SolutionsSolutions

Chapter 6Chapter 6

What Are Solutions?What Are Solutions?

• Solution: homogeneous mixture of 2 Solution: homogeneous mixture of 2 or more substancesor more substances– Solid, liquid, or gasSolid, liquid, or gas– Solvent: dissolving mediumSolvent: dissolving medium– Solute: substance that dissolvesSolute: substance that dissolves– When in solution, you cannot distinguish When in solution, you cannot distinguish

solvent and solutesolvent and solute

What is a Solution?What is a Solution?

• Soluble – a substance that can Soluble – a substance that can dissolve in a given solventdissolve in a given solvent– Miscible: two liquids that can dissolve in Miscible: two liquids that can dissolve in

each othereach other•Example: water and antifreezeExample: water and antifreeze

• Insoluble – substance cannot dissolveInsoluble – substance cannot dissolve– Immiscible: two liquids that cannot Immiscible: two liquids that cannot

dissolve in each otherdissolve in each other•Example: oil & waterExample: oil & water

Why Do Some Substances Why Do Some Substances Dissolve and not Others?Dissolve and not Others?

• To dissolve, solute particles must To dissolve, solute particles must dissociate from each other and mix dissociate from each other and mix with solvent particleswith solvent particles– Attractive forces between solute and Attractive forces between solute and

solvent must be greater than attractive solvent must be greater than attractive forces within the soluteforces within the solute

• Process of surrounding solute Process of surrounding solute particles with solvent particles is particles with solvent particles is called SOLVATIONcalled SOLVATION– In water, it is also called HYDRATIONIn water, it is also called HYDRATION

Aqueous Solutions of Ionic Aqueous Solutions of Ionic CompoundsCompounds

• Remember:Remember:– Water molecules are polar (+ and – ends)Water molecules are polar (+ and – ends)– Water molecules are in constant motionWater molecules are in constant motion

• When you put salt in water, water When you put salt in water, water molecules collide with surface of crystalmolecules collide with surface of crystal– Charged ends of water attract ions of saltCharged ends of water attract ions of salt– Dipole interaction (water/salt) is stronger Dipole interaction (water/salt) is stronger

than ions in crystal, so it pulls them awaythan ions in crystal, so it pulls them away

SolvationSolvation

Aqueous Solutions of Molecular Aqueous Solutions of Molecular CompoundsCompounds

• Water is also a good solvent for many Water is also a good solvent for many molecular compounds (Example: molecular compounds (Example: sugar)sugar)– Sugar has many O-H bonds (polar)Sugar has many O-H bonds (polar)– When water is added, the O-H bond When water is added, the O-H bond

becomes a site for hydrogen bonding with becomes a site for hydrogen bonding with waterwater

– Water’s hydrogen bonds pulls the sugar Water’s hydrogen bonds pulls the sugar molecules apartmolecules apart

– Oil is not a good solute because it has Oil is not a good solute because it has many C-H bonds (not polar) and few or no many C-H bonds (not polar) and few or no O-H (polar) bondsO-H (polar) bonds

Factors that Affect Solvation Factors that Affect Solvation RateRate• Increase Solvation Rate (Dissolve Increase Solvation Rate (Dissolve

Faster) by:Faster) by:– Agitation (stirring)Agitation (stirring)– Increase surface area (make particles Increase surface area (make particles

smaller)smaller)– Temperature (make it hotter)Temperature (make it hotter)

• All these increase the number of All these increase the number of collision between water and the collision between water and the solute solute

SolubilitySolubility• Only a limited amount of solute can Only a limited amount of solute can

dissolve in a given amount of solventdissolve in a given amount of solvent

• Every solute is unique for the solventEvery solute is unique for the solvent

• This is ‘Solubility’ – the amount of This is ‘Solubility’ – the amount of solute that can dissolve in a given solute that can dissolve in a given amount of solvent at a specified amount of solvent at a specified temperature and pressuretemperature and pressure

Solubility ContinuedSolubility Continued

• Solubility can also be understood at the Solubility can also be understood at the particle level:particle level:– As particles collide, some particles are As particles collide, some particles are

deposited back to the solutedeposited back to the solute– Some particles are removed from the solute.Some particles are removed from the solute.– When the rate of deposit equals the rate of When the rate of deposit equals the rate of

solvation, then the solution is SATURATEDsolvation, then the solution is SATURATED

• Saturated Solution – no more solute can Saturated Solution – no more solute can be dissolved in the solvent at this be dissolved in the solvent at this temperature and pressuretemperature and pressure

