Lesson 8.1: A solution is a type of mixture

Lesson Focus

Solutions are special mixtures where substances blend completely. We'll explore what defines a solution, its parts, and how they interact to change physical properties like freezing and boiling points.

Learning Objectives

• Differentiate solutions from other mixtures by their unique, uniform properties.
• Identify and define the two main parts of any solution: the solute and the solvent.
• Explain how adding a solute changes a solvent's physical properties, like its boiling and freezing points.
• Observe how the components of a solution can be physically separated through experimentation.

A solution is a special mixture where substances are so evenly blended they look like one thing. Unlike a sand-and-water mix, you can't see the separate parts. This type of mixture is called homogeneous because all portions of it have the same properties. Think of saltwater versus a chunky salad.

A solution has two parts: the solute and the solvent. The solute is the substance that dissolves, like sugar. The solvent is the substance that does the dissolving, like water. Usually, the substance present in the greater amount is the solvent. Water is called the universal solvent for this reason.

Ionic compounds, like salt (NaCl), dissolve by breaking into charged ions. Covalent compounds, like sugar (C₁₂H₂₂O₁₁), dissolve by separating into whole molecules. If particles are too large to dissolve, they form a cloudy suspension, which is not a solution. Can you filter a suspension but not a solution?

Adding a solute to a solvent makes the solution harder to freeze. The solute particles get in the way, lowering the freezing point. This is why salt is spread on icy roads—it creates a saltwater solution that remains liquid below 0°C. The more solute, the lower the freezing point.

A solute also makes a solvent harder to boil. The solute particles hold onto the solvent molecules, raising the boiling point. This is why adding antifreeze to a car's radiator helps prevent the water from boiling over in the summer. The solution can stay liquid at temperatures above 100°C.