Forming Solutions 15.1: Pgs 520 - 527. Objectives To understand and describe the process of dissolving To understand and describe the process of dissolving.

Forming SolutionsForming Solutions

15.1: Pgs 520 - 52715.1: Pgs 520 - 527

ObjectivesObjectives

• To understand and describe the To understand and describe the process of dissolvingprocess of dissolving

• To learn why certain substances To learn why certain substances dissolve in water and others won’tdissolve in water and others won’t

• To understand factors that affect the To understand factors that affect the rate at which a solid dissolvesrate at which a solid dissolves

Important Solution Vocab:Important Solution Vocab:• SolutionSolution - a homogenous mixture. - a homogenous mixture.

– SolventSolvent - the substance present in the - the substance present in the largest amount.largest amount.

– SolutesSolutes - the other substance or substances. - the other substance or substances.

•Aqueous solutions - solutions with water as the solvent.

solvent

solute

solution

Solubility of Ionic Solubility of Ionic SubstancesSubstances

• What is occurring when something What is occurring when something dissolves? dissolves?

•The ionic forces that hold the salt together are The ionic forces that hold the salt together are overcome by the attractions between the ions and overcome by the attractions between the ions and the polar water molecules.the polar water molecules.

Ionic CompoundsIonic Compounds

• The resulting solution contains ions that The resulting solution contains ions that can move about independently!can move about independently!– Ex: NaCl Ex: NaCl (s)(s) Na Na++ (aq)(aq) + Cl + Cl-- (aq)(aq)

Step 1 of LabStep 1 of Lab1.1. Obtain a CLEAN test tube!!!Obtain a CLEAN test tube!!!2.2. Place 2.2 grams of NaPlace 2.2 grams of Na22SOSO44 into a test tube into a test tube

and add 10mL of water.and add 10mL of water.3.3. Dissolve the solid: Heat the test tube in a Dissolve the solid: Heat the test tube in a

bunsen burner until you hear a “pop”. bunsen burner until you hear a “pop”. Remove the test tube from the bunsen Remove the test tube from the bunsen burner and stir the contents with a burner and stir the contents with a stirring rod. Continue this process until stirring rod. Continue this process until the solid dissolves. the solid dissolves.

4.4. Place the test tube into the test tube rack Place the test tube into the test tube rack when the solid is dissolved. when the solid is dissolved.

5.5. Place 2 ice cubes into a beaker and fill Place 2 ice cubes into a beaker and fill the beaker with water (You will use this the beaker with water (You will use this later)later)

6.6. Sit back in your seat.Sit back in your seat.

Factors Affecting the Rate of Factors Affecting the Rate of DissolvingDissolving• Surface areaSurface area

– The larger the surface area, the The larger the surface area, the

faster the dissolving will occurfaster the dissolving will occur

• Stirring will increase the rate of Stirring will increase the rate of

dissolutiondissolution

• TemperatureTemperature– Dissolving occurs faster at higher Dissolving occurs faster at higher

temperaturestemperatures

Solubility of Polar Solubility of Polar SubstancesSubstances

• Water can also dissolve nonionic Water can also dissolve nonionic substancessubstances– Examples: sugar, ethanolExamples: sugar, ethanol

•These have polar O-H groups in their These have polar O-H groups in their structure that are attracted to the waterstructure that are attracted to the water

Polar OH groups that allow sugar to dissolve in water!

Substances Insoluble in Substances Insoluble in WaterWater

• Not all substances are soluble in Not all substances are soluble in waterwater– Ex: Petroleum Ex: Petroleum What happens when it What happens when it

leaks from an oil tanker in the ocean?leaks from an oil tanker in the ocean?

• Nonpolar substances are Nonpolar substances are not not compatiblecompatible with polar water with polar water moleculesmolecules– Ex: Oil and water don’t mix! Ex: Oil and water don’t mix!

An oil layer floating on water.An oil layer floating on water.

How Substances DissolveHow Substances Dissolve

• Follow the saying “Follow the saying “like dissolves like”like dissolves like”– A solvent usually dissolves solutes that have A solvent usually dissolves solutes that have

polarities similar to its own.polarities similar to its own.•Ex: Water and Sugar:Ex: Water and Sugar:

– Since water and sugar are both polar, the solute – Since water and sugar are both polar, the solute – solvent interactions formed are similar to the water – solvent interactions formed are similar to the water – water interactions present in the pure solventwater interactions present in the pure solvent

As we add more salt to the solution, As we add more salt to the solution, eventually there won’t be enough water to eventually there won’t be enough water to surround each ion. What will happen then?surround each ion. What will happen then?

Step 2 of the LabStep 2 of the Lab• 1 person from your team must place your 1 person from your team must place your

test tube into the beaker of ice water and test tube into the beaker of ice water and then return to your seat.then return to your seat.

Solution CompositionSolution Composition

• SaturatedSaturated – When a solution contains as – When a solution contains as much solute as will normally dissolve at much solute as will normally dissolve at that temperature.that temperature.

