S o l u t i o n s Solute A solute is the dissolved substance in a solution. Salt in salt water Sugar in soda drinks Carbon dioxide in soda drinks Solvent A solvent is the dissolving medium in a solution. Water in salt water Water in soda “Like Dissolves Like” Nonpolar solutes dissolve best in nonpolar solvents Fats Steroids Benzene Hexane Waxes Toluene Polar and ionic solutes dissolve best in polar solvents Inorganic Salts Sugars Water Small alcohols Acetic acid Solubility Trends The solubility of MOST solids increases with temperature. The rate at which solids dissolve increases with increasing surface area of the solid. The solubility of gases decreases with increases in temperature. The solubility of gases increases with the pressure above the solution. Therefore… Solids tend to dissolve best when: o Heated o Stirred o Ground into small particles Gases tend to dissolve best when: o The solution is cold o Pressure is high Solubility Chart Saturation of Solutions A solution that contains the maximum amount of solute that may be dissolved under existing conditions is saturated. A solution that contains less solute than a saturated solution under existing conditions is unsaturated. A solution that contains more dissolved solute than a saturated solution under the same conditions is supersaturated. Definition of Electrolytes and Nonelectrolytes An electrolyte is: A substance whose aqueous solution conducts an electric current. A nonelectrolyte is: A substance whose aqueous solution does not conduct an electric current. Electrolytes vs. Nonelectrolytes The ammeter measures the flow of electrons (current) through the circuit. If the ammeter measures a current, and the bulb glows, then the solution conducts. If the ammeter fails to measure a current, and the bulb does not glow, the solution is non-conducting. Try to classify the following substances as electrolytes or nonelectrolytes… 1.Pure water 2.Tap water 3.Sugar solution 4.Sodium chloride solution 5.Hydrochloric acid solution 6.Lactic acid solution 7.Ethyl alcohol solution 8.Pure sodium chloride Answers to Electrolytes ELECTROLYTES: NONELECTROLYTES: Tap water (weak) Pure water NaCl solution Sugar solution HCl solution Ethanol solution Lactate solution (weak) Pure NaCl Ionic Compounds “Dissociate” NaCl(s) Na+(aq) + Cl-(aq) AgNO3(s) Ag+(aq) + NO3-(aq) MgCl2(s) Mg2+(aq) + 2 Cl-(aq) Na2SO4(s) 2 Na+(aq) + SO42-(aq) AlCl3(s) Al3+(aq) + 3 Cl-(aq) Ions tend to stay in solution where they can conduct a current rather than re-forming a solid. The reason for this is the polar nature of the water molecule… Positive ions associate with the negative end of the water dipole (oxygen). Negative ions associate with the positive end of the water dipole (hydrogen). Some covalent compounds IONIZE in solution Covalent acids form ions in solution, with the help of the water molecules. For instance, hydrogen chloride molecules, which are polar, give up their hydrogens to water, forming chloride ions (Cl-) and hydronium ions (H3O+). Strong acids such as HCl are completely ionized in solution. Other examples of strong acids include: Sulfuric acid, H2SO4 Nitric acid, HNO3 Hydriodic acid, HI Perchloric acid, HClO4 Weak acids such as lactic acid usually ionize less than 5% of the time. Many of these weaker acids are “organic” acids that contain a “carboxyl” group. The carboxyl group does not easily give up its hydrogen. Because of the carboxyl group, organic acids are sometimes called “carboxylic acids”. Other organic acids and their sources include: o o o o o o Citric acid – citrus fruit Malic acid – apples Butyric acid – rancid butter Amino acids – protein Nucleic acids – DNA and RNA Ascorbic acid – Vitamin C This is an enormous group of compounds; these are only a few examples.