Chapter 5: Reactions Between Ions in Aqueous Solutions • A solution is a homogeneous mixture in which the two or more components mix freely • The solvent is taken as the component present in the largest amount • A solute is any substance dissolved in the solvent
Formation of a solution of iodine molecules in ethyl alcohol. Ethyl alcohol is the solvent and iodine the solute . Solutions have variable composition. They may be characterized using a solute- to-solvent ratio called the concentration .
• For example, the percentage concentration is the number of grams of solute per 100 g of solution • The relative amounts of solute and solvent are often given without specifying the actual quantities The dilute solution on the left has less solute per unit volume than the (more) concentrated solution on the right. Concentrated and dilute are relative terms.
• There is usually a limit to the amount of solute that can dissolve in a given amount of solvent – For example, 36.0 g NaCl is able to dissolve in 100 g of water at 20°C • A solution is said to be saturated when no more solute can be dissolved at the current temperature • The solubility of a solute is the number of grams of solute that can dissolve in 100 grams of solvent at a given temperature
• Solubilities of some common substances Substance Formula Solubility (g/100 g water) Sodium chloride NaCl 35.7 at 0°C 39.1 at 100°C Sodium hydroxide NaOH 42 at 0°C 347 at 100°C Calcium carbonate CaCO 3 0.0015 at 25°C A solution containing less solute is called unsaturated because it is able to dissolve more solute.
• Solubility usually increases with temperature • Supersaturated solutions contain more solute than required for saturation at a given temperature • They can be formed, for example, by careful cooling of saturated solutions • Supersaturated solutions are unstable and often result in the formation of a precipitate
• A precipitate is the solid substance that separates from solution • Precipitates can also form from reactions • Reactions that produce a precipitate are called precipitation reactions • Many ionic compounds dissolve in water • Solutes that produce ions in solution are called electrolytes because their solutions can conduct electricity
• An ionic compounds dissociates as it dissolves in water Ions separate from the solid and become hydrated or surrounded by water molecules. The ions move freely and the solution is able to conduct electricity. Ionic compounds that dissolve completely are strong electrolytes
• Most solutions of molecular compounds do not conduct electricity and are called nonelectrolytes The molecules of a nonelectrolyte separate but stay intact. The solution is nonconducting because no ions are generated.