A giant silver mirror
A solution containing silver nitrate (Tollen’s reagent) and a reducing sugar (glucose) react to form silver, which is deposited as a mirror-like coating on the inside of the reaction vessel.
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.
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), 2.2 g
'880' Ammonia solution (35% w/v) (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT), 30 cm3
Concentrated nitric(V) acid (OXIDISING, CORROSIVE), 100 cm3
Purified (distilled or deionised) water, 800 cm3
Refer to Health & Safety and Technical notes section below for additional information.
Eye protection for the teacher
Round- or flat-bottomed flask, 1 dm3 (Note 1)
Rubber stopper, to fit flask
Beaker (250 cm3)
Measuring cylinders (25, 100 and 250 cm3)
Access to a fume cupboard (for handling '880' ammonia solution)
Health & Safety and Technical notes
Before performing this demonstration, the teacher should consult the CLEAPSS Hazcard 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.
Wear googles and disposable nitrile gloves.
Silver nitrate, AgNO3(s), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Potassium hydroxide, KOH(s) (CORROSIVE) - see CLEAPSS Hazcard.
Glucose (dextrose), C6H12O6(s) - see CLEAPSS Hazcard.
Ammonia solution, NH3(aq), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Concentrated nitric(V) acid, HNO3(aq), (OXIDISING, CORROSIVE) - see CLEAPSS Hazcard.
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 M solution
- dissolve 11.2 g of potassium hydroxide in 250 cm3 of purified water to make a 0.8 M 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.
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.
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 M sodium hydroxide solution (IRRITANT) to 1 cm3 of 0.1 M silver nitrate(V). Add 1 M 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
For another description of the demonstration:
Chemguide - a website that sets the reactions in this demonstration in the wider context.
Wikipedia - for further discussion of Tollen's reagent.
Wikipedia - for a brief outline of the history of silvering mirrors.
Page last updated on 30 December 2011