Lesson 8.2: The amount of solute that dissolves can vary

Lesson Focus

Ever wonder why some drinks are stronger than others? This lesson explores concentration and solubility, revealing how temperature, pressure, and molecular structure determine how much of a substance can dissolve in another.

Learning Objectives

• Explain how a solution's concentration can vary, from dilute to saturated, based on the amount of solute it contains.
• Describe how factors like temperature and pressure can change a solute's solubility in a solvent.
• Recognize why "like dissolves like" and how molecular structure determines a substance's solubility.
• Design an experiment to demonstrate how temperature affects the solubility of a solid in water.

A solution's concentration measures the amount of solute in a solvent. Adding more solute makes it concentrated. A dilute solution has little solute, while a saturated one holds the maximum possible. A supersaturated solution, made by changing conditions like temperature, holds even more. Think: Is ocean water a dilute or concentrated solution?

Solubility is a property that measures how much solute can dissolve in a solvent at a given temperature. Ammonia has high solubility in water, so a lot dissolves. Carbon dioxide has low solubility, so little dissolves. A saturated solution of a highly soluble substance will be concentrated. Can a saturated solution also be dilute?

Changing temperature alters a solute’s solubility. For most solids like sugar, higher temperatures mean more can dissolve. The opposite is true for all gases; as temperature rises, gas solubility decreases, which is why warm soda goes flat. This explains why heated lake water can harm fish by holding less dissolved oxygen.

Pressure greatly affects how gases dissolve in liquids. Higher pressure forces more gas into a solution, increasing its solubility. This is why soda is bottled under high pressure. When you open the can, the pressure drops, the gas becomes less soluble, and it escapes as bubbles. This is why divers avoid rising too fast.

Why do oil and water not mix? It depends on molecular structure. Water is a polar solvent and dissolves other polar solutes like sugar. Oil is nonpolar and won't dissolve in water. The guiding principle is “Like dissolves like.” Based on this, would you predict that salt is polar or nonpolar? Why?