Solutions and Concentration

Objective: Identify components and distinguish homogeneous mixtures.

A solution is a homogeneous mixture at the scale of observation, with one or more solutes distributed through a solvent. The solvent is generally the component present in the greater amount in familiar liquid solutions; it need not always be water. Dissolving salt in water disperses ions rather than creating new elements. Solutions also include gases such as air and solid alloys. A suspension can settle, while a colloid contains dispersed particles that do not readily settle and can scatter light. Physical separation and composition help distinguish a mixture from a pure compound. The terms describe how substances are arranged, not a fixed recipe.

Objective: Calculate concentration with clearly stated units.

Concentration compares an amount of solute to a specified amount of solution or solvent. Mass concentration may be expressed as grams of solute per liter of solution; for 10 g dissolved to make 2.0 L total solution, it is 5.0 g/L. Mass percent equals solute mass divided by total solution mass times 100; 5 g solute in 100 g solution is 5%. Molarity is moles of solute per liter of solution, and requires converting mass to moles if mass is given. Always distinguish final solution volume from starting solvent volume and include units. A concentrated solution has more solute per specified amount than a dilute one, without implying saturation.

Objective: Use the simple dilution relationship appropriately.

Adding solvent to a solution lowers concentration when the amount of solute remains constant. In compatible molarity and volume units, C₁V₁ = C₂V₂ follows from equal initial and final solute amounts. For 100 mL of 2.0 mol/L stock diluted to 200 mL total volume, the final concentration is 1.0 mol/L. The final volume is the total volume after dilution, not the volume of solvent added. Volume additivity can be imperfect in real mixtures, so the target final volume should be measured. This equation does not describe a chemical reaction that consumes solute, and should be used only when its concentration definitions and conditions match.