Analysis of Simple Mixtures
Academic Chemistry
Name _______________________
Introduction
In the study of chemistry, a distinction is made between compounds and mixtures, each being
composed of molecules. A compound consists of molecules joined together through chemical
bonding. A bond is the “glue” that holds the compound together. Compounds do not have the
same physical properties as their original components. Water, for example, is a compound
composed of two atoms of hydrogen and one of oxygen, both of which are gases. When these
molecules are joined under the appropriate conditions, a chemical bond is formed, “gluing” the
atoms together. The result of this bonding is the formation of water, a liquid. Mixtures are not
“glued” together by chemical bonds. If any chemical bonds do exist, they are very weak and are
easily broken at room temperature by the random motion of each molecule. Mixtures are much
more easily separated into their original components than compounds. An example of a mixture
is a mix of salt and pepper. When the salt and pepper mixture is placed in water, the salt will
dissolve, leaving the pepper behind. The mixture is now separated.
A mixture can be described as either homogeneous, having visually indistinguishable parts or
heterogeneous, having visually distinguishable parts. Sometimes it is difficult to see the
heterogeneous mature of a mixture. For example, air is a mixture, containing many different
substances, yet it appears homogeneous. Other mixtures, such as granite, consist of materials
that are clearly distinguishable from each other.
To distinguish mixtures from compounds, one must try to separate the material into its
components. Mixtures are more easily separated into parts than are compounds. Also, simply
mixing the separated materials together will regenerate the original mixture. Compounds,
however, are very hard to separate, since they cannot be broken down without breaking the
“glue” of the chemical bonds. Once the bonds have been broken, the must be re-formed to
produce the original material. A compound cannot be re-formed by simply mixing the
components together.
Most of the materials which we use in everyday life come to us ready to use. When you buy a
bottle of turpentine or a gallon of gas, you don’t need to separate the material from all of the
other substances it used to be mixed with. The companies that make materials such as gasoline
and paint thinner do the work for you. But how are materials that are all mixed together
separated? This activity is designed to help you understand how mixtures are separated.
To separate a mixture into its different parts, we have to find a method which will react
differently with the different components. There are five basic techniques which you will use in
this activity. Each of them is described, along with the instructions for performing the
separation.
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1. MAGNETIC SEPARATION – Many materials contain iron. These are called ferrous
metals. You probably know that iron can be attracted by a magnet. If one of the
materials in a mixture contains iron, and the other does not, then we can separate the
mixture by bringing it near a magnet.
2. SIZE SEPARATION – If a mixture is composed of pieces that have different sizes, we
can separate them by sifting.
3. FLOTATION – If one of the materials in a mixture is less dense than water, it will float.
If the other material is denser than water it will sink, and we can separate the mixture. In
some cases, dependent on the density of the materials, a liquid other than water may be
used for this type of separation.
4. SOLUBILITY – Solubility refers to the fact that some materials dissolve in a liquid and
others do not. If one part of a mixture can be dissolved in water (or some other liquid) it
can be separated from the rest of the mixture. If the liquid containing the dissolved
mixture is poured off, and the liquid evaporated, then the pure substance can be reformed.
5. MELTING/RECRYSTALLIZATION – By melting part of the mixture, and pouring off
the liquid, we can separate the mixture. Usually, the two parts of a mixture will have
different melting points, so they can be separated easily. We could also separate two
liquids by boiling off one of the liquids.
Pre-Lab Questions
1. How can you tell if a mixture has been completely separated?
2. Some mixtures appear to be a single substance, yet are actually mixtures. Such a
substance is described as being homogeneous. Think of two such materials that are really
mixtures. How would you separate each mixture?
3. Using reference materials, find out what separation techniques are used to manufacture
iron from iron ore and gasoline from crude petroleum.
Materials
1 – Magnet
1 – Spatula
3 – Small Plastic Cups
1 – 16 x 100 mm Test Tubes
1 – Red Cap
3 – Beral Pipets
3 – 20 x 150 mm Test Tubes (In your lab drawer)
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3 – Watch Glasses (In your lab drawer)
1 – Wire Gauze (In your lab drawer)
1 – Thick Plastic Cup
1 – Mesh Screen
1 – Blue Plastic Mesh Holding Lid
Liquid Soap
Mixture A
Mixture B
Mixture C
Procedures
1. Locate the jar labeled “A”. Place several spatulas of the sample material in one of the
small plastic cups, up to a height of 2 to 3 cm. Examine the material carefully. Is there
any evidence that the material is made up of more than one substance? Be sure to record
which material you have taken. Run through all the experimental steps using the same
material.
2. Place the magnet in a second small plastic cup. Place the cup with the magnet inside the
cup with the material, and gently rub the material against cup holding the magnet. If any
of the substances in the material are iron-containing, they will be attracted to the upper
cup. The purpose of the cup is to keep the iron material from sticking directly to the
magnet. Record your results in the data table.
3. Place some of your sample in the thick plastic cup that holds the mesh screen. Secure the
mesh screen across the top with the blue plastic mesh holding lid. Hold the cup over a
white paper towel, turn the cup upside down, and shake the sample. If all the material is
of the same size, it will either fall through or be held back. If the materials are of
different sizes they will separate. Record the results in the data table.
4. Fill another plastic cup ⅔ full of water. Add 1 drop of soap to the water to make sure the
floatation is not due to the surface tension of the water. Place ½ spatula of the material
into the water and stir. Does any of the mixture float? Sink? Record your results in the
data table.
5. Put enough material into a 16 x 100 mm test tube so that the material to a height of
approximately 0.5 to 1.0 cm. Fill the test tube ⅔ full of water. Cap the tube, and shake it
well. This test is to see if some of the material has dissolved. (This is called going into
solution). Allow the test tube to sit for five minutes to allow any suspended material to
settle. Use a beral pipet to remove some of the clear solution and place the solution on a
watch glass. Allow the liquid to evaporate overnight. The next day observe if there were
any dissolved materials and record your results in the data table.
6. In a 20 x 150 mm test tube, place a 1 – 2 spatulas of the material to a height of 1 to 2 cm.
Heat the mixture over the Bunsen burner. Keep the tube moving over the flame, and
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keep the open end pointing away from yourself and others. When the mixture starts to
bubble remove it from the heat. Slowly tip the tube, do not invert completely. Lay the
tube on its side on your table on your wire gauze heat protective mat. The melted part
will collect on the side and the solid will remain on the bottom. Record your results in
the data table.
7. Place any of the dry substances and the original material in the garbage. Place the water
and wet material from step number 5 in the appropriate disposal beaker. Wash all
materials, rinse in distilled water and dry them. Dispose of the test tube from step
number 6 in the broken glass receptacle. See your teacher for a replacement test tube.
Repeat the procedure with each of the other materials.
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Data and Observations
Property
Mixture A
Mixture B
Mixture C
Magnetic
Size
Floatation
Solubility
Melting/Recrystallization
Post-lab Questions
1. Where A, B and C mixtures?
2. If appropriate how many components are in each mixture? What evidence do you have
to support this, for each mixture?
3. If appropriate, which methods of separation were the most effective at separating mixture
A? B? C?
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