Compounds in Aqueous Solution

Introduction

What Happens When Compounds Dissolve?

When compounds dissolve, their individual molecules or ions separate from one another and become surrounded by solvent molecules — a process called solvation (or hydration when water is the solvent). The video below shows the dissolution of potassium permanganate (KMnO₄), and the animation provides a molecular-scale view of what is happening.

Dissolution of KMnO₄ — video by Charles D. Winters

Think about it: What do you see happening at the molecular scale that you cannot observe in the macroscopic video?

Strong Electrolytes

The presence of hydrated ions in a solution affects its electrical conductivity. Compounds that dissolve completely into ions are called strong electrolytes. These include soluble ionic compounds and strong acids.

Watch the animations of dissolution for the strong electrolytes NaCl, NaNO₃, and HNO₃. Notice how each compound separates completely into its constituent ions.

NaCl — ionic compound (strong electrolyte)

Key idea: Strong electrolytes dissociate completely in water. The solution contains essentially no intact formula units — only free hydrated ions.

Nonelectrolytes

There are many examples of compounds that dissolve in water but do not form ions — such as sucrose (C₁₂H₂₂O₁₁) and ethanol (C₂H₅OH). These are called nonelectrolytes. Their aqueous solutions do not conduct electricity.

Watch the animation of the nonelectrolyte methanol (CH₃OH) and compare it with the strong electrolyte NaCl. In what ways are they similar? In what ways do they differ?

CH₃OH — nonelectrolyte

Compare: In the NaCl animation, individual Na⁺ and Cl⁻ ions are pulled away by water molecules. What do you see happening differently with CH₃OH?

Weak Electrolytes

Weak electrolytes, such as acetic acid, are compounds whose solutions conduct electricity only slightly. These compounds dissolve but ionize only partially — the solution contains a mixture of intact molecules and a small concentration of ions.

Compare the weak acid HNO₂ with the strong acid HNO₃. Notice the difference in the extent of ionization shown in each animation.

HNO₂ — weak acid (weak electrolyte)

Notice: In the HNO₂ animation, most molecules remain intact. In HNO₃, essentially all molecules have ionized. This difference in ionization extent defines strong vs. weak electrolytes.