Silver Mirror Test Tube Experiment
Chemistry students love unusual and exciting demonstrations. This is certainly no revelation but the challenge is how to utilize these demonstrations to effectively teach useful content that students will retain. This demonstration uses an oxidation-reduction reaction to convert an ordinary test tube into a beautifully silvered mirror.

The silver-producing reaction in this demonstration is one that is commonly used in basic organic laboratory classes to identify aldehydes. The reaction called "The Silver Mirror Test" or Tollens' Test is accomplished by mixing aqueous silver nitrate with aqueous ammonia to produce a solution known as Tollens' reagent. Although this solution contains only a very weak oxidizing agent, it is strong enough to oxidize the aldehyde functional group. As this oxidation occurs, silver is reduced from the +1 oxidation state to metallic silver. This metallic silver is deposited on the walls of the test tube producing a reflective "mirror". The essential part of the overall reaction occurs as follows:

CH₂OH(CHOH)₄CHO +
(dextrose)

2[Ag(NH₃)₂]⁺ + 3OH^- →
(Tollens' reagent)

2Ag_(s) + CH₂OH(CHOH)₄COO^- + 4NH₃ + 2H₂O
("silver mirror")

Materials Needed

• 10 mL of 0.25 M dextrose
• 30 mL of 0.10 M AgNO₃ (silver nitrate)
• 15 mL of 0.80 M KOH (potassium hydroxide)
• 3 or 4 mL of concentrated NH₄OH (ammonium hydroxide)
• Large (8 inch) test tube
• Demineralized water
• Solid rubber stopper to fit the test tube
• 100 mL beaker
• Graduated cylinder
• Glass stirring rod
• Clean eyedroppers

Procedure

1. Measure 10 mL of the 0.25 M dextrose solution into a VERY clean 8 inch test tube.
2. Measure 30 mL of the 0.10 M silver nitrate solution into a 100 mL beaker.
3. With gentle but constant stirring, add concentrated ammonium hydroxide dropwise to the silver nitrate solution until a grey-black silver hydroxide, AgOH, precipitate forms. Continue adding concentrated ammonium hydroxide dropwise with gentle stirring until the solution JUST becomes clear and colorless.
4. Add 15 mL of the 0.80 M potassium hydroxide solution to the beaker. At this point, the silver hydroxide usually precipitates again, so add concentrated ammonium hydroxide dropwise with gentle stirring until the solution JUST becomes clear.
5. Pour the contents of the beaker into the large 8 inch test tube, place the stopper on the tube, and swirl the tube gently to insure the liquid contacts the entire inner surface. Continue to swirl and within 5 - 6 minutes, the entire inner surface of the tube will be coated with a silver mirror. An alternate procedure would be to rest the test tube at an angle and allow the majority of the silver to accumulate on the bottom surface. This will produce much heavier silvering but on a smaller portion of the internal surface.
6. Pour the liquid from the test tube down the drain with plenty of water. Rinse the test tube thoroughly with demineralized water.

Reminders

Always mix the chemicals fresh and dispose of them immediately after use with large amounts of tap water. The mixed chemicals can form an explosive precipitate if heated and then stored for several hours. This activity will work best by using a brand new test tube. It's almost impossible to get an old used one clean enough to silver evenly. If the interior of the tube is contaminated, the silver will be released as a grey precipitate rather than a silver mirror.

Safety Precautions

Mixtures of silver nitrate, ammonium hydroxide, and potassium hydroxide may form an explosive precipitate if heated and then stored for several hours. Silver nitrate is toxic by ingestion, and will stain skin and clothing. Potassium hydroxide is strongly corrosive as a solid and in solution. Avoid all contact with skin and eyes. Ammonium hydroxide is toxic by ingestion and inhalation. Dispense in a fume hood. Wear chemical splash goggles, chemical-resistant gloves, and chemical-resistant apron. The resulting mixture of solutions should be disposed of properly by flushing down the drain with excess tap water.

For more detailed instructions, consult your chemical demonstration hand book or general organic chemistry lab manual.

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