Today is Monday,June (!) 1st, 2015

Pre-Class:Where have you seen solutions in everyday life?

(I mean the chemistry kind of solution, FYI)

Solution!

Stuff You Need:Calculator

Periodic TablePolyatomic Ion

List

In This Lesson:Unit 6

Solutions and Solubility

(Lesson 1 of 4)

Today’s Agenda

• Solutions• Solubility Curves• Why I get so angry at TV sometimes.

• Where is this in my book?– P. 471 and following…

By the end of this lesson…

• You should be able to describe solutions and colloids in terms of their properties.

• You should be able to identify solubility trends based on pressure, agitation, and temperature.

Wait…what’s a solution?

• Recall that a solution is a homogeneous mixture.– If you’ve ever stirred sugar into tea or coffee, you’ve

made a solution.• A solute (usually either a gas or a solid, but

sometimes a liquid) is what’s dissolved into a liquid, called the solvent.– Example: Salt water

• Solute: Salt• Solvent: Water

– Example: Seltzer water• Solute: CO2

• Solvent: Water

Solutions Up Close

• Water doesn’t really understand personal boundaries.

• On a molecular level, when something dissolves, water (or a different solvent) completely surrounds the solute particles.– It forms a hydration shell.

• As a result, we get a very easy-to-remember solubility rule (next slide).

http://www.elmhurst.edu/~chm/vchembook/images2/171saltdissolve2.gif

“Like Dissolves Like” Video

• Here’s an introduction to an important solubility concept…

“Like Dissolves Like”

• Non-polar solutes dissolve best in non-polar solvents:– Fats, steroids, waxes into benzene, hexane,

toluene.• Polar and ionic solutes dissolve best in polar

solvents:– Inorganic salts and sugars into water, small

alcohols, acetic acid.

Hydrophobic/Hydrophilic

• Hydrophilic (polar) substances can dissolve easily in water.

• Hydrophobic (non-polar) substances cannot.– Oil, for example, doesn’t. That’s why you see this:

http://www.ecofriendlymag.com/wp-content/plugins/wp-o-matic/cache/9e8b0_bp-leak-gusher.jpg

Hydrophobic/Hydrophilic

http://static.guim.co.uk/sys-images/Environment/Pix/columnists/2010/5/20/1274366784453/Deepwater-Horizon-oil-spi-006.jpg

Hydrophobic/Hydrophilic

http://www.newsoxy.com/images/0706/tar-balls-3.jpg

Hydrophilic/Hydrophobic

http://www.yoganonymous.com/wp-content/uploads/2010/06/save-the-sea-turtles.jpghttp://inapcache.boston.com/universal/site_graphics/blogs/bigpicture/oil_06_03/o02_23681001.jpghttp://inapcache.boston.com/universal/site_graphics/blogs/bigpicture/oil_06_03/o01_23681845.jpg

Hydrophobicity in Practice

• TED: Mark Shaw – One Very Dry Demo

Solutions

• Also remember that solutions are considered aqueous, but the solute does not actually change state.– For example, sugar stays a solid when dissolved

into tea; carbon dioxide stays a gas in soda.• Lastly, solubility is the word chemists use to

describe the maximum amount of solute that can dissolve in a certain quantity of solvent.

IMPORTANT

• Recall that ionic compounds will dissociate (break down) into their component ions when they dissolve.

• Example:– NaCl becomes Na+ and Cl- in water.– PbCl2 becomes Pb2+ and 2Cl- in water.• Note that there is a coefficient of 2 in front of Cl-, since

there are twice as many chloride ions as lead ions in solution.

Dissociation

CaCl

Cl

Ca2+Cl-

Cl-

Bound ions in… …component ions out.

Dissociation Practice

• Into what does H2SO4 dissociate?

• What’s the compound made of, and how many of each?– 2H and 1SO4

• Write their charges in:– 2H+ and 1SO4

2-

• Ta-da!

Dissociation Practice

• Into what does CaCl2 dissociate?– Ca2+ and 2Cl-

• Into what does Na2S dissociate?– 2Na+ and S2-

• Into what does MgO dissociate?– Mg2+ and O2-

• Into what does Mg(NO3)2 dissociate?– Mg2+ and 2NO3

-

Solubility Rate

• As you may have guessed, not all solutions are created equally.

