Experiment 1

Pure substances are either element or compounds.  Compounds have fixed composition but mixtures can have virtually any combination of the different pure substances which make them up.  For example, brass is composed of about 70% copper and 30% zinc, but the proportions of each metal can vary, and still be called brass.  In contrast, a pure substance, such as water (H₂O) always has the same composition -- two atoms of hydrogen are combined with one atom of oxygen -- never any other combination (the Law of Constant Composition).  As long as you don't perform a chemical reaction to change the chemical nature of the components of a mixture, you can usually separate the different components of the mixture into pure substances.

For this experiment we will attempt to separate three pure substances from each other: iron (Fe), SiC (silicon carbide) and NaCl (sodium chloride, common table salt).  You will need to devise a separation protocol taking advantage of some of the physical properties of these different chemicals.  You can obtain information about different chemicals from the text, chemical reference books, MSDS (material safety data sheets) or online.  You can find useful information, including molar mass and physical constants online at http://www.chemfinder.com.  If you are asked to register, or enter a user name, enter mccchem, and for the password use mccmcc on Chemfinder.com.

Prior to doing this experiment, you should ask yourself the following questions to come up with a separation protocol that will separate Fe, SiC, and NaCl:

• Which components are soluble or insoluble in water?
• Which can be melted?  
• Which has magnetic properties?
• Which can be recovered by dissolving and evaporating the water?
• Which floats or sinks in water?
• Any other information that might be used to separate the mixture components.

Once you come up with answers to some of the above questions, you should be able to easily separate the three components.  You will be graded on how well you design the experiment, and in some cases the order that you remove one component from the others.  You need to be certain that when you remove one item from the others, that you do not have carry-over of unwanted components.

Often in an experimental protocol, you are asked to weigh out a certain amount of starting material.  For example, for today's experiment, you should weigh out "about" 5 grams of mixture.  You do not need to get exactly 5.000 g.  Usually, any amount between about 4.7-5.3 grams will suffice.  Therefore, it does not matter how much you have, but you need to record the exact amount you do have.  That is, it is okay to weigh out 4.894 g of mixture, and record this value, you don't need exactly 5.000 g.

Using our balances, it is easy to determine how much mixture you are weighing out.  If you weigh the material in a small beaker, place the beaker on the balance, and press the "Re-Zero" key, which returns a value of 0.000 in the display window.  Now, add enough mixture to give about 5 g.  Record the amount of material added, to three decimal places. Also, it is convenient to pre-weigh your evaporating dish, before doing an evaporation procedure, so that after you are through, weigh the entire contents, evaporating dish and recovered solid material, and then subtract the mass of the evaporating dish from the total to determine how much solid has been recovered.  It is also convenient to pre-weigh filter paper before you use it in later steps of the experiment.

To do the experiment, you need to isolate one component from the other(s).  You design this procedure.  If you use water to dissolve something, use 10 mL or less, and don't use much water to rinse the containers or to wash the filter paper, otherwise it will take a long time to evaporate the water.

During the second lab period, in order to reinforce the correct use of an evaporation dish, you will obtain solution containing NaCl of unknown concentration.  You will use an evaporating dish setup to determine the percent concentration of NaCl in this solution.

Prelab Questions:

Do the prelab questions for Experiment 1.  Put these questions and answers in your laboratory notebook.

As a prelab exercise, list at least five (5) physical properties for each substance, SiC, Fe, and NaCl, that might be useful in separating them from the other components.

Is there a problem using a magnet to separate the iron from the original mixture?  If so, what might be the problem?

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Copyright © Donald L. Robertson (Modified: 09/20/2006)