• UnsaturatedUnsaturated – When a solution contains – When a solution contains less solute than can normally dissolveless solute than can normally dissolve

• SupersaturatedSupersaturated – it contains more solute – it contains more solute than a solution will normally hold at that than a solution will normally hold at that temperaturetemperature

http://www.youtube.com/watch?v=HnSg2cl09PI&feature=fvw

http://www.youtube.com/watch?v=aC-KOYQsIvU&feature=related

Solution CompositionSolution Composition

• Supersaturated solutions are Supersaturated solutions are very very unstableunstable!!– A saturated solution can be forced to A saturated solution can be forced to

precipitateprecipitate by adding one crystal of by adding one crystal of the solid.the solid.•Precipitation will continue until the solution Precipitation will continue until the solution

reaches the saturation pointreaches the saturation point

Last step of the labLast step of the labGENTLYGENTLY remove the test tube remove the test tube

from the cold water. Add a small from the cold water. Add a small amount of amount of NaNa22SOSO44 and observe.and observe.

Clean up the lab: All Clean up the lab: All solution/compounds can be rinsed solution/compounds can be rinsed down the drain.down the drain.

Sit back in your seat.Sit back in your seat.

What just happened???What just happened???

• How would you classify the solution How would you classify the solution BEFORE the extra solid was added to BEFORE the extra solid was added to the test tube?the test tube?– What does this mean?What does this mean?

• What did the addition of the extra What did the addition of the extra solid do to your solution?solid do to your solution?

• How would you classify your solution How would you classify your solution now???now???

Yesterday we covered…Yesterday we covered…

• 4 factors that will increase the rate of 4 factors that will increase the rate of dissolutiondissolution

Today we will cover… How temperature not only increases

the rate of dissolution, temperature also affects the amount that can dissolve.

SolubilitySolubility

• SolubilitySolubility:: the amount of a solute the amount of a solute that can dissolve in a given amount that can dissolve in a given amount of solvent at a specific temperatureof solvent at a specific temperature

• The solubility of a substance depends The solubility of a substance depends on TWO things:on TWO things:– The nature of the soluteThe nature of the solute– The temperature of the solventThe temperature of the solvent

Factors Affecting SolubilityFactors Affecting Solubility

• Solubility is VERY temperature Solubility is VERY temperature dependent!dependent!– Ex: 36.2g of NaCl dissolves in 100g Ex: 36.2g of NaCl dissolves in 100g

HH22O at 25O at 25ooC, HOWEVER 39.2g NaCl C, HOWEVER 39.2g NaCl can dissolve in 100g Hcan dissolve in 100g H22O at 100O at 100ooC!C!

• For most substances, solubility For most substances, solubility increases as temperature of the increases as temperature of the solvent increasessolvent increases

Expressing SolubilityExpressing Solubility

• Solubility is expressed as follows:Solubility is expressed as follows:

• Can use solubility like to set-up Can use solubility like to set-up proportion problems.proportion problems.

re temperatua @solvent of g

solute of gC25@

OH g 100

NaCl g 2.36

2

o

Problem:Problem:

• I dissolved 5.2g of a I dissolved 5.2g of a substance into 16g of water. substance into 16g of water. What is the solubility of this What is the solubility of this substance in units of g / 100g substance in units of g / 100g of water?of water?

Solubility Solubility CurvesCurves

• What substance What substance has the highest has the highest solubility at solubility at 5050ooC? C? (exclude (exclude KI)KI)

• What has the What has the lowest at 50lowest at 50ooC?C?

Solubility Curve LabSolubility Curve Lab

• Not all substances are stable Not all substances are stable enough to create a enough to create a supersaturated solution. Most supersaturated solution. Most substances can only form a substances can only form a saturated solution.saturated solution.

• We will complete this as a class. We will complete this as a class. Each pair of students will be Each pair of students will be assigned a specific amount of salt assigned a specific amount of salt to use and we will collaborate our to use and we will collaborate our data to create a graph.data to create a graph.

DirectionsDirections• Heat the test tube in hot water on a hot plate. Heat the test tube in hot water on a hot plate.

Make certain all the solid has dissolved.Make certain all the solid has dissolved.

• Remove the test tube from the hot water and Remove the test tube from the hot water and place a thermometer into the test tube.place a thermometer into the test tube.

• Continue to stir the contents slowly until you see Continue to stir the contents slowly until you see some crystals (solids) begin to form. Record the some crystals (solids) begin to form. Record the temperature for when the solid begins to form in temperature for when the solid begins to form in the data table on the board.the data table on the board.

• Repeat steps 1-3 in order to record a total of 2 Repeat steps 1-3 in order to record a total of 2 trials.trials.

• Calculate and record the solubility of KNOCalculate and record the solubility of KNO33in units in units of “g/100g of water” in the data table.of “g/100g of water” in the data table.

• When you are done, work on textbook homework When you are done, work on textbook homework and worksheet that is due tomorrow.and worksheet that is due tomorrow.