Salting Out - OMSI

Figure 1. After addingsalt, the alcohol andwater should separateinto two layers. Themarker ink stays withthe alcohol layer that ison top. The salt stayswith the water layerthat is on the bottom.Ongoing Assessment What can happen when you combine two substances? They canmix, they can stay separate, or they can react to make somethingnew. How do you know if something has dissolved? It so finely dispersesas to become invisible in the liquid. The liquid is clear, you can seethrough it, but it may have a color. At this point, each molecule ofsalt is completely surrounded by liquid molecules. Why do you think some chemicals dissolve and some don’t? Theyhave different chemical properties; in this case, things dissolve ifthey have similar charges. The charges on the water are attractedto the charges on the alcohol. Can you explain why the alcohol and water separate when youadd salt? The salt strongly attracts the water and the salty waterhas too much charge to dissolve the alcohol.Safety and Disposal InformationIf available, goggles are recommended for this activity. Waste isopropyl alcohol should be kept away from all sparks andflames. The small amounts used in this experiment may be poureddown the sink drain with lots of water. All other materials can be thrown away as solid waste. CAUTION: Isopropyl alcohol is flammable and poisonous. Keep awayfrom heat and open flames.CLASSROOM DISCUSSIONAsk for studentobservations andexplanations. Letthe studentsguide thediscussion andpresent theirhypothesesbefore discussingexplanations.Choose questionsthat areappropriate foryour classroom.Did the salt dissolve in the alcohol? Did the salt dissolve in the water?Answers will vary. Some salt actually does dissolve in the alcohol but notvery much. The alcohol is less polar than the water, so its charges are lesswell attracted to the charged particles in salt. Much more salt dissolves inthe water, but the water can only hold so much salt.Permanent markers are designed to not dissolve in water. Did the inkdissolve in the water? Did it dissolve in the alcohol? Why does it dissolve inthe water and alcohol solution?The permanent ink is more like the alcohol than the water. However,because the alcohol can dissolve with the water, the alcohol carries themarker ink into the water when it dissolves in the water. The alcohol actsas a bridge in the solution to keep the marker ink dissolved.When you added the salt to the colored water and alcohol, whathappened to the salt? Did it dissolve in the water or the alcohol?Answers will vary. The salt dissolved mostly in the water, but some in thealcohol. It’s hard to tell where which liquid dissolved the salt just bySalting OutGrades 6-8A - 72Chemistry in the K-8 Classroom 2007, OMSI

looking. One way to find out is to take one drop of each liquid and let itevaporate, to see which leaves behind more salt.At the end of the experiment, which layer was alcohol and which waswater? How do you know? Why do you say so?The marker ink helps us guess the alcohol layer is on top. Another way tofigure out which layer is which is to add a bit more water. If the waterpasses through the top layer and collects on the bottom, the top layer isalcohol. If the water stays on the top and increases the width of the top,the top layer is water.The alcohol layer seems to be smaller than the water layer, but we startedout with more alcohol, right? Why do you think it is smaller?Answers will vary. The layer on top is small because some of the alcohol isdissolved in the lower water layer, even though the water is salty. In actualfact, neither the alcohol nor the water layers are completely pure—somealcohol remains dissolved in the water layer, and vice versa. NOTE:Observant students might point out that the water layer must havedissolved more of the alcohol, since the alcohol layer is smaller.To further investigate this last question, students can complete Extension B.EXPLANATIONThis background information is for teachers. Modify and communicate to studentsas necessary.In this activity, students make a colored solution of permanent marker ink, water, andalcohol. Then they cause the solution to form separate, colored layers by adding salt.Like Dissolves Like—Salt in waterSalt dissolves in water because it is “like” water in a chemical way. Students explore thisidea in the activity when they add salt to both water and alcohol. The salt dissolveseasily in the water but does not dissolve very well in the alcohol. This demonstrates acommon phenomenon in chemistry, that “like dissolves like,” or that chemicals thathave similar properties will dissolve in each other. Thus, we see that salt and water have“something” more in common than salt and alcohol. Another place to see this is in theTeacher Demonstration, Oil and Water. Oil and water do not mix at all; they areinsoluble. This implies that water and oil have very different chemical properties.Salting OutGrades 6-8A - 73Chemistry in the K-8 Classroom 2007, OMSI

