Separating Mixtures
Objective: In this experiment you will make a mixture of a known percent composition using two assigned substances; then you will separate it by a process you will devise. After assessing the success of your separation method, you will either modify and retry your procedure or further purify the components.
Pre-lab: Look up the properties of your substances using trusted internet sources, a CRC handbook, Lange’s handbooks, or a Merck Index. Look up the following properties of your two assigned substance: melting point, boiling point, solubility in water, solubility in ethanol. Note temperatures and units indicated for the solubilities. Some handbooks may simply say soluble or insoluble rather than giving a number. Try to find a number for the solubility if you can. If you find a number, note what the units are. Note if a substance will sublime; this is often indicated under the boiling point column. Also note if a substance decomposes when heated; that information is often indicated under the melting point column. Write down web addresses and/or handbook titles, editions and date numbers for the reference.
To use the CRC or Lagne’s handbook: If the substance contains carbon, but is not a carbonate or hydrogen carbonate, then we classify it as organic and you should look in the handbook index for Organic with a subheading of “Properties of.” Compounds that contain no carbon or only carbon in the form of carbonate, hydrogen carbonate or carbon dioxide are a classified as inorganic, and you should look in the handbook index for a table heading of Inorganic, with a subhead of “Properties of.” Look at the beginning of the tables (not the beginning of the handbook) to see what abbreviations mean. Note that different handbooks may use slightly different wording (for examples, one might say Organic
Compounds while another might say Organic Substances). Properties that you can’t find will have to be measured in lab.
To use the Merck Index, look in the back index for the compound name. The number beside the name is a section number, not a page number. The sections are listed numerically.
BEFORE the lab period, come up with a tentative procedure you could use to separate a mixture of your two substance; to come up with something, you need to use a property that is different for the two substance. You can check with your instructor before you start the lab about whether your proposed procedure would work or is safe.
Background Information:
In real life, mixtures are much more common than pure substances. There are many situations in which scientists want to isolate and study the properties of a particular substance. For instance, if a drug is synthesized, it must be separated to be studied. Isolation of substances from mixtures can often be difficult and it’s always harder in lab than it looks on the printed page. There are several common separation methods. Some are describe below.
Sublimation followed by deposition. Sublimation is the direct conversion of a substance from a solid to a gas without it melting or changing to another substance. To perform a separation using sublimation and deposition, the sample is often heated, but sometimes sublimation occurs at room temperature. Moth balls, for instance, sublime readily. If one component of a mixture will sublime and the other doesn’t readily melt or sublime, then heating the mixture will result in vaporizing one substance while the second remains a solid. The solid remains in the bottom of the container and the vapor generally rises. After the vapor reaches a different part of the container, it is cooled so it reforms a solid and the two components are thus separated. The cooling part is called deposition and is not part of sublimation.
To see if a solid will sublime and then deposit, a small amount of it is placed in a 250 or 400mL beaker that is supported on a ring stand. A pre-weighted watch glass is placed on top of the beaker and some ice is put into the watch glass. The flame of a Bunsen burner is waved under the beaker (if the beaker is heated in only one spot, it often cracks). If the solid deposits on the sides of the flask instead of the watch glass, a smaller beaker is needed. Note if a substance turns brown when heat, it has decomposed.

When the vapor reaches the watch glass, the cold glass surface cools the vapor and it condenses on the watch glass. It may be recovered by removing the watch glass (being careful that water does not get into the hot beaker and break it) and scraping the solid off the watch glass with a scoopula.
Extraction: If only some of the components of a mixture are soluble in a liquid, this property can be used as a basis of separation. A simple example would be using water to separate a mixture of salt and sand. The salt would dissolve in water while the sand would not. The sand could then be isolated by filtration and the salt recovered by evaporating the water. The difference in solubility is being used as a basis of separation.
