Properties of Solutions Objectives: Identify the factors that determine the rate at which a solute dissolves. Identify the factors that affect solubility

Definitions Solution - homogeneous mixture Solute - substance being dissolved Solvent – does the dissolving (present in greater amount)

Solutions What the solute and the solvent are determines whether a substance will dissolve and how much will dissolve. Stirring In order to dissolve the solvent molecules must touch the solute. Solvent molecules hold on to and surround the solute Stirring moves fresh solvent next to the solute. Dissolves faster Particle Size The solvent touches the surface of the solute. Smaller pieces increase the amount of surface of the solute. Solvent and solute touch each other more often Smaller particles dissolve faster Temperature Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often. More pieces are broken off Speeds up dissolving. Usually increases the amount of solid that will dissolve.

Solution Definitions solution: a homogeneous mixture in which the solute is dissolved in the solvent -e.g., Salt water alloy: a solid solution of metals -- e.g., bronze = Cu + Sn; brass = Cu + Zn refers to two gases or two liquids that form a solution; more specific than “soluble” miscible: Solutions – A homogeneous mixture in which substances present in lesser amounts, called solutes, are dispersed uniformly throughout the substance in the greater amount, the solvent – Aqueous solution — a solution in which the solvent is water – Nonaqueous solution — any substance other than water is the solvent – Water is essential for life and makes up about 70% of the mass of the human body. – Many of the chemical reactions that are essential for life depend on the interaction of water molecules with dissolved compounds. -- e.g., food coloring and water

Solutions Solute Solvent Solution Gaseous Solutions Liquid Solutions air (nitrogen, oxygen, argon gases) humid air (water vapor in air) Liquid Solutions solid carbonated drinks (CO2 in water) vinegar (CH3COOH in water) salt water (NaCl in water) Solid Solutions dental amalgam (Hg in Ag) sterling silver (Cu in Ag) Solutions are not limited to gases and liquids; solid solutions also exist. • Amalgams, which are usually solids, are solutions of metals in liquid mercury. • Network solids are insoluble in all solvents with which they do not react chemically; covalent bonds that hold the network together are too strong to be broken and are much stronger than any combination of intermolecular interactions that might occur in solution. • Most metals are insoluble in all solvents but do react with solutions such as aqueous acid or base to produce a solution; in these cases the metal undergoes a chemical transformation that cannot be reversed by removing the solvent. Charles H.Corwin, Introductory Chemistry 2005, page 369

Factors Affecting the Rate of Solution As To , rate 1. temperature As size , rate 2. particle size More mixing, rate 3. mixing Formation of a solution from a solute and a solvent is a physical process, not a chemical one. Both solute and solvent can be recovered in chemically unchanged form using appropriate separation methods. Dissolution of a solute in a solvent to form a solution does not involve a chemical transformation. Substances that form a single homogeneous phase in all proportions are said to be completely miscible in one another. If two substances are essentially insoluble in each other, they are immiscible. 4. Amount of solute already dissolved ↓ conc ↑ rate

Non-Solution Definitions insoluble: “will NOT dissolve in” e.g., sand and water immiscible: refers to two gases or two liquids that will NOT form a solution e.g., water and oil suspension: appears uniform while being stirred, but settles over time e.g., flour and water

Classes of Solutions aqueous solution: solvent = water water = “the universal solvent” amalgam: solvent = Hg e.g., dental amalgam tincture: solvent = alcohol (alcohol won’t dissolve salt but will dissolve iodine which won’t dissolve in water) e.g., tincture of iodine (for cuts) organic solution: solvent contains carbon e.g., gasoline, benzene, toluene, hexane

Solubility Experiment 1: Add 1 drop of red food coloring A B A B Before Water HOT AFTER COLD A B Miscible – “mixable” two gases or two liquids that mix evenly Water COLD Water HOT You should observe that temperature effects the rate of solution. As the temperature of the liquid solvent increases, the molecules move faster, and the food coloring dissolves more quickly. A B

