Heating sugar, two house cleaners, and silver nitrate to make a mirror
In March 2023, due to our reputation for being science enthusiasts and always messing around in the lab, our school chose us (my friend Máximo and I) to conduct an experiment for its inaugural scientific congress. A few months prior, we had performed the Tollens’ test for fun (as usual) to create a silver mirror on a round-bottomed flask. Hence, for our scientific project, we decided to investigate how the reaction temperature affects the deposition of silver.
The experiment is very straightforward. First, a few drops of a dilute sodium hydroxide solution (NaOH), used as a household cleaner, are added to a 0.1 mol dm-3 aqueous solution of silver nitrate (AgNO3). This converts the silver nitrate to silver(I) oxide (Ag2O) precipitate via the following reaction:
2AgNO3(aq) + 2NaOH (aq) → Ag2O(s) + 2NaNO3(aq) + H2O (l)
Then, an adequate amount of ammonia solution (NH3) (also used as a household cleaner!) is added to convert the silver(I) oxide into the diamminesilver(I) coordination complex ([Ag(NH3)2]+), the main component of Tollen’s reagent. This happens through the following reaction:
Ag2O(s) + 4NH3 (aq) + 2NaNO3(aq) + H2O (l) → 2[Ag(NH3)2]NO3(aq) + 2NaOH(aq)
The flasks containing the diamminesilver(I) solutions were placed in a water bath and monitored using a thermometer until the desired temperature was reached.
Now comes the fun part: to make the silver mirror, we only need to add a reducing sugar (glucose). Yes, you heard just right, the solution only requires some sweetening to make the flask shine! We used glucose, but any reducing sugar (containing aldehyde groups) will do.
The reaction that takes place is what Tollens’ test consists of: an aldehyde (R-CHO) is oxidised to a carboxylic acid (R-COOH), and the diamminosilver(I) complex is reduced to elemental silver (Ag), which deposits on the surface of the flask.
2[Ag(NH3)2]+(aq) + R−CHO(aq) + H2O(l) → 2Ag(s) + 4NH3(aq) + R−COOH(aq) + 2H+(aq)
We made mirrors on the inside of the flasks at five different temperatures and then weighed them (knowing their initial mass).
After performing the experiment, we produced a graph illustrating the relationship between the mass of deposited silver and the reaction temperature. We found that at higher temperatures, more silver was deposited, probably due to a faster reaction rate (we only allowed the reaction to proceed for a limited time). We then created a poster and a presentation and presented our results at the scientific congress.
Chemistry doesn’t only bond atoms, it also bonds people!
With a wealth of additional scientific knowledge, two years after presenting it, we stood next to our poster in admiration, reviving all our lab adventures together. We took a picture so that in a few years, we can look back and admire how far we’ve come on our journey as pursuers of nature’s secrets.
Leave a Reply