Like Dissolves Like—Permanent marker ink in alcoholPermanent marker dissolves in alcohol because it is “like” alcohol in a chemical way. Itdoes not dissolve in water because it is chemically unlike water. Students explore this ideawhen they add permanent marker to both the water andalcohol. *After the solutions separate, students notice that the color from the permanent markerstays in the alcohol layer, not the water layer. Permanent marker ink is designed to resistwashing with water (i.e., water does not dissolve it), but the ink dissolves easily inalcohol. Permanent marker ink must share some chemical properties with alcohol andnot with water.Alcohol and WaterEven though alcohol and water act differently when compared with salt or with markerinks, they are similar enough to dissolve in each other quite well. Students investigatethis when they mix their colored alcohol and water: they find the color is evenlydistributed through the entire mixture. This shows that the alcohol and water are soluble(will dissolve in each other). It also shows that permanent marker ink will dissolve in waterwhen alcohol is present.Students might construct a continuum like the following:Salt.Water . Alcohol.Permanent Marker InkChemicals near each other on this chart are similar and will dissolve together, butchemicals that are far apart will not.Salting Out—Separating alcohol and waterAdding salt to water/alcohol mixtures can separate them into layers. Even thoughwater and alcohol mix well together, alcohol and salt do not. When salt is added to thesolution, the salt dissolves in the water but not in the alcohol. Two layers are formed,with the alcohol layer on top. The marker ink allows us to see the separation moreclearly, as it travels with the alcohol layer.NOTE: The alcohol layer floats on the saltwater layer because it is less dense than wateror saltwater. For a more detailed discussion of density, see the Explanation section ofDensity Rainbow.Note: Students might find that brand new permanent marker ink seems to dissolve in water.However, this isn’t really the case. Marker ink contains some alcohol, and a new marker hasenough alcohol that some of the alcohol leaks into the water, carrying some of the dissolved ink.This allows the ink to mix in the water to a certain extent.*Salting OutGrades 6-8A - 74Chemistry in the K-8 Classroom 2007, OMSI

EXTRA INFORMATIONCharges on MoleculesWhen chemists say that “Like Dissolves Like,” they really mean “like charges dissolve likecharges.” In other words, to understand whether solids or liquids dissolve together (i.e.,are soluble), it is necessary to look at the electrical charges on the molecules. Thesecharges come from how the electrons (negatively charged particles) are sharedbetween atoms.When atoms combine to make molecules,they can share their electrons in one of threeways: the atoms can share the electronsequally, one atom can take the electronscompletely, or they can share the electronsunequally. The way the electrons are shareddetermines the overall charge of themolecule (see Figure 2).Equal Sharing CovalentAn example of almost equal sharing is foundin oil. Molecules of vegetable oil have verylittle overall charge because most the atomsshare the electrons almost equally. In mineraloil the sharing is even more equal, and theoverall charge is even smaller. This type ofsharing between atoms results in covalentchemical bonds.equal sharingcovalentno charge -no sharingionichigh charge -unequal sharingpolarlow chargeFigure 2. How atoms can shareNo sharing Ionicelectrons.An example of no sharing of electrons isfound in table salt, where one atom(chlorine) completely takes the electron and acquires a strong negative charge. Theatom that donated the electron (sodium) ends up with a strong positive charge. Theseopposite charges attract to form an ionic bond.Misconception Alert: Even though the parts of the molecule have positive and negativecharges, they cancel out, so each molecule has a total net chargeof zero.Unequal Sharing PolarExamples of unequal sharing are found in water and alcohol, where the atoms sharethe electrons unequally. Because the electrons are held nearer to one of the atoms, itacquires a partial negative charge, while the other atom gets a partial positive chargeBecause the atoms partially share the electrons, this type of bond is considered acovalent bond; however, it is called a polar bond because it is partially charged.It is important to note that water shares its electrons much more unevenly than alcohol,so its molecules are much more charged (but still not completely charged like the salt).Salting OutGrades 6-8A - 75Chemistry in the K-8 Classroom 2007, OMSI

