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.

“Silver mirror” (on the left) [Wikipedia]

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.

Ball-and-stick model of the diamminesilver(I) complex [Wikipedia]

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?

An aldehyde group formula [Wikipedia]

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).