• Some solvents can hold a lot of a certain substance, some can’t hold any.

Solubility Trends

• Solubility of (most) solids increases with:– Increase in temperature– Increase in surface area

• Solubility of gases increases with:– Decrease in temperature– Increase in pressure• In other words, push harder and more will dissolve…

Therefore…

• Solids dissolve best when:– Heated– Stirred– Ground into small particles

• Gases dissolve best when:– Chilled– Under high pressure

• This explains why many beer commercials describe their drinks as being “frost-brewed” – it’s because they have to be!

Gas Dissolution in Soda

• When a soda bottle is capped, vapor pressure above the liquid keeps the carbon dioxide dissolved.

• Opening the bottle decreases the pressure allowing CO2 to escape.– Hence, bubbling.

Aside: Henry’s LawNot on your exam.

• The concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution:

• S – Solubility (g/L)• P – Pressure

• Applies most accurately for dilute solutions of gases that do not dissociate or react with the solvent:– Good for CO2, N2, O2

– Bad for HCl, HI

2

2

1

1

P

S

P

S

Aside: The Bends

• “The bends,” also known officially as decompression sickness, occurs when divers go to extreme depths.

• At these depths, water pressure force-dissolves nitrogen gas into blood vessels.

• If those divers then rise to the surface too quickly, the nitrogen gas bubbles out of the blood (like CO2 from a freshly-opened soda bottle).– These N2 bubbles restrict oxygen flow and cellular function.

• The result? Crippling pain forcing people to bend over, resulting in permanent damage or even death.

Gas Solutions

• Interestingly, biochemists have been playing around with the possibility of creating a liquid that has so much oxygen dissolved in it that it’s possible to breathe.– That’s right, breathing a liquid.– Like that scene in The Abyss.

• Cue the video!

Terms to Describe a Stadium

• If a stadium has 50,000 seats and there are 50,000 people there, how do we describe it?– At capacity.

• If a stadium has 50,000 seats and there are 49,999 people there, how do we describe it?– Under capacity.

• If a stadium has 50,000 seats and there are 60,000 people there, how do we describe it?– Over capacity.

Terms to Describe a Solution

• Saturated– Full of solute.

• Unsaturated– Not full of solute.• It can have some solute, just not the maximum.

• Oversaturated– More solute than can dissolve (some not dissolved).

• Supersaturated– More solute than can dissolve (all dissolved).• Typically, a supersaturated solution has been heated.

Supersaturated Solutions

• Making a supersaturated solution is like putting 60,000 fans in a 50,000 seat stadium and getting them all in seats.

• Supersaturated solutions aren’t very stable. The solute will actually fall out of solution (precipitate) if disturbed.– AKA those extra 10,000 fans stand up.

• Video!

IMPORTANT

• Supersaturated is not as simple as “more solute than saturated.”

• Typically, supersaturated solutions are heated to allow more solute to dissolve, then allowed to cool gently to a lower temperature.– Any disturbance, however, and the solute will

precipitate.• If you don’t heat it and simply dump in so much

solute that it sits at the bottom and doesn’t dissolve, that’s an oversaturated solution.

Concentration

• Concentration is the relative amount of solute and solvent.– High concentration means more solute, less solvent.– Low concentration means less solute, more solvent.

• When you concentrate something, you make it a higher concentration.

• When you dilute something, you make it a lower concentration.

• Concentrations are indicated with [brackets].– Example: [H+] means “concentration of hydrogen ions.”

Solubility Curves

• So, since there are so many different things that affect solubility, not to mention how much solvent you have in the first place, chemists created the solubility curve.

• Solubility curves are lines on a graph that represent the point at which a given quantity of a solvent is saturated at a given temperature.– Often, these are graphed with “grams of solute per

100 grams of water” on the Y-axis.

Solubility Curves

• How much potassium bromide can be dissolved in 100 g of H2O at 80°C?– About 100 grams.

Solubility Curves

• How much potassium bromide can be dissolved in 150 g of H2O at 80°C?– About 150 grams.

