Introduction

Sodium hydroxide is added to a cylinder of aluminum nitrate, precipitating out aluminum hydroxide.  When additional base is added, the precipitate dissolves.  The reactions can then be reversed with the addition of hydrochloric acid.  The precipitate will reappear and then redissolve.

Equations

 Al(NO3)3(aq) + 3NaOH(aq) → Al(OH)3(s) + NaNO3(aq)

 Al(OH)3(s) + OH-(aq) → Al(OH)4-(aq)

 Al(OH)4-(aq) + H+(aq) → Al(OH)3(s)

 Al(OH)3(s) + 3H+(aq) → Al3+ (aq) + 3H2O(l)

Supplies

Short List

  •   Goggle and gloves
  •   1000 mL cylcinder
  •   2 x 600 mL beakers
  •   Stir plate and stir bar

Chemicals

  •   50mL of 1M Al(NO3)3
  •   400mL of 1M NaOH
  •   400mL of 1M HCl

Prior to Lecture  (prep time ~ 5 min.)

1. Prepare the following chemicals:

  1.  50mL of 1M Al(NO3)3 (aluminum nitrate)
  2.    37.5g Al(NO3)3.9H2O / 100mL
  3.   400mL of 1M NaOH
  4.   400mL of 1M HCl

2. Assemble the following equipment:

  1.  1000mL cylinder
  2. Two 600mL beakers
  3.  stir plate and stir bar
  4.  goggles

To Conduct Demonstration:

  1.  Turn on stir plate and add NaOH solution until Al(OH)3 precipitates.
  2. Add more NaOH until the hydroxide precipitate dissolves.
  3.  Add HCl until the precipitate is visible.
  4. Continue to add HCl until the hydroxide is re-dissolved.

Safety 

Goggles should be worn. 

 Chemicals may be disposed of with the 1131 Qual Lab.

Reference:

Shakhashiri, B.Z. Chemical Demonstrations:  A Handbook for Teachers of Chemistry; 1983.