Demonstration 3: Indicators and the pH Scale

Different indicators change colors at different pH values.

• Materials
  
  Universal indicator solution
  Bromthymol blue indicator solution
  Phenol red indicator solution
  Other indicator solutions if desired, or solutions of known pH: 1, 4, 7, 10

• Safety
  
  Goggles should be worn even though these solutions are weak acids and bases.

• Directions
  
  Line up twelve petri dishes (or small beakers) on the overhead projector as shown here:
  Place a separate solution of pH 1, 4, 7, and 10 in each row. Add universal indicator to each petri dish in the first row, bromthymol blue to each dish in the second row, and phenol red to each dish in the third row. Add one to three drops (or the amount of indicator needed to produce an observable color) to each petri dish. Either invite students to make their own observations and interpretations or, alternatively, interpret the results for them.

Demonstration 4: Conductivity

Acid and bases conduct an electric current.

• Materials
  
  Conductivity apparatus (see Instrumentation)
  Beaker, 250 mL
  10 Beakers, 50 mL
  0.01 M or 0.1 M Sulfuric acid, H2SO4
  0.01 M or 0.1 M Barium hydroxide, Ba(OH)2
  Phenolphthalein solution, 1%
  0.1 M Hydrochloric acid, HCl
  0.1 M Acetic acid, HC2H3O2
  0.1 M Aqueous ammonia, NH3
  0.1 M Sodium hydroxide, NaOH
  Glacial acetic acid, HC2H3O2
  Selected household products (e.g., Drano(TM), vinegar, etc.)

• Safety
  
  Many commercial products can be corrosive to the skin or damage clothing. Apron and goggles should be worn.

• Conductivity Test Directions
  
  Test the 0.1 M solutions and selected household products for their relative conductivities (do not test H2SO4 or Ba(OH)2; save these for the conductivity titration). Use a clean beaker for each solution. Test glacial acetic acid last in this manner: test its conductivity, then double the volume by adding distilled water and test the conductivity again. Continue in this fashion until adding water no longer changes the conductivity of the acetic acid.

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