Chapter 5Solutions

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Solution• homogeneous mixtures

composition may vary from one sample to anotherappears to be one substance, though really contains

multiple materials

• most homogeneous materials we encounter are actually solutionse.g., air and sea water

• nature has a tendency toward spontaneous mixinggenerally, uniform mixing is more energetically

favorable

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Solutions• solute is the dissolved substance

seems to “disappear”“takes on the state” of the solvent

• solvent is the substance solute dissolves indoes not appear to change state

• when both solute and solvent have the same state, the solvent is the component present in the highest percentage

• solutions in which the solvent is water are called aqueous solutions

4

Seawater• drinking seawater will dehydrate you and give

you diarrhea

• the cell wall acts as a barrier to solute moving

• the only way for the seawater and the cell solution to have uniform mixing is for water to flow out of the cells of your intestine and into your digestive tract

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Common Types of Solution

Solution PhaseSolute Phase

Solvent Phase Example

gaseous solutions gas gas air (mostly N2 & O2)

liquid solutions

gas

liquid

solid

liquid

liquid

liquid

soda (CO2 in H2O)

vodka (C2H5OH in H2O)

seawater (NaCl in H2O)

solid solutions solid solid brass (Zn in Cu)

• solutions that contain Hg and some other metal are called amalgams

• solutions that contain metal solutes and a metal solvent are called alloys

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BrassType Color % Cu % Zn Density

g/cm3

MP

°C

Tensile

Strength

psi

Uses

Gilding redish 95 5 8.86 1066 50K pre-83 pennies,munitions, plaques

Commercial bronze 90 10 8.80 1043 61K door knobs,grillwork

Jewelry bronze 87.5 12.5 8.78 1035 66K costume jewelry

Red golden 85 15 8.75 1027 70K electrical sockets,fasteners & eyelets

Low deep yellow

80 20 8.67 999 74K musical instruments,clock dials

Cartridge yellow 70 30 8.47 954 76K car radiator cores

Common yellow 67 33 8.42 940 70K lamp fixtures,bead chain

Muntz metal yellow 60 40 8.39 904 70K nuts & bolts,brazing rods

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Solubility• when one substance (solute) dissolves in another

(solvent) it is said to be solublesalt is soluble in waterbromine is soluble in methylene chloride

• when one substance does not dissolve in another it is said to be insolubleoil is insoluble in water

• the solubility of one substance in another depends on two factors – nature’s tendency towards mixing, and the types of intermolecular attractive forces

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Spontaneous Mixing

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Solubility• there is usually a limit to the solubility of one

substance in anothergases are always soluble in each othertwo liquids that are mutually soluble are said to be

miscible alcohol and water are miscibleoil and water are immiscible

• the maximum amount of solute that can be dissolved in a given amount of solvent is called the solubility

• the solubility of one substance in another varies with temperature and pressure

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Mixing and the Solution ProcessEntropy

• formation of a solution does not necessarily lower the potential energy of the system the difference in attractive forces between atoms of

two separate ideal gases vs. two mixed ideal gases is negligible

yet the gases mix spontaneously

• the gases mix because the energy of the system is lowered through the release of entropy

• entropy is the measure of energy dispersal throughout the system

• energy has a spontaneous drive to spread out over as large a volume as it is allowed

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Intermolecular Forces and the Solution ProcessEnthalpy of Solution

• energy changes in the formation of most solutions also involve differences in attractive forces between particles

• must overcome solute-solute attractive forces endothermic

• must overcome some of the solvent-solvent attractive forcesendothermic

• at least some of the energy to do this comes from making new solute-solvent attractionsexothermic

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Intermolecular Attractions

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Relative Interactions and Solution Formation

• when the solute-to-solvent attractions are weaker than the sum of the solute-to-solute and solvent-to-solvent attractions, the solution will only form if the energy difference is small enough to be overcome by the entropy

Solute-to-Solvent >Solute-to-Solute +

Solvent-to-SolventSolution Forms

Solute-to-Solvent =Solute-to-Solute +

Solvent-to-SolventSolution Forms

Solute-to-Solvent <Solute-to-Solute +

Solvent-to-SolventSolution May or May Not Form