Separation Of Mixtures And Compounds For K Through 12

Patricia A. Riley                Lincoln Park High School
Viva Henson                      Andrew Jackson Elementary School
Patricia Doyle                   Peck Elementary School

Objectives:

    1.  To distinguish between mixtures and compounds.
    2.  To separate mixtures by using such techniques as solubility differences 
        in water, filtration and evaporation.
    3.  To decompose various compounds either by heating or electrolysis.

Materials needed:

white rice, uncooked       7 small beakers         dilute sulfuric acid
white sand                 table sugar             copper(II) chloride
table salt                 hydrogen peroxide       bromothymol blue indicator
distilled water            small raw potato        6 electrical leads with 
ground black pepper        1 medium beaker             alligator clips
iron filings               matches                 3 6-volt batteries
ground charcoal            hot plate               4 graphite electrodes
coffee filter papers       sodium sulfate          2 U-shaped drying tubes
2 utility clamps           2 ring stands           Hoffman electrolysis 
plastic spoons             strainer                    apparatus

Strategy:

    1.  Review definitions of element, compound and mixture while showing 
students examples of each (e.g. aluminum foil, sugar and Kool-Aide). 

    2.  Hold up a small bottle of sand and another of white rice.  Ask students 
to name some properties of each.  Now pour some of each into the same small 
beaker and stir.  How have the properties of each changed?  What have we made?  
How could we separate them?  Students may suggest separating them by picking out 
the rice; have a student try this.  What would be a faster way?  Use a strainer. 

    3.  Hold up a bottle of distilled water and another of table salt.  Again 
ask students to name their properties.  Pour small portions of each into the 
same small beaker and stir.  How have the properties of each changed?  What have 
we made?  How could we separate them?  Place the beaker on the hot plate and 
observe what happens with heating.  What is left in the beaker?  What left the 
beaker? 

    4.  Hold up samples of table salt and ground black pepper.  Compare their 
properties.  Mix small portions of each together in a small beaker.  What have 
we made?  Add water to the beaker.  What have we made?  What happened to the 
salt?  How can we separate the three ingredients from each other?  Use coffee 
filter paper and then evaporation on the hot plate.  Hold up the beaker to show 
the recovered salt.

    5.  Hold up samples of iron filings and charcoal.  Mix small portions 
together.  Have the students write out a procedure for separation.

    6.  Place a small beaker of sugar on the hot plate and slowly heat it while 
holding up a second small sample for students to observe.  What are the 
properties of sugar?  Students will eventually notice the smell of heating 
sugar.  Ask for observations.  What is in the beaker?  What might the moisture 
droplets on the side of the beaker be?  Light a match and then blow it out.  
Pass the burnt match about for inspection.  What does the tip of the match look 
like?  What element might it be?  Is there any resemblance to the heated sugar?
What elements compose sugar?  Is sugar a compound or a mixture? 

    7.  Hold up a bottle of hydrogen peroxide.  What is it used for?  Is it a 
compound or a mixture?  Cut a small potato in half.  Fill a medium sized beaker 
about half full of the peroxide and drop in the potato.  What observations can 
be made?  What might be causing the bubbles?  Explain that the potato cells 
contain the enzyme catalase which speeds up the decomposition of hydrogen 
peroxide into oxygen gas and water. 

    8.  Hold up a small beaker of pure water.  What about water itself?  Is it a
compound or mixture?  Would heating separate it?  Place the beaker on the hot 
plate with a watch glass over it; heat to boiling.  What is observed?  Show the 
students a Hoffman apparatus that has been operating long enough for the 
hydrogen gas column to be clearly twice that of the oxygen.  Explain the 
circuitry.  How many gases have been produced?  Which is which?  Write the 
equation for each electrode on the board. 

    9.  Repeat the electrolysis of water using a U-tube filled with a mixture of 
distilled water, sodium sulfate and bromothymol blue indicator.  Fasten the tube 
to a ring stand with a utility clamp.  What is happening at the yellow 
electrode?  What about at the blue?  What is happening in the green region?  Now 
add a drop of phenolphthalein indicator to each arm.  What happens?  Why was 
this indicator added?  (To prove that hydroxide ions are actually being made at 
the hydrogen gas electrode.)  Explain about acid/base indicators and write the 
reactions on the board. 

   10.  Can other compounds besides water be decomposed by electrolysis?  Pass 
around a small sample of copper(II) chloride.  Then pour a mixture of the 
chloride, water and phenolphthalein into another U-tube.  If water is being 
electrolyzed, then one arm of the tube should be pink, showing the presence of 
hydroxide ions and two gases will be seen.  What do the students actually see?