Crystallization: The experimental conditions are adjusted so one substance in a dissolved mixture forms a solid while the other stays in solution. One way to crystallize a substance is to slowly add a new liquid to a solution of the desired substance and other stuff. If the desired substance is insoluble in the new liquid and the undesired stuff is soluble, sometimes crystals will form when the new liquid is added. For example, if you have two things soluble in water and one of them is insoluble in hexane, then you would add hexane to the aqueous solution of the mixture. Hopefully the one insoluble in hexane would crystallize while the other stayed in solution. Then filtration would be used to separate the two.
If a solid is almost pure, then recrystallization may be used to purify it more. In this process, the solid is dissolved and conditions are adjusted to encourage the desired substance to crystallize while the impurities stay in solution. One adjustment is to cool the solution since solids are virtually always less soluble at lower temperatures. If you try this method and don’t get crystals, use a clean glass stirring rod and scratch the inside beaker wall as this sometimes starts the crystallization process. Another adjustment is to slowly evaporate the solvent so new crystals form slowly. Crystals grown slowly from a solution are normally quite pure and the impurities are left in solution. The solvent should not be completely evaporated, or the nonvolatile impurities would be in the solid along with the crystals of the desired substance.
In using crystallization or recrystallization, keep in mind that some substance is always lost; the more excess liquid used, the greater the loss.
Procedure: You will be assigned one set of substances from the list below.
Benzoic acid/Calcium carbonate Copper II sulfate pentahydrate/Copper(II) carbonate
Copper II chloride dehydrate/Benzoic acid
Naphthalene/ Copper(II) carbonate
Boric acid/Biphenyl
Boric acid/ Iodine
Boric acid/Copper (II) sulfate pentahydrate
Urea/Biphenyl
Urea/ Copper(II) carbonate
Naphthalene/Copper(II) sulfate pentahydrate
You will need to use the data you looked up to develop a good separation technique. You will have two laboratory periods (approximately 90 minutes each) to separate your mixture and complete your laboratory report.
Solubility Test (in water and ethanol)
1.
Create a data table to record amount of substance initially, and amount of substance in the end and for each solvent and each assigned substance.
2.
To determine the solubility of a compound, first accurately weight out approximately 0.5 g of the compound.
3.
Measure out 10 ml of one solvent (water or ethanol).
4.
Now add very small bits of the weighed sample to a test tube that contains 10mL of the solvent (water or ethanol).
Shake the test tube well between additions of compound. When no further compound will dissolve, weight the leftover compound.

5.
Using the initial and final masses determine how much dissolved. Then divide the amount dissolved by 10mL to determine the solubility of each substance.
Create your mixture and separate
1.
Weigh the two components you have been assigned. You will need about 1g (preferably less) of each. Do not be very fussy about getting exactly 1 g. Instead, be careful about recording your value.
2.
Grind the two substances together to mix them well. This will also make them easier to dissolve. You can grind with a mortar and pestle or sandwich the mixture between two watch glasses.
3.
Weight out a part of the ground mixture and carry out the separation, recovering as much as you can of each component. If your method doesn’t seem to work well, modify it or try another method. Repeating a separation procedure on a partially separated sample often results in a better separation.
Lab Notebook Notes: Record all of your steps and observations as you go. Be detailed; for example, describe the reactants, the intensity of colors (dark, light, etc.), approximate sizes (1mm, 3mm , etc…) of crystals, whether changes were immediate or took a few seconds, whether a noticeable temperature change occurred (feel the test tube), and so on. Also, keep in mind that “clear” is not a color. Water is clear and colorless but grape Kool-Aid might be clear and purple. Keep a good record of everything you do! It is easier to write a report when everything is written down in neat data tables.
Your laboratory notebook will include: your assigned substances for the mixture, a data table including all of your properties including references, a data table for determining solubility for each solvent and each substance in the mixture, your procedure used for the separation, a data table including description of your mixture for each step in your separation method, and a conclusion. Your conclusion should discuss the evidence you have that your separation procedure was successful and a discussion of any improvements that you could make in the separation method. Stating that you instructor should write the procedure for you is grounds for a full letter grade reduction.