Solubility Experiment 2: Add oil to water and shake T0 sec T30 sec AFTER Before Immiscible – “does not mix” two liquids or two gases that DO NOT MIX Oil You should observe that for solutions to mix they must be chemically similar. Polar and polar molecules will mix, non-polar and non-polar molecules will mix, but polar and non-polar molecules will not mix. The reasons for this will be explained later. Remember, ‘like dissolves like’. polar dissolves polar non-polar dissolves non-polar Water Water T0 sec T30 sec

Muddy Water: Dissolved Solids Experiment 3: Add soil to water, shake well, and allow to settle AFTER Before Muddy Water Water T1 min T5 min

Centrifugation Spin sample very rapidly: denser materials go to bottom (outside) Separate blood into serum and plasma Serum (clear) Plasma (contains red blood cells ‘RBCs’) Check for anemia (lack of iron) AFTER Before RBC’s Serum Blood When you donate blood, the phlebotomist will check for anemia before drawing your blood. A small amount of blood will be taken from your fingertip into a thin glass tube. The blood may be placed in a centrifuge as described above to check for sufficient amounts of red blood cells. An alternate method is to drop the blood in a solution of copper sulfate. If the blood contains enough RBC’s the blood drop will sink to the bottom of the container, if it is not dense enough (lacks adequate number of RBC’s), the blood will float and you will not be allowed to donate blood. A B C

Blood plasma (a solution) White blood cells Red blood cells Blood (a suspension) Centrifuge Blood plasma (a solution) White blood cells Red blood cells Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Solubility Solubility= maximum quantity of a solute that can be dissolved in a certain quantity of solvent at a specified temperature. Ex. 36 g of salt will dissolve in 100 g of water at 20oC.

SUPERSATURATED SOLUTION Solubility UNSATURATED SOLUTION Contains less solute than can be dissolved at a given temperature SUPERSATURATED SOLUTION Has dissolved in it more solute than it can normally hold at a given temperature. SATURATED SOLUTION Has dissolved in it all the solute that it can normally hold at a given temp. Maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure is its solubility. – Solubility is expressed as the mass of solute per volume (g/L) or mass of solute per mass of solvent (g/g) or as the moles of solute per volume (mol/L). – Solubility of a substance depends on energetic factors and on the temperature and, for gases, the pressure. • A solution that contains the maximum possible amount of solute is saturated. • If a solution contains less than the maximum amount of solute, it is unsaturated. When a solution is saturated and excess solute is present, the rate of dissolution is equal to the rate of crystallization. • Solubility increases with increasing temperature — a saturated solution that was prepared at a higher temperature contains more dissolved solute than it would contain at a lower temperature, when the solution is cooled, it can become supersaturated. increasing concentration

3 factors that effect solubility 1. Nature of the solute and solvent 2. Temperature 3. Pressure

1. Nature of solute and solvent “Rule”: LIKE DISSOLVES LIKE polar solvents dissolve polar solutes. (ex. Salt in water) Non-polar solvents dissolve non-polar solutes. (Styrofoam in acetone) Water is polar (because it’s bent). It will therefore tend to dissolve other polar molecules or ions. For example, most salts, alcohols and sugars dissolve in water. Alcohols and sugars all contain the O-H part of a molecule which makes them polar: O X . . H

2. Temperature Increase in temp = increase in solubility of a solid substance Increase in temp = decrease in solubility of gaseous substance (pop when warmed loses its fizz faster)

