Exp 2: Identification of a Compound: Physical Properties

1
Exp 2 Identification of a Compound Physical
Properties

• Introduction
• Chemistry is based on the relation between
  experimental observations and theory
• Experiments lead to theories lead to other
  experiments
• Accurate, precise and complete observations are
  the basis for successful experiments
• Without good observations there is no basis for
  developing a coherent theory
• Purpose
• Identify an unknown compound based on properties
  of known compounds
• Practice proper and accurate observations and
  focus on the conclusions that can be drawn from
  those observation
• Correct and complete observations are the only
  tools that you need for this experiment

2
Exp 2 Identification of a Compound

• Successful qualitative analysis
• . . . when you have excluded the impossible,
  whatever remains must be the truth.
• (Sherlock Holmes)

3
Exp 2 Identification of a Compound

• Experimental Concept
• Determine the identity of an unknown compound
• observe chemical reaction of a series of
  solutions of known solid compounds
• observe the reactions of an unknown compound and
  compare to the known compounds
• Known compounds
• sodium chloride, NaCl
• sodium carbonate, Na2CO3
• magnesium sulfate, MgSO4
• ammonium chloride, NH4Cl
• water, H2O

4
Exp 2 Identification of a Compound

• Experimental Concept (contd)
• Test compounds for
• reaction of solutions with silver nitrate
  reagent, AgNO3
• reaction of solutions with sodium hydroxide
  reagent, NaOH
• reaction of solutions with hydrochloric acid
  reagent, HCl
• Observe Evidence of a Reaction (chemical change)
• change of color in a solution
• change in odor of a solution
• evolution of a gas
• bubbles forming in the solution (not just a few,
  but a lot)
• appearance or disappearance of a precipitate
  (insoluble material)
• solids can be so fine that they are sometimes
  hard to see
• a precipitate will eventually settle at the
  bottom of a test tube

5
Formation of an Insoluble Precipitate
Pb(NO3)2 Na2CrO4 ? PbCrO4 2 NaNO3
6
Exp 2 Identification of a Compound

• Preparation Put 10 drops of each of the
  solutions (NaCl, Na2CO3, MgSO4, NH4Cl and water)
  in separate wells of the clean, marked test
  plates (spot plates). These are numbered 1-5
  across the length of the plate. Label the wells
  along the width with A,B and C for each test
• Testing with Silver Nitrate
• Add 3-5 drops of the silver nitrate (AgNO3)
  solution. Add 5-10 more drops to those that seem
  to change.
• Record your observations including color, thick
  or thin precipitate!
• Testing with Sodium Hydroxide
• Slowly add 5-10 drops of the sodium hydroxide,
  NaOH, solution. Check for gas, odor (use proper
  techniques) and formation of precipitates
• Record your observations including color, thick
  or thin precipitate!

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Exp 2 Identification of a Compound

• D. Testing with Hydrochloric Acid
• 1. Slowly add 5-10 drops of the hydrochloric
  acid reagent, HCl, to
• each solution. Check for gas, odor (use
  proper techniques).
• 2. Observe closely Record your
  observations including color, odor
• if any
• Testing the unknown solution
• 1. Place 10 drops of the unknown solution in
  three wells
• of column 6
• 2. Test with the reagents, using the same
  proportions as with the
• known solutions. Compare results and
  identify your unknown.
• Clean up
• Dispose solutions in the liquid waste container
  in the fume hood
• Rinse the plates with tap water and DI water

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Exp 2 Identification of an Unknown Compound
Reaction/Observation Worksheet
NaCl Na2CO3 MgSO4 NH4Cl H2O Unknown _____
AgNO3 test
NaOH test
HCl test


indicate number or letter and name of the
unknown compound

• Key c color w,y,bl,br,gr
• pc precipitate color g gas, no odor
• cc cloudy color go gas, odor
• nr no reaction

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Exp 2 Identification of a Compound

• Next week
• SPRING BREAK Study for Lecture Quiz Monday
• Tuesday June 9 Turn in Pre-lab for Exp 10
  Vinegar Analysis
• Read procedure and safety precautions