ACT Science: Solubility, Saturation, and Concentration Basics

Solubility is the maximum amount of solute that can dissolve in a solvent at a given temperature. A saturated solution contains the maximum amount of dissolved solute (at equilibrium). An unsaturated solution contains less than the maximum. Solubility is usually expressed as grams of solute per 100 mL of solvent. Key insight: solubility changes with temperature. For most solids, increasing temperature increases solubility. For gases, increasing temperature decreases solubility. This relationship is tested on ACT Science passages involving solution properties.

Example: At 25°C, sugar solubility is 200 g/100 mL water. A solution with 150 g sugar/100 mL is unsaturated (room for more). A solution with 200 g sugar/100 mL is saturated (maximum dissolved).

Scenario 1 (Temperature effect): At 20°C, salt solubility is 36 g/100 mL. At 40°C, solubility is 47 g/100 mL. A saturated solution at 20°C (with 36 g dissolved) is heated to 40°C. It becomes unsaturated because the solubility increased. More salt could dissolve. Scenario 2 (Saturation check): NaCl solubility is 36 g/100 mL at 25°C. You dissolve 30 g in 100 mL. Is it saturated? No, unsaturated (30<36). Scenario 3 (Gas solubility): CO₂ solubility in water decreases as temperature increases. A warm soda has fewer bubbles than a cold soda because CO₂ is less soluble at higher temperature. For each scenario, compare the dissolved amount to the solubility limit.

Practice: If solubility is 50 g/100 mL and 60 g is dissolved in 100 mL, the solution is supersaturated (unstable). Crystals will precipitate until saturation is reached.

Problem 1: At 30°C, KNO₃ solubility is 46 g/100 mL. You dissolve 40 g in 100 mL. Is the solution saturated? No, 40<46; unsaturated. Problem 2: A solution is saturated with 70 g of salt in 100 mL at 25°C. You heat it to 50°C, where solubility is 80 g/100 mL. Is it still saturated? No, now unsaturated because solubility increased. Problem 3: At 20°C, AgNO₃ solubility is 122 g/100 mL. You dissolve 100 g in 100 mL. Is it saturated? No, unsaturated (room for 22 g more). Complete all three daily until you instantly identify saturated vs. unsaturated.

Remember: saturated = dissolved amount = solubility limit. unsaturated = dissolved amount < solubility limit.

Solubility and saturation questions appear in 1-2 ACT Science chemistry passages. They test conceptual understanding of solution properties. Mastering saturation and solubility concepts yields 1-2 guaranteed points because the definitions are clear and the logic is straightforward.

Review this topic one day before the test. By test day, you will answer solubility questions with confidence.