A giant silver mirror

Read our standard health & safety guidance

Lesson organisation

A teacher demonstration which takes 5 minutes to perform. There are also possible extension activities, each of which would take a similar time.

Although the reaction involved is related to the post-16 study of aldehydes and reducing sugars, this dramatic demonstration will fascinate a wider age range.

The reaction may also be used on a small-scale as a student experiment for post-16 advanced studies in chemistry – see Teaching Notes below.

Apparatus and chemicals

Eye protection for the teacher

Round- or flat-bottomed flask, 1 dm³ (see note 1)
Rubber stopper, to fit flask
Beaker (250 cm³)
Measuring cylinders (25, 100 and 250 cm³)
Dropping pipette
Glass rod
Access to a fume cupboard (for handling '880' ammonia solution)

The quantities listed below are sufficient for three demonstrations:

Silver nitrate (Corrosive, Dangerous for the Environment), 8.5 g
Potassium hydroxide (Corrosive), 11.2 g
Glucose (dextrose), (Low hazard), 2.2 g
'880' Ammonia solution (35% w/v) (Corrosive, Dangerous for the Environment), 30 cm³
Concentrated nitric acid (Oxidising, Corrosive), 100 cm³
Purified (distilled or deionised) water, 800 cm³

Technical notes

Silver nitrate (Corrosive, Dangerous for the Environment) Refer to CLEAPSS Hazcard 87
Potassium hydroxide (Corrosive) Refer to CLEAPSS Hazcard 91
Ammonia solution (‘880’ or 35% w/v) (Corrosive, Dangerous for the Environment) Refer to CLEAPSS Hazcard 6
Concentrated nitric acid (Oxidising, Corrosive) Refer to CLEAPSS Hazcard 67

1 Thorough cleaning of the 1 dm³ flask is vital if the demonstration is to succeed:

• first use detergent and a brush, then rinse with water.
• next rinse with concentrated nitric acid
• finally wash out several times with purified water.

2 Make up the three solutions needed as follows:

• dissolve 8.5 g of silver nitrate in 500 cm³ of purified water to make a 0.1 mol dm^–3 solution.
• dissolve 11.2 g of potassium hydroxide in 250 cm³ of purified water to make a 0.8 mol dm^–3 solution.
• dissolve 2.2 g of glucose in 50 cm³ of purified water.

Do NOT mix any of the solutions together before the lesson.

After the lesson, the silver can be removed from the silvered flask using concentrated nitric acid. Work in a fume cupboard as nitrogen dioxide (Toxic) is formed.

Procedure

HEALTH & SAFETY: Before performing this demonstration, the teacher should consult CLEAPSS Hazcard 87 for advice on the hazards of the silver compounds used and formed in this demonstration. Do NOT prepare Tollen's Reagent in advance. It is likely to explode on standing.

After the demonstration, do NOT save the silver solution in a silver residues container. The solution must be disposed of down the sink with plenty of cold water within 30 minutes of mixing at the start of the demonstration. This is to avoid any chance of the formation of a deposit of silver fulminate, a dangerously explosive substance.

a Place 150 cm³ of the silver nitrate solution in a 250 cm³ beaker.

b Working in a fume cupboard, add ‘880’ ammonia using a dropping pipette until the brown precipitate first formed re-dissolves to give a clear, colourless solution. Less than 5 cm³ of ammonia solution should be needed. The solution then contains the colourless complex ion, [Ag(NH₃)₂]⁺(aq).

c Add 75 cm³ of the potassium hydroxide solution. A dark brown precipitate of silver(I) oxide will form. Add more ammonia solution dropwise until this re-dissolves to give a clear, colourless solution. About 5 cm³ of ammonia will be needed. This solution is sometimes called Tollen’s reagent.

d Pour this solution into the 1 dm³ flask and add 12 cm³ of the glucose solution. Stopper the flask and swirl the solution so that the whole of the inner surface of the flask is wetted. The solution will turn brown. Continue swirling until a mirror forms in about 2 minutes.

e When a satisfactory mirror has formed, wash the solution down the sink with plenty of water. Rinse out the flask well with water and discard the washings down the sink. The flask can now be passed around the class.

An alternative to plating the inside of a flask is to silver plate the outside of small glass objects which can be suspended in the plating solution by hanging them on threads. These objects must be thoroughly cleaned beforehand.

Teaching notes

For pre-16 students this demonstration is more likely to be performed for its spectacular nature rather than for the chemical changes that take place. However for post-16 students, these chemical changes may form part of their studies, and the teacher may wish to consider a variant as a student experiment:

1 In a new, clean test-tube add one drop of 0.4 mol dm^-3 sodium hydroxide solution (Irritant) to 1 cm³ of 0.1 mol dm^-3 silver nitrate(V). Add 1 mol dm^-3 ammonia solution dropwise until the precipitate just re-dissolves.

2 To this solution, add a few drops of glucose solution and warm gently in a beaker of hot water (do NOT heat the tube directly in a flame) with occasional swirling until the silver mirror forms.

3 Discard the solution within half an hour of preparation by washing it down a sink followed by a large amount of water.

The experiment may be repeated as either the large-scale demonstration or as the student version using an aldehydes such as ethanal or propanal. In contrast, a ketone such as propanone does not react, illustrating an important chemical difference between aldehydes and ketones.

Aldehydes reduce Ag⁺(aq) ions to metallic silver. The aldehydes are oxidised to carboxylate ions. The reaction that occurs using propanal ( a safer alternative to ethanal) is:

CH₃CH₂CHO(aq) + 2[Ag(NH₃)₂]⁺(aq) + 3OH^-(aq) → 2Ag(s) + CH₃CH₂COO^-(aq) + 4NH₃(aq) + 2H₂O(l)

Health & Safety checked, June 2008

Web Links

For another description of the demonstration:
http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Tollens-e.htm

A website that sets the reactions in this demonstration in the wider context, see:
http://www.chemguide.co.uk/organicprops/carbonyls/oxidation.html

For further discussion of Tollen’s reagent:
http://en.wikipedia.org/wiki/Tollens'_reagent

For a brief outline of the history of silvering mirrors:
http://en.wikipedia.org/wiki/Silvering

(Last accessed July 2008)