Identifying an Unknown Compound - Tests and Observations

Tests and Observations

The following are tests and observations that you should make on your unknown
and on the standards that you characterize:
Observations of the physical appearance of the compound.


Solubility of the compound

 When a solid compound dissolves in a particular liquid, we say that the compound is soluble in that particular solvent. If a compound does not dissolve in a particular solvent, we say that it is insoluble. If some of a compound dissolves in the solvent, we say it is slightly soluble. In quantitative terms, the following ranges for number of milligrams of solid which dissolve
per milliliter of solvent can be used to define the terms:
>30 mg/mL soluble (a significant amount dissolves)
<10 mg/mL insoluble (no detectable amount dissolves)
10-30 mg/mL slightly soluble (a moderate amount dissolves)
These limits are, however, approximate and the solubility of a solid compound is usually determined quickly and easily in the lab in the following manner: Place a "spatula-tip" amount of the compound in a small test tube. You can look at the samples of 0.010 g of solid that are provided in the lab, to get an idea of how much should be used. Add about 10 drops of the solvent to the test tube and agitate the contents by shaking the bottom of the test tube back and forth for a minute or so. Observe whether or not


Density

The density of a compound is an intrinsic property, which means that it is a characteristic property of the compound that can be used to help identify it. As you know from Experiment 1, density can be determined by measuring the mass and volume of a sample and dividing:
d = m/V, where d = density in g/mL; m = mass in g; V = volume in mL
With your teammates, design a procedure for determining the density of a
solid compound. Check this procedure with your TA before beginning.

Acidity/Basicity


Acids are compounds which:
-taste sour
-react with active metals (such as zinc and iron) to dissolve the metal and
 produce hydrogen gas
-react with bases to form water and ionic compounds called salts
-can donate a hydrogen ion

Bases are compounds which:
-taste bitter
-feel slippery or soapy on the skin
-react with acids to form water and a salt
-can accept a hydrogen ion

Both acids and bases are corrosive materials that should never be tasted in the laboratory and should not come into contact with your skin. They have an additional property wherein they will react with certain dyes known as acidbase indicators. A very common acid-base indicator is called litmus. An acid will react with litmus to produce a red color, while the reaction of a base with litmus produces a blue color. Some compounds are neither acidic nor basic and are referred to as neutral compounds. These compounds will not change the color of neutral litmus paper. Thus, a simple and safe test for determining the acidity or basicity of a solid compound is to dissolve it in water and test the resulting solution with litmus paper. Note: Make sure that you have "neutral" litmus paper in your drawer, not "red" or "blue". If you do not have the correct paper, trade it in at the stockroom. The proper technique for testing a solution with indicator paper is to remove a drop of the solution with a stirring rod and place it on a piece of the paper. Do not place the paper directly into the solution being tested. Be sure to rinse off the stirring rod with water before placing it in the next solution you test. Before testing any of the compounds, place a drop of water, a neutral compound, on a piece of neutral litmus paper. The color of the paper in water provides you with a reference against which you can compare the color of the paper that results from other tests. In addition to the litmus test, and especially if the compound is insoluble.

In water, it is useful to observe its behavior in acidic and basic solutions. If the compound is acidic, it will react with a solution of a base, such as sodium hydroxide, NaOH. If it is basic, it will react with a solution of an acid, such as hydrochloric acid, HCl. Evidence that a reaction has occurred can include dissolution of the solid, evolution of a gas (bubbles) or generation of heat. A neutral compound may or may not react with an acid or base. Compounds that have both acidic and basic properties also exist and are called amphoteric.

To test how a compound interacts with an acid or a base, place a “spatulatip” amount in a test tube. Add about 10 drops of HCl solution or 10 drops of NaOH solution to the test tube, and agitate the contents of the tube by shaking the bottom of the test tube back and forth for a minute or so. Record your observations. Using these tests and observations, develop an experimental procedure that will allow you to identify and characterize your unknown compound. Do not use more than 2 g of your unknown compound for the tests. Return the unused portion in the test tube to your TA. Do not contaminate the contents of this test tube. The following resources will be provided for you:

Standards
baking soda
aspirin
corn starch
fertilizer ingredient
caffeine
chalk
caustic lime (corrosive!)
Epsom salt
Borax

Solvents
acetone
ethanol
hexane
don't forget water!
10% aqueous solution of hydrochloric acid, HCl (50 mL per group)
10% aqueous solution of sodium hydroxide, NaOH (50 mL per group)

Note: a 10% solution contains 10 g of the compound dissolved in 90 g of the solvent (water, in this case).
Equipment: 10 mL graduated cylinder with a precision of ±0.02 mL

Precautions:
Hydrochloric acid, HCl, and sodium hydroxide, NaOH, are both corrosive
chemicals. If either of these solutions comes into contact with your skin, rinse it with copious amounts of water. Some of the unknowns are also corrosive. Always assume that an unknown compound is toxic and potentially dangerous and use the proper precautions. Acetone, hexane and ethanol (organic solvents) are all flammable. Keep away from open flames and heat sources!

Questions
1. Describe the procedure that your team followed in performing this experiment and
indicate what your role was in carrying out this procedure (what tasks you
performed and which observations were yours).
2. Write the code number that was on your unknown. What is the identity of your
unknown? What results prompted you to arrive at this conclusion? List the
characteristics of your unknown.
3. Discuss sources of error associated with the procedure your team used for
determining the density of a solid compound and the effects these errors might
have on the results