Misconception Alert: Even though the parts of the molecule have small positive andnegative charges, they cancel out so each molecule has a total net charge of zero.Charge and SolubilityThe charges on the molecules in chemicals like water, oil, alcohol, and salt affect howmuch they dissolve in each other. Salt molecules are very charged, so they dissolveeasily in water, which has slightly charged molecules. Salt dissolves less easily in alcohol,because alcohol molecules have less charge than water.Alcohol also has a portion of its molecule that has no charges, i.e., it is non-polar, like oil.This portion is less compatible with water and more compatible with non-polarmolecules. Under normal circumstances, alcohol can dissolve in both polar and nonpolar substances. This implies that permanent marker ink has molecules that have littleor no charge since they dissolve in alcohol but not in water. Because alcohol dissolveswith both water and permanent marker ink, the alcohol acts as a bridge between thewater and the permanent marker ink allowing all three to mix together.Finally, salt doesn’t dissolve in oil at all because oil has practically no charge at all.Some of these relationships are shown in Figure 3.Figure 3. Charge differences explain solubility.A difference in charge also explains why oil and water will not mix. Since oil moleculesare almost entirely uncharged, they won’t mix with charged water molecules. Wax, fat,and rubber are also made of uncharged molecules and are similarly “waterproof.”Salting OutGrades 6-8A - 76Chemistry in the K-8 Classroom 2007, OMSI

Some chemicals can dissolve in two different chemicals that don’t dissolve in eachother. For example, soap and alcohol can dissolve in both water and oil, even thoughwater and oil don’t mix. What’s more, when soap (for instance) mixes with oil and watertogether, it helps the oil and water mix. (This is how soap and water remove cooking oilfrom pans.) In this way, soap acts as an emulsifier since it is helping two liquids mix thatnormally won’t.Salting Out RevisitedWhen students add salt to the mixture of water, alcohol, and permanent marker ink, themixture separates into two separate layers. One layer has alcohol and permanentmarker ink, and one layer has salt and water. With its high charges, the salt dissolvesmuch more easily in the water. This makes the saltwater mixture too charged for thealcohol to stay in solution. The salt effectively repels the alcohol, and the mixtureseparates.Salting Out ApplicationsChemists use salt to alter the charge of water solutions when they want to precipitateproteins, separate polar liquids from water, or concentrate dissolved chemicals (like themarker ink) in a non-water layer.Scientists take advantage of how salt dissolves in water to purify proteins. Whenbiochemists study the chemistry of the body, they often focus on the proteins, becauseunderstanding how proteins work can help them design new medicines, diagnosediseases, and understand how genes work. Unfortunately, though, most of theinteresting proteins and other biological molecules of the body are dissolved in thewatery interior of the cell. It can be very difficult to separate one protein from thethousands of other proteins and molecules.One way to isolate proteins for study is to add precise amounts of salt to “salt out” theproteins. This forces the proteins to precipitate, or leave the water solution, similar to theway the colored alcohol does in the students’ experiment. Adding salt increases thecharge of the overall solution, and, because all proteins have different charges, theyprecipitate from the salt water solution at different concentrations of salt.EXTENSIONSExtension A: Candy ColorThe dyes in many candies will dissolve in the water layer, not the alcohol. Students canuse these candies to color the water layer in the jars. If the students want to color boththe layers at the same time (that is, use candies to color the water and markers to colorthe alcohol), they should avoid using candies that are the same color as the marker.Extra Supplies water soluble dyes: M&M’sTM candies, SkittlesTM candiesExtra Instructions Follow the same procedure as the general activity. Substitute (or add) candies in Step #7 for the permanent marker.Salting OutGrades 6-8A - 77Chemistry in the K-8 Classroom 2007, OMSI

Students should use two candies for each jar and leave them in the solution briefly.They should remove the candies before the colored coating dissolves completely(because, for example, the chocolate in M&M’sTM will dissolve and make a murkysolution).Extension B: Inquiry Opportunity—Test SolubilityAsk students what other compounds they think will dissolve in only water or in onlyalcohol. Why or why not? For example, “waterproof” or “water resistant” compounds(such as permanent markers) are more likely to dissolve in only alcohol. Othercompounds are more likely to dissolve in water.Extra Supplies dyes of unknown solubility: dry erase markers, overhead pens, colored markers,squares of paper with color printed from a computer, candies (as in Extension Aabove)Extra InstructionsStudents can repeat the entire procedure, but use another dye in place of marker inks.When students complete the procedure, they can evaluate whether their dye wassuccessfully separated into either the alcohol or water layer.Extension C: Measure LayersAfter the salting out procedure is finished, students should add more of one ingredientto the layered solution. What will happen? Will both layers get bigger? Will one layergrow and the other shrink? By measuring the width of each layer beforehand, studentscan see the difference and calculate ratios.Extra Suppliesruler (1 per group) extra alcohol extra water extra salt Extra Instructions1. Add alcohol After completing the original Scientific Procedure, students should measure thedepth of each layer in the jar. Add one teaspoon of alcohol to the layers in Jar 3. Recap the jar and shake for20–30 seconds Re-measure the layers. Students can repeat adding alcohol and re-measuring until the jar is full.Salting OutGrades 6-8A - 78Chemistry in the K-8 Classroom 2007, OMSI

