the solution electrically conductive.
Not all ionic compounds are soluble—you will learn the solubility rules later in this lesson to predict which ones dissolve and which do not.
The 7 Strong Acids
Acids begin with hydrogen (H) in their chemical formulas. The strong acids dissociate 100% into ions in aqueous solution. You must memorize these 7 strong acids:
| Strong Acid Formula | Name |
|---|---|
| HCl | Hydrochloric acid |
| HBr | Hydrobromic acid |
| HI | Hydroiodic acid |
| HClO₄ | Perchloric acid |
| HClO₃ | Chloric acid* |
| H₂SO₄ | Sulfuric acid |
| HNO₃ | Nitric acid |
Note: HClO₃ (chloric acid) is sometimes excluded from strong acid lists. Check your textbook or instructor's preference.
Once you memorize the 7 strong acids, you can identify weak acids by elimination: any acid not on this list is weak by default. For instance, HF (hydrofluoric acid) begins with H and is therefore an acid, but it does not appear on the list—so it is a weak acid. Similarly, HNO₂ (nitrous acid) is weak because only HNO₃ made the cut.
Strong Bases: Group 1 and Group 2 Hydroxides
The strong bases are all hydroxide bases—compounds containing OH⁻. They include most Group 1 and Group 2 metal hydroxides, and they dissociate completely in aqueous solution.
5 Group 1 metal hydroxides (all strong bases):
- Lithium hydroxide (LiOH)
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Rubidium hydroxide (RbOH)
- Cesium hydroxide (CsOH)
3 Group 2 metal hydroxides (strong bases):
- Calcium hydroxide (Ca(OH)₂)
- Strontium hydroxide (Sr(OH)₂)
- Barium hydroxide (Ba(OH)₂)
What About Magnesium Hydroxide? Mg(OH)₂ is a borderline case. The Group 2 hydroxides are not as soluble as the Group 1 hydroxides, but to the extent that they do dissolve, they dissociate completely. Most instructors include calcium, strontium, and barium hydroxide as strong bases but exclude magnesium hydroxide. Follow your instructor's convention.
2 Types of Weak Electrolytes
Weak electrolytes dissociate only partially into ions when dissolved in water. They produce fewer ions than strong electrolytes, resulting in lower electrical conductivity. Weak electrolytes fall into 2 main categories: weak acids and weak bases.
Weak Acids
A weak acid is any acid that is not on the strong acid list. If a compound begins with H and is an acid, but you do not find it among the 7 strong acids, it is weak by default.
Example: Identifying Weak Acids
HF — Starts with H (acid), not on the strong acid list → weak acid
HNO₂ — Starts with H (acid), not on the strong acid list → weak acid
H₂CO₃ — Starts with H (acid), not on the strong acid list → weak acid
There is one additional category to recognize: organic acids (also called carboxylic acids). These end with the functional group COOH (sometimes written as CO₂H). Unlike other acids, organic acids do not begin with H in their formula—they end with it. All organic acids are weak acids.
Common Mistake: Do not confuse compounds ending in OH (alcohols or bases) with those ending in COOH (organic acids). Only the COOH group makes a compound an acid.
Weak Bases: Ammonia and Amines
Weak bases are harder to identify by a simple rule. The most important weak base to know is ammonia (NH₃). Compounds that resemble ammonia—where one or more hydrogen atoms on nitrogen are replaced by carbon-containing groups—are also weak bases. These are called amines.
Example: Weak Bases
NH₃ (ammonia) — the classic weak base
CH₃NH₂ (methylamine) — one hydrogen on nitrogen is replaced by a CH₃ group
In general, if you see a nitrogen-containing molecular compound that is not an acid, it is likely a weak base.
Nonelectrolytes: Molecular Compounds That Produce No Ions
Nonelectrolytes may dissolve in water, but they do not dissociate into ions. A nonelectrolyte solution does not conduct electricity.
To identify a nonelectrolyte, apply a process of elimination: if a compound is not an acid, not a base, and not an ionic compound, it is a nonelectrolyte. In other words, nonelectrolytes are molecular compounds (composed entirely of nonmetals) that are neither acids nor bases.
Example: Methanol (CH₃OH)
Methanol is composed entirely of nonmetals and is a molecular compound. Although it ends in OH, it does not end in COOH—so it is an alcohol, not an organic acid. Methanol dissolves readily in water but produces no ions, making it a nonelectrolyte.
Example: Glucose (C₆H₁₂O₆)
Glucose is a molecular compound made entirely of nonmetals. Despite being highly soluble in water, glucose does not dissociate into ions. It is therefore a nonelectrolyte.
Conductivity Test: If you dissolve equal concentrations of a strong electrolyte, a weak electrolyte, and a nonelectrolyte in water, the strong electrolyte solution will be the most conductive, the weak electrolyte solution will show moderate conductivity, and the nonelectrolyte solution will remain nonconductive.
