Dichotomous Key Lab

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Vocabulary:

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dichotomy - the process of dividing into two mutually exclusive or contradictory groups

A starting point for the scientific investigation of anything is to observe and find similarities and differences among things.  Scientific work must be able to be understood and repeated by others studying the same thing in order to be useful.  One way that scientists accomplish these goals is to classify things according to their traits.  To help others to use their method of classification, scientists often construct a dichotomous key.

A dichotomy is a division into two parts.  In a dichotomous key there are a series of paired statements.  If one is true about the thing you wish to classify, you go on to another pair of statements.  The alternative answer leads you to a different pair of statements.  In this way, by choosing one or the other of paired statements, you are led to a final category for the thing you are observing.

This lab covers both the concept and use of a dichotomous key and the classification of matter.  Using informational questions and vials containing various types of matter, you will classify the vials into mixtures; compounds; elements; etc.  This flowchart provides a visual reference on how to classify matter and this flowchart provides a visual reference on all the properties of matter.  Both of these charts must be included in the data section of your lab report (for a better printing, set your page setup to "landscape" and then print.)

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Procedure:

            This lab exercise uses a dichotomous key to help you classify the contents of small vials into the proper type of matter.  The classification of matter is a fundamental step in physical science.  As you cannot directly observe all of the traits you will be looking for, you must rely on previous knowledge; charts within the science lab room and any information that others may share with you during the classification procedure.  As you make each choice for the contents of a vial, write the number and letter of each statement you choose in the column labeled "justification" on the line for that vial.  Once you reach your final category, the statements you chose will be justification for your choice.  The chart of your findings should look like this.

Materials:

 pen/pencil  Dichotomous Key Chart  matter flowchart
 student worksheet  vial of sand  vial of glue
 vial of gravel

 vial of sugar (C12H22O11)

 vial of salt in water
 vial of sulfur (S)  vial of lead (Pb)  vial of beads
 vial of milk of magnesia  vial of bromothymol blue in water  vial of copper (Cu)
 vial of water (H2O)    

A Dichotomous Key to the Classification of Matter

1. a) the contents of the vial have mass and take up space....................................................go to Q2
  b) the contents of the vial does not have mass or does not take up space....................it is ENERGY
2. a)

the contents of the vial have a variable composition, a sample of this material is not uniform or different samples could differ in the proportions of the things that make it up.............................. ....................................................................................................it is a MIXTURE; go to Q3

  b) the contents of the vial have a definite composition; every sample of this material will have the same composition............................................................it is a PURE SUBSTANCE; go to Q6
3. a) the contents of the vial are uniform throughout, although their proportions could differ from one sample to another..............................................it is a HOMOGENEOUS MIXTURE; go to Q5
  b) the contents of the vial vary in composition or traits from one part of the sample to another; they do not appear uniform or are known to vary from one part of a sample to another or to settle out over time....................................................it is a HETEROGENEOUS MIXTURE; go to Q4
4. a) the contents of the vial may appear uniformly distributed, but solid particles will settle out of the liquid portion of the mixture.....................................................................it is a SUSPENSION
  b) the contents of the vial visibly vary in composition........it is a HETEROGENEOUS MIXTURE
5. a) the particles of the homogeneous mixture are atoms, ions, or molecules; in a transparent mixture they will not scatter a beam of light.............................................................it is a SOLUTION
  b) the particles of the homogeneous mixture are larger than molecules, they make the mixture appear cloudy or milky or they scatter light in a transparent mixture...................it is a COLLOID
6. a) the substance in the vial is made up of a single type of atom that has a symbol and appears on the Periodic Table of the Elements....................................................................it is an ELEMENT
  b) the substance in the vial is made up of two or more atoms chemically combined in a fixed ratio........................................................................................................it is a COMPOUND

Student Work Sheet - for a printer friendly version, click here.

Vial Number & Substance What is it? Justification
 1. gravel    
 2. sugar (C12H22O11)    
 3. salt in water    
 4. sulfur (S)    
 5. lead (Pb)    
 6. beads    
 7. milk of magnesia    
 8. sand    
 9. copper (Cu)    
 10. water (H2O)    
 11. bromothymol blue in water    
 12. glue    

When doing the lab report write-up, be sure to follow the guidelines.

Lab Notes  >