Solutions Chemistry – Chapter 15

What Are Solutions?  Characteristics of Solutions  A substance that dissolves in a solvent is soluble in that solvent.  Sugar is soluble in water  A substance that does not dissolve in a solvent is insoluble in that solvent.  Sand is insoluble in water  Immiscible – two liquids that do not mix  Miscible – two liquids that do mix

 Solvation in Aqueous Solutions  Solvation – process of surrounding solute particles with solvent particles to form a solution  Hydration – solvation in water  “Like dissolves like” is the general rule to determine whether solvation will occur  Bonding and polarity of particles and intermolecular forces between particles  Aqueous solutions of ionic compounds  Ex: for sodium chloride, charged ends of water molecules attract positive sodium ions and negative chloride ions  Solvation continues until entire crystal has dissolved and all ions are distributed throughout the solvent  Aqueous solutions of molecular compounds  Ex: sucrose (sugar) dissolves in water b/c it has many O-H bonds, which are polar

 Three common ways to increase rate of solvation: 1. Agitating the mixture – moves dissolved solute particles away from contact surfaces quickly, allowing new collisions between solute and solvent particles 2. Increasing the surface area of the solute 3. Increasing the temp. of the solvent – increases kinetic energy, resulting in more frequent collisions  Heat of solution – overall energy change that occurs during solution formation  Energy is required to overcome the attractive forces within the solute and within the solvent (endothermic)  When solute and solvent particles mix, the particles attract each other and energy is released (exothermic)

 Solubility – amount of solute that will dissolve in a given amount of solvent at a specified temp. and pressure  Saturated solution - contains the max amount of dissolved solute for a given amount of solvent at a specific temp. and pressure  Unsaturated solution – contains less dissolved solute for a given temp. and pressure (more solute can be dissolved)  Factors That Affect Solubility  Temperature  Many substances are more soluble at high temps than at low temps  Exception – solubility of a gaseous solute decreases as temp increases

 Supersaturated solution – contains more dissolved solute than a saturated solution at the same temp  A saturated solution is formed at a high temp and then cooled slowly to make a supersaturated solution  Very unstable  Crystallization can occur if inside of container is scratched or the solution undergoes a physical shock  Cloud seeding  Pressure  Solubility of a gas in any solvent increases as its external pressure (pressure above the solution) increases  Ex: carbonated beverages  Henry’s Law – states that at a given temp, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid S 1 = S 2 P 1 P 2

Solution Concentration  Concentration – measure of how much solute is dissolved in a specific amount of solvent  Expressing Concentration – table 15-3  Using % to Describe Concentration  % by mass usually involves a solid dissolved in a liquid % by mass = mass of solute/mass of solution x 100  % by volume is used when both solute and solvent are liquids % by volume = volume of solute/volume of solution x 100

 Molarity (M) – number of moles of solute dissolved per liter of solution (molar concentration) M = moles of solute/liters of solution  Preparing Molar Solutions  Must use slightly less than one liter of water to make one liter of solution  Ex: You cannot add 513 g of sugar to one liter of water to make 1.5M solution b/c sugar takes up space and adds volume to the solution  Diluting solutions (in lab) M 1 V 1 = M 2 V 2  Volume of solutions can change w/ temp., altering molarity  Molality (m) – ratio of number of moles of solute dissolved in one kilogram of solvent m = moles of solute/ kg of solvent

Heterogeneous Mixtures  Suspension – mixture containing particles that settle out if left undisturbed  Gravity acts on suspended particles in a short time  Cornstarch and water is a thixotropic mixture  Settle out cornstarch particles form a solid-like state on the bottom of the container  When stirred the solid-like state quickly begins flowing like a liquid  Colloid – heterogeneous mixture of intermediate size particles (between size of solution particles and suspension particles)  Ex: milk  Can’t separate its components by settling or by filtration

 Brownian motion – jerky, random, and erratic movement of colloid particles  Results from collisions of particles of the dispersion medium w/ dispersed particles  The Tyndall effect – scattering of light caused by dispersed colloid particles  Ex: rays of sunlight passing through smoke; lights through fog