Comparing All 3 Electrolyte Types
| Property | Strong Electrolyte | Weak Electrolyte | Nonelectrolyte |
|---|---|---|---|
| Dissociation | Complete (100%) | Partial | None |
| Ions in solution | Many | Few | None |
| Conductivity | High | Low | None |
| Examples | NaCl, HCl, NaOH | HF, NH₃, CH₃COOH | C₆H₁₂O₆, CH₃OH |
| How to recognize | Soluble ionic compounds, 7 strong acids, Group 1/2 hydroxides | Acids not on the strong list, ammonia and amines, organic acids (COOH) | Molecular compounds that are not acids or bases |
Quick Reference: How to Identify Electrolytes
Strong Electrolytes
1. Soluble Ionic Compounds
2. Strong Acids (HCl, HBr, HI, HClO4, HClO3, H2SO4, HNO3)
3. Strong Bases (Group I metal hydroxides, Ba(OH)2, Sr(OH)2, Ca(OH)2)
Weak Electrolytes
1. Weak Acids (ex. HF, HCN, CH3COOH, etc.)
2. Weak Bases (ex. NH3, CH3NH2, etc.)
Nonelectrolytes
Molecular compounds (except acids/bases) (ex. C6H12O6 and CH3OH)
Solubility Rules for Ionic Compounds in Water
Knowing which ionic compounds are soluble matters because only soluble ionic compounds act as strong electrolytes in aqueous solution. The solubility rules classify ionic compounds into two categories—soluble and insoluble—based on the identity of their anion.
In reality, solubility exists on a spectrum. These rules draw a convenient line: compounds more soluble than roughly 0.01 M are called "soluble," and those less soluble are called "insoluble." Even "insoluble" compounds dissolve to a tiny extent, and even "soluble" compounds eventually reach a saturation limit.
Do You Need to Memorize the Solubility Rules? About half of general chemistry courses require memorization; the other half provide them on exams. Check with your instructor.
6 Mostly Soluble Anions
If a salt contains one of the following 6 anions, it is usually soluble:
| Anion | Solubility | Exceptions (Insoluble) |
|---|---|---|
| Nitrate (NO₃⁻) | Always soluble | No exceptions |
| Acetate (CH₃COO⁻) | Always soluble | No exceptions |
| Chloride (Cl⁻) | Usually soluble | Ag⁺, Hg₂²⁺, Pb²⁺ |
| Bromide (Br⁻) | Usually soluble | Ag⁺, Hg₂²⁺, Pb²⁺ |
| Iodide (I⁻) | Usually soluble | Ag⁺, Hg₂²⁺, Pb²⁺ |
| Sulfate (SO₄²⁻) | Usually soluble | Ba²⁺, Ca²⁺, Sr²⁺, Hg₂²⁺, Pb²⁺ |
Example: Applying the Solubility Rules
MgCl₂ — Contains Cl⁻. Magnesium is not Ag, Hg, or Pb → soluble
Hg₂I₂ — Contains I⁻. Mercury(I) is an exception → insoluble
K₂SO₄ — Contains SO₄²⁻. Potassium is not an exception → soluble
BaSO₄ — Contains SO₄²⁻. Barium is an exception → insoluble
Cation-Based Shortcuts: All Group 1 metal salts (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and all ammonium salts (NH₄⁺) are soluble, regardless of the anion. These rules are not separate from the anion-based rules—they are consistent with them.
4 Mostly Insoluble Anions
If a salt contains one of the following 4 anions, it is usually insoluble:
| Anion | Solubility | Exceptions (Soluble) |
|---|---|---|
| Sulfide (S²⁻) | Usually insoluble | NH₄⁺, Group 1, Ca²⁺, Sr²⁺, Ba²⁺ |
| Carbonate (CO₃²⁻) | Usually insoluble | NH₄⁺, Group 1 |
| Phosphate (PO₄³⁻) | Usually insoluble | NH₄⁺, Group 1 |
| Hydroxide (OH⁻) | Usually insoluble | NH₄⁺, Group 1, Ca²⁺, Sr²⁺, Ba²⁺ |
Example: Applying the Solubility Rules
CuS — Contains S²⁻. Copper is not an exception → insoluble
(NH₄)₂S — Contains S²⁻. Ammonium is an exception → soluble
Na₂CO₃ — Contains CO₃²⁻. Sodium is Group 1 → soluble
CaCO₃ — Contains CO₃²⁻. Calcium is not an exception for carbonates → insoluble
Ba₃(PO₄)₂ — Contains PO₄³⁻. Barium is not an exception → insoluble
KOH — Contains OH⁻. Potassium is Group 1 → soluble
Sulfides vs. Carbonates: Notice that sulfide and hydroxide exceptions include the larger Group 2 metals (Ca²⁺, Sr²⁺, Ba²⁺), while carbonate and phosphate exceptions do not. Calcium carbonate, for example, is insoluble—it is the main component of limestone and seashells.
Key Takeaways
- A solution consists of a solvent (usually water) and one or more solutes, which can be solids, liquids, or gases.
- An electrolyte is any substance that dissociates into ions in water, making the aqueous solution electrically conductive. Pure water alone does not conduct electricity.
- Strong electrolytes dissociate completely and include soluble ionic compounds, the 7 strong acids (HCl, HBr, HI, HClO₃, HClO₄, H₂SO₄, HNO₃), and Group 1/Group 2 metal hydroxides.
- Weak electrolytes dissociate only partially and include weak acids (any acid not on the strong list, plus organic acids ending in COOH) and weak bases (ammonia and amines).
- Nonelectrolytes dissolve in water but produce no ions. They are molecular compounds that are neither acids nor bases.
- The solubility rules predict whether an ionic compound dissolves in water based on its anion. Nitrate and acetate salts are always soluble; sulfide, carbonate, phosphate, and hydroxide salts are generally insoluble except with Group 1 cations and ammonium.
- For the halides (Cl⁻, Br⁻, I⁻), watch for the silver, mercury(I), and lead exceptions. For sulfates, watch for barium, calcium, strontium, mercury(I), and lead.