To separate a heterogeneous mixture based on each component’s physical properties.




                Matter can be classified according to the following divisions:





MIXTURES                                                                                             PURE SUBSTANCES                            

                678                                                                                                                   678


                           HOMOGENEOUS              HETEROGENEOUS                                                                                                                     COMPOUNDS               ELEMENTS


A mixture is a physical combination of two or more pure substances wherein each substance retains its own chemical identity.  For example, each component in a salt-water mixture possesses the same chemical properties as in the pure state; water is H2O molecules and salt is sodium ions (Na+) and chloride ions (Cl-).


                Mixtures are either homogeneous or heterogeneous.  Some homogeneous mixtures are also called  solutions.  Solutions consist of two or more substances mixed at a molecular-sized level forming a system having uniform properties and composition throughout.  Air is an example of a homogeneous mixture of gases.  A heterogeneous mixture consists of two or more distinct regions or phases each having its characteristic properties – salt crystals/sand and water/oil are heterogeneous mixtures.


                Most substances found in nature, and many prepared in the laboratory, are impure; that is, they are a part of a mixture.  A part of chemical research is to devise methods for separating mixtures to remove impurities from the chemical substance of interest.  The method chosen for separating the mixture is based on the differences in the chemical and/or physical properties of the mixture’s components.

Some common physical methods for separating a mixture’s components include:


(a)                 distillation:  vaporizing a liquid from a solid (or another liquid) and condensing the vapor;

(b)                 crystallization: forming a crystalline solid by decreasing its solubility as a result of cooling the solution, evaporating the solvent, or adding a solvent in which it is less soluble so that solid crystals form;

(c)                 extraction: removing a substance form a solid or liquid mixture by adding a solvent in which the substance is more soluble;

(d)                 filtration:  removing a solid substance from a liquid by passing the suspension through a filter;

(e)                 centrifugation: removing a substance from a solution by means of centrifuge;

(f)                  sublimation:  vaporizing a solid and condensing its vapor (not all solids sublime, however);

(g)                 chromatography: separating a mixture’s components because of their differing adsorptive tendencies on a stationary phase as the mixture is passed over or through it.


In this experiment, we will separate an ammonium chloride, sodium chloride and sand mixture using sublimation, distillation, and extraction methods.  Some physical properties of each chemical in the mixture are listed below.





Physical Properties of Some Pure Substances




g / 100 g H2O at 25 oC








white crystals




sublimes at 340

white crystals





white crystals


A.                                          MASS OF SAMPLE


Obtain in about 5 g of unknown sample from your laboratory instructor.  Record the weight     (0.001g) of a clean, dry, evaporating dish.  Transfer unknown sample to the dish, weigh ( 0.001g) and record.


B.                                           MASS OF AMMONIUM CHLORIDE


1.                              Place the evaporating dish and sample on a wire gauze as shown in figure 1.  Heat the

Evaporating dish slowly; gradually intensify the heat until the white fumes of NH4 Cl cease to be evolved (approximately 15 minutes).  Occasional stirring with a glass stirring rod aids in its complete removal.  Cool the evaporating dish to room temperature.  Weigh (0.001 g) dish and its content at room temperature and record.   CAUTION:   Do not set the hot dish on the desk top.


2.                          Let the evaporating dish cool and then add 10 mL of distilled water and gently stir the residue.  Decant (pour off the liquid) the liquid into a cleaned dry beaker which you have previously weighed.  Add 10 mL of water to the dish two more times and repeat the decantings into your weighed beaker.























Figure 7.1




C.                                           MASS OF SAND


Return the evaporating dish containing the remaining solid to the wire gauze.  Heat slowly to evaporate the water and dry the residue.  You may choose to put a watch glass on the dish to prevent spattering of the residue which can happen if you heat the dish too fast.  When your sample is dry and you have made sure to account for all of your dried sample, let the dish cool to room temperature and weigh it to 0.01 g.


D.                                          MASS OF SODIUM CHLORIDE


                            Place the beaker containing your salt solution on the wire gauze and allow the liquid to gently boil until all of the liquid has evaporated leaving a dry whitish residue in the beaker.  When the evaporation is complete, let the beaker cool to room temperature and weigh it to 0.01 g.

DATA:        UNKNOWN SAMPLE USED ___________


A.  MASS OF SAMPLE                                                                       TRIAL 1                       TRIAL 2

      1.  Mass of evaporating dish (g)                                                      __________                  _________

      2.  Mass of dish and sample (g)                                                       __________                  _________

      3.  Mass of sample (g)                                                                      __________                  _________



      4.  Mass of dish and sample after heating (g)                                  __________                 _________

      5.  Mass of sample after heating (g)   (4 - 1)                                   __________                  _________

      6.  Mass of NH4Cl in sample (g)  (2 - 5)                                         __________                  _________



      7.  Mass of dish and dry sand (g)                                                    __________                   _________

      8.  Mass of SiO2 (g)  (7 - 1)                                                             __________                   _________



      9.  Mass of residue plus the beaker (g)                                           ___________                  _________

     10.  Mass of clean/dry beaker (g)                                                    ___________                  _________

     11.  Mass of NaCl (g)  (9 - 10)                                                        ___________                  _________







    12.  Percent NH4Cl in the sample  (6 / 3  X 100)                             ___________                  _________

    13.  Percent SiO2 in the sample      (8 / 3  X 100)                            ___________                  _________

    14.  Percent NaCl in the sample      (11 / 3  X 100)                         ___________                  _________                


1.                     9.87-g mixture contains 6.31 grams of NaCl and 3.47 grams of SiO2; the remaining is NH4Cl. 

Determine the percent by weight of each component in the mixture.                                                         



                                                                                                                                _____________ % NaCl


                                                                                                                                _____________ % SiO2


                                                                                                                                _____________ % NH4Cl








2.               If water contains a disagreeable volatile substance, how can it be removed?











3.        In this experiment, identify the substance that sublimes.