• Unsaturated Solution – there is still room Unsaturated Solution – there is still room for more solute to be dissovedfor more solute to be dissoved

Factors that Affect SolubilityFactors that Affect Solubility

• Most substances are MORE soluble at Most substances are MORE soluble at high temperature than at lowhigh temperature than at low– If you dissolve a substance until If you dissolve a substance until

saturated at high temperature and then saturated at high temperature and then reduce the temperature, the solution reduce the temperature, the solution becomes “supersaturated”becomes “supersaturated”

– Supersaturated solutions are unstableSupersaturated solutions are unstable•A small change makes the solute reappearA small change makes the solute reappear

•Rock candy worked that way. How?Rock candy worked that way. How?

Gas Solubility is Opposite Gas Solubility is Opposite SolidsSolids• Gas solubility DECREASES with Gas solubility DECREASES with

temperaturetemperature

• Gas solubility decreases with shakingGas solubility decreases with shaking

Colligative Properties of Colligative Properties of SolutionsSolutions

Electrolytes and Colligative Electrolytes and Colligative PropertiesProperties

• When solutions are made, the When solutions are made, the physical properties of the solutions physical properties of the solutions are affected by the number of are affected by the number of particle dissolvedparticle dissolved– Colligative: depending on the collectionColligative: depending on the collection

Colligative PropertiesColligative Properties

1.1. Electrolytes vs non-electrolytesElectrolytes vs non-electrolytes– Ionic compounds ARE electrolytes Ionic compounds ARE electrolytes

because they form ions in solution that because they form ions in solution that conduct electricityconduct electricity

– Molecular compounds ARE NOT Molecular compounds ARE NOT electrolytes because they do not electrolytes because they do not conduct electricityconduct electricity

2.2. Vapor Pressure LoweringVapor Pressure Lowering• Adding a non-volatile solute lowers the Adding a non-volatile solute lowers the

vapor pressure of the solution (vs. vapor pressure of the solution (vs. solvent)solvent)

• More solute More solute more vapor pressure more vapor pressure loweringlowering

Colligative PropertiesColligative Properties3.3. Boiling Point ElevationBoiling Point Elevation

• Because vapor pressure is lowered, it takes Because vapor pressure is lowered, it takes more energy to make it boilmore energy to make it boil• Boiling point temperature is raisedBoiling point temperature is raised• Boiling point elevation is directly proportional to Boiling point elevation is directly proportional to

solution molalitysolution molality• What is the benefit of adding salt to boiling water for What is the benefit of adding salt to boiling water for

pasta?pasta?

4.4. Freezing Point DepressionFreezing Point Depression• Freezing point temperature is loweredFreezing point temperature is lowered• Solute particles interfere with attractive forces Solute particles interfere with attractive forces

of solventof solvent• Freezing point of a solution is always lower Freezing point of a solution is always lower

than the freezing point of a pure solventthan the freezing point of a pure solvent• FP Depression is directly proportional to FP Depression is directly proportional to

molalitymolality

Colligative PropertiesColligative Properties

5.5. Osmosis and Osmotic PressureOsmosis and Osmotic Pressure• Diffusion: mixing of gasses or liquids Diffusion: mixing of gasses or liquids

through random motionsthrough random motions

• Osmosis is diffusion of solvent through a Osmosis is diffusion of solvent through a semi permeable membrane from high semi permeable membrane from high solvent concentration to lower solvent solvent concentration to lower solvent concentrationconcentration– Living cells use this to get materials in/out of Living cells use this to get materials in/out of

cellscells

Colligative PropertiesColligative Properties• Example: Salt/waterExample: Salt/water

– During Osmosis, Water molecules move During Osmosis, Water molecules move both directions through membraneboth directions through membrane•But only water can move through the But only water can move through the

membranemembrane•So pure water builds up on one side of the So pure water builds up on one side of the

membranemembrane•Water/salt builds up on the other.Water/salt builds up on the other.

– Higher concentration of water on one Higher concentration of water on one side creates:side creates:•Osmotic pressureOsmotic pressure•A pressure or push to equalize the water/salt A pressure or push to equalize the water/salt

concentrationsconcentrations•Pressure depends on concentration of solutePressure depends on concentration of solute

Colligative Properties - Colligative Properties - AntifreezeAntifreeze

Antifreeze Colligative Properties

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Heterogeneous MixtureHeterogeneous Mixture