2. Add water Start over. Recreate the layers in Jar 3 by redoing the original ScientificProcedure (students can skip any reference to Jar 1 and Jar 2). Measure the depth of each layer in Jar 3. Add one teaspoon of water. Recap Jar 3 and shake for 20–30 seconds. Re-measure the layers. Students can repeat adding water and re-measuring until Jar 3 is full.3. Add salt Start over again! Recreate the layers in Jar 3 by redoing the original ScientificProcedure (students can skip any reference to Jar 1 and Jar 2). Add one teaspoon of salt. Recap Jar 3 and shake for 20–30 seconds. Re-measure the layers. Students can repeat adding salt and re-measuring until no more salt will go intosolution.Explanation When alcohol is added: The alcohol layer increases in depth while the water layerdecreases. This happens because the larger amount of alcohol is able to absorb(i.e., dissolve) some of the water. The water layer will shrink as the alcohol layergrows. Eventually the layers combine into one mixture of alcohol, water, and salt. When water is added: This is more or less the opposite of adding alcohol. The waterlayer increases in depth and the alcohol layer decreases—the increased amount ofwater can dissolve more of the original alcohol. The alcohol layer will shrink as thewater layer grows. Eventually the layers combine into one mixture of alcohol, water,and salt. When salt is added: the salt dissolves mostly in the water, changing the water andmaking it less able to mix with the alcohol. This causes the alcohol layer to grow. Thewater layer appears to shrink, but it is hidden inside the growing pile of salt at thebottom of the jar.Salting OutGrades 6-8A - 79Chemistry in the K-8 Classroom 2007, OMSI

CROSS-CURRICULAR CONNECTIONSBIOLOGYCell BiologyResearch the parts of the cell, the process of membrane transport, andthe contents of the cytoplasm. Find out how the chemicals in thecytoplasm guide the cell in deciding what enters and leaves the cellthrough the membrane.SOCIAL STUDIESThe Importance of SaltThe impact of salt on economies, trade, and culture is vast. Researchthe important role salt has had in food preservation, the growth andstrength of cultures, and even in war.RESOURCESWeb – http://en.wikipedia.org/wiki/Polar moleculeAn introduction to the concept of polar and non-polar molecules, with pictures and diagrams.Web – http://chem4kids.comAn excellent website with information on all chemical concepts. Their site map has a well-organized topicindex. The topics “Solutions,” “Mixtures 1,” and “Mixtures 2” are relevant tothis activity.Web – http://www.saltinstitute.org/38.htmlThe Salt Institute has an extensive site explaining the place salt has in history, politics, and economics.Frankel, Jill and Kline, Michael, Super Science Concoctions: 50 Mysterious Mixtures forFabulous FunReading Level: 3rd to 8th gradeWell-organized, detailed recipes, fun activities, and thorough, accurate scientific explanations. The firstsection focuses on solutions and mixtures; later sections explore other chemistry topics.VOCABULARYbiochemists:scientists who study the chemistry of life processescovalent bonds:connections between atoms where the electrons are equallyshared; molecules with these bonds tend to be balancedelectricallydense:describes how tightly packed matter (molecules, people) is in aspace; dense is the adjective, density is the nounSalting OutGrades 6-8A - 80Chemistry in the K-8 Classroom 2007, OMSI

dissolve:when the molecules of one substance separate and becomecompletely surrounded by the molecules of another substanceelectron:negatively charged particle found in atomsemulsifier:something added to two liquids that normally won’t dissolve tokeep them uniformly mixedlike dissolves like:the principle in chemistry where substances with similar moleculesmix wellinsoluble:unable to dissolveionic bonds:connections between atoms where the electrons are transferredfrom one atom to another; molecules with these bonds tend to behighly chargednon-polar:describes neutral molecules whose charges are electricallybalancedpolar bonds:connections between atoms where the electrons are not equallyshared; molecules with th

The molecules in water have a small positive and a small negative charge, a bit like the poles on a magnet. We call such molecules with partial charges polar molecules. The molecules in oil have almost no positive or negative charge. We call these molecules non-polar molecules. Different kinds of molecules do not mix well with each other. Polar