Solubility Curve Saturation

• Before we practice, remember:– On the line is saturated– Above is oversaturated– Below is unsaturated

Saturated

Oversaturated

Unsaturated

Practice

• Solubility Curve Packet– Page 1 (hints on next slides)– Page 2 (#10 is difficult)– Page 4 (#3 – A and E are difficult)

100 g H2O

Solubility Curves Packet #2• How many grams of potassium

chloride can be dissolved in 200 g of water at 80°C?

• Let’s look at the graph – at 80°C, the curve for KCl corresponds to 50 g on the Y-axis.

• What does that mean?• That means you can fit 50 g of

solute (KCl)…• …in 100 g water.• So you can fit 100 g KCl in 200 g

H2O.200 g H2O = 100 g KCl50 g KCl

Solubility Curves Packet #5• At 30°C, 90 g of sodium

nitrate is dissolved in 100 g of water. Is this solution saturated, unsaturated, or supersaturated?

• Let’s look at the graph – at 30°C, the curve for NaNO3 corresponds to about 92 g per 100 g H2O on the Y-axis.

• Since we only added 90 g NaNO3 to 100 g of water, this solution is unsaturated.

Solubility Curves Packet #6• A saturated solution of potassium

chlorate is formed from one hundred grams of water. If the saturated solution is cooled from 80°C to 50°C, how many grams of precipitate are formed?

• Let’s look at the graph – at 80°C, the curve for KClO3 corresponds to 40 g on the Y-axis.

• At 50°C, the curve corresponds to 20 g on the Y-axis.

• That means that as the solution cools, 20 g of KClO3 precipitate are formed.• 40 g – 20 g = 20 g

Electrolytes

• An electrolyte is a substance whose aqueous solution conducts an electric current.– Usually an ionic compound.

• A non-electrolyte is one that, well, doesn’t.

Electrolytes?

• Pure Water– No

• Tap Water– Yes, there are dissolved solutes in there

• Sugar solution– No, sugar is not ionic

• Sodium chloride solution– Yes, NaCl is an ionic compound

Electrolytes?

• Hydrochloric acid solution– Yes, it’s an ionic compound

• Lactic acid solution– Yes, though weak

• Ethanol solution– No, it’s not ionic

• Pure sodium chloride– Only if molten

Suspensions and Colloids

• Two things that aren’t solutions are:• Suspensions: Particles are so large they will

settle if not constantly stirred.• Colloids: Particles are between suspension

and solution size.

Tyndall Effect• There’s one weird

property that allows for detecting a solution versus a suspension, and that’s called the Tyndall Effect.

• If light is shined into a colloid/solution, colloids will scatter light, solutions will not.

Which is the solution?

ColloidSolution

Types of ColloidsExample Dispersing Medium Dispersed

SubstanceColloid Type

Fog, aerosol sprays Gas Liquid AerosolSmoke, airborne

germs Gas Solid Aerosol

Whipped cream, soap suds Liquid Gas Foam

Milk, mayonnaise Liquid Liquid EmulsionPaint, clays, gelatin Liquid Solid Solid

Marshmallow, Styrofoam Solid Gas Solid Foam

Butter, cheese Solid Liquid Solid EmulsionRuby, glass Solid Solid Solid

Aside: The Fog of…Colloid?

• You may have noticed that fog is a colloid.• That means it scatters light.• That’s why when you’re driving you shouldn’t use your

high beams (the “brights”) as it will simply scatter more light and blind you or an oncoming driver.– You wouldn’t want to blind them right into your car.

Closure (1 of 4)

• Which is the best drawing for magnesium chloride in water?

Magnesium Chlorine

A B C D

Closure (2 of 4)

• How would that drawing have changed if it were magnesium oxide instead of magnesium chloride?a) The ion ratio would be the same as for MgCl2.

b) The ratio would be 1 Mg : 1 O.c) The ratio would be 2 Mg : 1 O.d) You would need different colors.

Closure (3 of 4)

• Which image best shows a saturated solution?

BA C D

Closure (4 of 4) and Homework

• You can dissolve approximately 135 grams of KI in 100 grams of water at 10 °C.

• If you fill a beaker with 100 grams of H2O and dissolve 134 grams of KI in it, how would you describe the solution? (assume it is at 10 °C)– Unsaturated.

• Tonight’s homework: Finish the solubility curve worksheets. These will be checked at the beginning of class tomorrow.