Experiment 4 Separation of a
Mixture
OBJECTIVES


To learn how to use physical properties of four different substances to design a
scheme for the separation of a mixture.
After the separation is achieved,, percent composition of each component in the
mixture will be calculated.
DISCUSSION
One of the most important laboratory techniques used by chemists is called separation.
Often when chemists encounter an unknown mixture, their first objective is to separate
and then identify the mixture into its constituent parts. But before we go on we need to
first define the difference between an element, compound, and a mixture.
Element: An element is a fundamental component of matter that can not be broken
down by simple chemical means. The periodic table, which we have talked about in
class, contains all the elements we know about up to this point. An example of an
element is copper (Cu). At room temperature copper is a solid, if the temperature is
raised above the melting point of copper it becomes a liquid. But the element has not
changed; it is still made up of copper atoms.
Compound: Substances that are made up of elements and can be broken down into
different elemental parts only by chemical means. A good example of a compound is
the metal brass. This soft metal is made up of different amounts of copper (Cu), zinc
(Zn), tin (Sn), lead (Pb), and iron (Fe). A brass block can not be physically separated
into these elemental parts, it will take chemistry to be able to do that.
Mixture: A combination of elements and /or compounds that do not react chemically
and can be separated by physical means. If a beaker contains a mixture of copper (Cu)
and iron (Fe) powder at room temperature these two elements can be separated. It
might take a while, but it can be done. Any suggestions on how you might do that?
Given an unknown mixture, a chemist knows that each of its components has their own
fundamental physical properties that enable us to separate and identify these individual
components and determine their percent composition. Some of the physical properties
that we will use in this lab to separate a mixture includes solubility, boiling point, and
sublimation.
The solubility of a substance indicates how easily a particular substance called a solute
dissolves in a solvent, which is often a liquid. The boiling point is the temperature at
which a substance changes from the liquid phase to the vapor phase. Finally,
sublimation is the ability of a substance to change from the solid phase to the gas
phase without ever entering the liquid phase. This doesn’t happen for many solids at
room temperature, which will help us in designing our separation procedure.
This leads to the different types of separations that are commonly used by chemist:
1- Distillation is the process of heating the liquid to its boiling point, condensing it (by
cooling), and collecting the vapors. This process uses the differing boiling points of
liquids for separation.
2- Extraction and Filtration utilizes the solubility of a substance to permit separation.
Extraction is the process of removing a substance with a greater solubility from a
substance with a lower solubility. Filtration is the process of straining a solid
suspended in a liquid.
3- Centrifugation is the process of separation a suspended solid in a liquid from the
liquid by high speed rotation.
4- Chromatography is the process by which substances pass through a stationary phase
(different type of molecules) and are separated by difference in physical attraction to
this phase. We will talk more about these methods later in the quarter.
In this experiment you will be given a mixture of iron filings (Fe), ammonium chloride
(NH4Cl), sodium chloride (NaCl), and sand which is silicon dioxide (SiO2) of unknown
composition (you will not know how much of each substance is present in the solid
mixture). By carefully examining the table below, which details the different physical
properties of the species you will be analyzing, you will then design a separation scheme
for your mixture.
Table of Physical Properties
Sodium
chloride
CHEMICAL
Ammonium
chloride
NaCl
NH4Cl
58.5
53.5
Silicon
dioxide
SiO2
Iron
Fe
EQUATION
MOLECULAR
60.1
55.8
WEIGHT
SOLUBILITY
(g ) in 100g
water at 25 C
MELTING
POINT C
35
37
801
sublimes 350
insoluble
1600
insoluble
1535
white
APPEARANCE
white
white
crystals
crystals
crystals
grains
dark
brown
We will at the beginning of lab talked extensively about the different separation
techniques and the above table, after which time you will be asked to design a flow
chart that will be used to separate the different components of an unknown mixture.
Your flow chart will be rather general to begin with since the detailed steps of the
separation techniques are, as of yet, unknown to you—we will discuss that in class.
But from examining the table you should be able to logically derive a procedure to
determine the percent composition of sand, salt, iron, and ammonium chloride.
You will be working with a partner in this lab. Carefully look at the chart and try and
find unique physical properties that could be use to separate these different
components. If you have any questions please ask the instructor. After you have
designed your procedure in the form of a flow chart show it to your instructor for final
approval. Once your procedure has been approved the instructor will help you figure out
the detailed procedure with the chemical techniques to be use in this experiment.
Once the different components have been isolated you will weigh each of them and find
the percent composition of each in the unknown mixture. After this has been
completed, ask the instructor for the real percentage values for each. This comparison
will be use then to discuss the possible errors involved in the experiment and how your
procedure could have been improved.
Pre-laboratory
Assignment
Separation of Mixtures
Experiment#4:
1. Give an example of an element involved in this lab.
2. One of the components to be separated is salt. What type of chemical bond is found
in salt?
3. Define solubility. Is salt very soluble in water?
4. You could go to an alternative food store and buy "Sea Salt" for 5 times what it cost
to by regular table salt. But you could also gather it yourself. Briefly describe how this
could be done.