The silver mirror reaction
Scientific description of an amazing experiment
The experiment that has the beautiful effect of a mirror surface forming on glass looks very impressive. Carrying out this reaction requires experience and patience. In our article you can read about the necessary reagents, how to prepare the equipment, and find out the chemical formulas of reactions.
The essence of the silver mirror reaction is the formation of metallic silver as a result of an oxidation-reduction reaction, in the interaction of an ammonia solution of silver oxide in the presence of aldehydes.
To make a durable silver layer you will need:
- a glass flask with a volume of up to 100 ml;
- 2,5-4% ammonia solution;
- silver nitrate 2%;
- formaldehyde solution (40%).
Instead of the ammonia and silver nitrate solutions, you can also use Tollens’ reagent – an ammonia solution of silver oxide. Add 1 gram of silver nitrate to 10 drops of water (if the liquid will be kept for a long time, keep it in a dark place or in a dark glass bottle). Immediately before the experiment, mix the solution (around 3 ml) in the proportion of 1/1 with a 10% sodium hydroxide solution. There may be a sediment of silver, which you can dilute by slowly adding ammonia solution. We also recommend you to conduct another spectacular experiment with ammonia solution and to print the «chemical photo».
The reaction takes place at room temperature. For the result to be successful result, the walls of the glass vessel must be perfectly clean and smooth. If there are tiny particles of dirt on the walls, the sediment created in the experiment will have the appearance of a crumbly layer of a black or dark grey color.
To clean the flask, use different types of alkaline solutions. First, use a caustic soda solution, and then rinse the flask with simple distilled water. Rinse the flask of the cleaning fluid many times.
Why it is so important for the vessel to be clean
This is because the colloidal particles of silver formed at the end of the experiment must firmly lock with the surface of the glass, and its surface should not have any fat or mechanical particles on it. The distilled water does not contain salts and is ideal for the final rinsing of the flask. You can make it at home, but it is easier to buy it in ready form.
Formula for describing the silver mirror reaction:
Ag₂O + 4 NH₃·Н₂О ⇄ 2[Ag(NН₃)₂]ОН + 3Н₂О
where [Ag(NН₃)₂]ОН is silver diamine hydroxide, obtained in the dissolution of the metal oxide in an ammonia solution.
NB Observe the requirements for the ammonia concentration, the reaction works at low concentrations!
You can examine the final stage of the reaction in the following formula:
R (any aldehyde)-CH=O + 2[Ag(NH₃)₂]OH → 2Ag (the silver colloid that forms the sediment) ↓ + R-COONH₄ + 3NH₃ + H₂O
It is better to carry out the second stage of the reaction by carefully heating the flask on a burner, then the process should go ahead without any problems
What the silver mirror reaction can show
This interesting chemical reaction not only let us examine certain states of a substance, but can also give make an accurate definition of aldehydes. This reaction solves the question: is there an aldehyde group in the solution or not?
For example, in this process we can find out what component the solution contains: glucose or fructose. Glucose gives a positive result, a “silver mirror” is formed, while fructose contains a ketone group and cannot form a silver sediment. To make an analysis, you must add a 10% glucose solution instead of a formaldehyde solution. Let’s use a formula to see why and how the dissolved silver turns into a solid sediment:
2[Ag(NH₃)₂]OH + 3H₂O + C₆H₁₂O₆ (glucose) = 2Ag↓+ 4NH₃∙H₂O + C₆H₁₂O₇ (gluconic acid forms).