Solubility vs. Temperature 200 180 160 140 120 100 80 60 40 20 KI KNO3 Solubility (g solute / 100 g H2O) NaNO3 The general rule of thumb is that solubility of solids increases with increases in temperature. Maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure is its solubility. – Solubility is expressed as the mass of solute per volume (g/L) or mass of solute per mass of solvent (g/g) or as the moles of solute per volume (mol/L). – Solubility of a substance depends on energetic factors and on the temperature and, for gases, the pressure. • A solution that contains the maximum possible amount of solute is saturated. • If a solution contains less than the maximum amount of solute, it is unsaturated. When a solution is saturated and excess solute is present, the rate of dissolution is equal to the rate of crystallization. • Solubility increases with increasing temperature — a saturated solution that was prepared at a higher temperature contains more dissolved solute than it would contain at a lower temperature, when the solution is cooled, it can become supersaturated. Solubility of a substance generally increases with increasing temperature No relationship between the structure of a substance and the temperature dependence of its solubility Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds This variation of solubility with temperature is used to separate the components of a mixture by fractional crystallization, the separation of compounds based on their solubilities in a given solvent Fractional crystallization is a common technique for purifying compounds; the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution Solubility of gases in liquids decreases with increasing temperature Attractive intermolecular interactions in the gas phase are essentially zero for most substances When a gas dissolves, its molecules interact with solvent molecules and heat is released when these new attractive interactions form, therefore, dissolving most gases in liquids is an exothermic process (Hsoln < 0) Adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas Na3PO4 NaCl 20 40 60 80 100 Temperature (oC) Timberlake, Chemistry 7th Edition, page 297

Gas Solubility Higher Temperature …Gas is LESS Soluble CH4 O2 2.0 O2 Higher Temperature …Gas is LESS Soluble CO Solubility (mM) 1.0 The general rule of thumb is that the solubility of gases decreases when temperature increases. Solubility of a substance generally increases with increasing temperature No relationship between the structure of a substance and the temperature dependence of its solubility Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds This variation of solubility with temperature is used to separate the components of a mixture by fractional crystallization, the separation of compounds based on their solubilities in a given solvent Fractional crystallization is a common technique for purifying compounds; the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution Solubility of gases in liquids decreases with increasing temperature Attractive intermolecular interactions in the gas phase are essentially zero for most substances When a gas dissolves, its molecules interact with solvent molecules and heat is released when these new attractive interactions form, therefore, dissolving most gases in liquids is an exothermic process (Hsoln < 0) Adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas He 10 20 30 40 50 Temperature (oC)

Solids dissolved in liquids Gases dissolved in liquids To Sol. To Sol. As To , solubility As To , solubility

3. Pressure Gases are more soluble at... high pressures (Henry’s Law). External pressure has very little effect on the solubility of liquids and solids, but the solubility of gases increases as the partial pressure of the gas above a solution increases. The concentration of molecules in the gas phase increases with increasing pressure, and the concentration of dissolved gas molecules in the solution at equilibrium is also higher at higher pressures. Relationship between pressure and the solubility of a gas is described quantitatively by Henry’s Law: C = kP, where C is the concentration of dissolved gas at equilibrium; P is the partial pressure of the gas; and k is the Henry’s law constant, which must be determined experimentally for each combination of gas, solvent, and temperature and has units of mol/(L•atm) = M/atm. Concentration of a dissolved gas in water at a given pressure depends strongly on its physical properties. For a series of related substances, London dispersion forces increase as molecular mass increases with the Henry’s law constants increasing smoothly. Gases that react chemically with water do not obey Henry’s law; all of these gases are much more soluble than predicted by Henry’s law

Henry’s Law Pressure does not effect solids and liquids (only gases) Henrys Law: As the pressure of the gas above a liquid increases, the solubility of the gas increases.

Carbonated beverages Bottled under high pressure to force the carbon dioxide gas into the liquid

Equation: S1 = S2 P1 P2 Units for Solubility: g/L, g/mL, g/g Units for Pressure: kPa or atm

Henry’s Law Example: If the solubility of a gas in water is 0.77g/L at 3.5 atm of pressure, what is its solubility (ing/L) at 1.0 atm of pressure? (Temp is held constant) Henry’s Law: S1 = S2 P1 P2 (0.77 g/L • 1.0 atm)= S2 (3.5 atm.) 0.77 g/L = S2 3.5 atm. 1.0atm. S2= 0.22g/L