Chapter 9 Properties of Water Properties of Water Surface Tension ...

Properties of WaterChapter 9Solutions• Because of the difference inelectronegativity, the O-H bond is polar• The oxygen has a partial negative charge(δ - ), while the H takes on a partial positivecharge (δ + )• Water is therefore a polar substanceProperties of Water• Hydrogen Bonds– Hydrogen is attracted to the stronglyelectronegative Oxygen atoms of anothermolecule– The bond is weaker than ionic or covalentbonds– The bond serves as an attraction betweenmolecules, holding water togetherSurface Tension• When the molecules on the surface of a liquidbecome more tightly packed than the rest of theliquid• Throughout the liquid, molecules are attracted toeach other from all directions• The result is that a needle can float on water,despite being more dense than the water• This is also the reason water drops are sphericalSurface Tension• Surfactant– Compound added to water which disrupts thehydrogen bonding between molecules– The result is loss of surface tension– Soaps and detergents are examples– Lungs produce a natural surfactant that allowsfor exchange of oxygen and carbon dioxide1

Solutions•A solution exists when one substance (thesolute) is uniformly dispersed throughoutanother substance (the solvent)• There is no reaction between the soluteand solvent• As such, they can be mixed in varyingquantitiesSolutions• Solvents and solutes may be solids,liquids, or gases• The solution has the same physical stateas the solvent• Gases easily form solutions because theparticles are moving so fast that there islittle interaction between gases• When solids or liquid form solutions, theremust be an attraction between particles• NaCl will dissolve inwater because the Na +and Cl - are attracted tothe dipoles of the watermolecule• Molecules such as I 2 , oils,or greases do notdissolve in water becausethey are nonpolarmolecules• “Like Dissolves Like”Solutions2

Solutions• Dissolving a solute– If we put a salt crystal in a beaker of water:– The dipoles of the water molecules begin towork upon the molecules of NaCl on the outeredges of the crystal– It takes the pull of many water molecules tobreak the bond between the Na and the Cl– Once broken, there is a constant pull of manywater molecules on each of the Na and Cl– This is the process of hydrationHydrates• Solid ionic compounds where watermolecules are part of the crystal structure• The water molecules are called water ofhydration• With the addition of heat, the bonds of thewater molecules are broken and the waterevaporates• The result is an anhydrantHydratesCuSO +4 • 5H2O(s)⇔ CuSO4(s)5H2O(g )Electrolytes• Solutions containing ions are conductors ofelectricity• Compounds from which ions come are calledelectrolytes• The degree dissociation of compoundsdetermines how strong/weak an electrolyte is– Strong electrolytes (i.e. NaCl) dissociate completely– Weak electrolytes (i.e. HF) do not; usually polarcovalent compounds• Strong ElectrolytesNaClMgCl• Weak ElectrolytesElectrolytesH2O( s)⎯⎯→H2O2( s)⎯⎯→NaIONIZATIONHF ⎯⎯ ⎯⎯ →H+( aq)MgRECOMBINATIONHF ←⎯⎯⎯⎯⎯⎯ H++ Cl2+( aq)+ F+−+ F−( aq)+ Cl−−( aq)Electrolytes• Nonelectrolytes– Polar covalent molecules that do notdissociate in water– Dissolve as whole moleculesC12H22O11( s)H2O⎯⎯→C12H22O11( aq)3

Equivalents• The amount of an ion needed to make 1 mole ofpositive or negative charge• For an ion with a charge of 2 + , there are 2equivalents for each moleSolubility• Amount of a solute that can dissolve in agiven amount of solvent at a giventemperature•Factors:– Type of solute or solvent– Temperature• Usually expressed grams of solute in 100grams of solventSolubility• If the solute dissolves easily, the solution is notat maximum capacity and is said to beunsaturated• When the solution contains all the solute it candissolve, it is saturated• Adding more solute to a saturated solution willresult in the solute sitting on the bottom of thecontainer• At saturation, an equilibrium exists between thedissolving and crystallizing processes4

Effect of Temperature• Solubility of solids increases withtemperature• Solubility of gases decreases withincreasing temperature– As the temperature increases, the moleculeshave more energy to escapeHenry’s Law• The solubility of a gas in a liquid is directlyrelated to the pressure of that gas above theliquid• For example, in a pop can, CO 2 is injected intothe space above the liquid, creating a highpressure, and forcing the CO 2 to dissolve• When the can is opened, the pressure drops,and so does the solubility. Gas bubbles escape.Insoluble Salts• Ionic compounds discussed up to nowhave been soluble in water – soluble salts• Ions that do not separate in water arecalled insoluble salts• Soluble salts almost always contain one ofthe following:–Li + , Na + , K + , NH 4+ , NO 3-• The attractive forces within insoluble saltsare too strong to be pulled apart5

Solubility• Formation of a solid– Mixing of two solutions may result in a solidprecipitate formingPercent ConcentrationConcentration =of a solutionamount of soluteamount of solution• Mass percent– Percent by mass of a solute within a solution8.0gKCl + 42.0g H2O= 50g solution– Both solution and solute are measured on abalanceMass % = mass of solute (g) × 100%8.0gof KClmass of solution (g) × 100% = 16%50gof solutionPercent Concentration• Volume Percentvolume (mL) solutevolume % =× 100%volume (mL) solution• Mass/Volume Percentmass (g) solutemass/volum e % =× 100%volume (mL) solution6

Molaritymoles of soluteMolarity (M) =Liters of solutionColloids• Large molecules in homogeneousmixtures that do not separate• Four types of colloids– Aerosols: liquid or solid dispersed in a gas– Foams: gas or liquid in solid– Emulsion: liquid in a liquid or solid– Sol: Solid in a liquid or solidSuspensions• Heterogeneous, non-uniform mixtures• Often have VERY large particles• Particles settle when agitation stops• Often include medicines or products whichdictate: Shake well before using7