The silver mirror reaction

The scientific description of an amazing experiment

The ex­per­i­ment that has a beau­ti­ful ef­fect of a mir­ror sur­face form­ing on glass looks very im­pres­sive. Car­ry­ing out of this re­ac­tion re­quires ex­pe­ri­ence and pa­tience. In our ar­ti­cle, you can read about the nec­es­sary reagents, how to pre­pare the equip­ment, and to find out the chem­i­cal equa­tions of re­ac­tions.

The essence of the sil­ver mir­ror re­ac­tion is the for­ma­tion of metal­lic sil­ver lay­er as a re­sult of an ox­i­da­tion-re­duc­tion re­ac­tion, by 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:

  • 100ml glass flask;
  • 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 is sup­posed to be stored for a long time, keep it in a dark place or in a dark glass bot­tle). 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 pre­cip­i­tate of sil­ver, which you can di­lute by slow­ly ad­di­tion of am­mo­nia so­lu­tion. We also rec­om­mend you to con­duct an­oth­er spec­tac­u­lar ex­per­i­ment with am­mo­nia so­lu­tion and to print the «chem­i­cal pho­to».

The re­ac­tion oc­curs at room tem­per­a­ture. For the suc­cess­ful re­sult, the walls of the glass flask must be per­fect­ly clean and smooth. If there are tiny par­ti­cles of dirt on the walls, the ob­tained pre­cip­i­tate will ap­pear as 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 dis­tilled wa­ter. Rinse the flask with the cleanser many times.

Why is it so im­por­tant for the ves­sel to be clean

The col­loidal par­ti­cles of sil­ver formed at the end of the ex­per­i­ment must firm­ly in­ter­lock with the glass sur­face, and it should not have any fat or me­chan­i­cal par­ti­cles on it. The dis­tilled wa­ter does not con­tain salts and is ide­al for the fi­nal flask rins­ing. You can make it at home, but it is eas­i­er to buy it.

Equa­tion which de­scribes the first stage of the sil­ver mir­ror re­ac­tion:

Ag₂O + 4 NH₃·Н₂О ⇄ 2[Ag(NН₃)₂]ОН + 3Н₂О

where [Ag(NН₃)₂]ОН is sil­ver di­amine hy­drox­ide, ob­tained by 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 is pos­si­ble at low con­cen­tra­tions!

You can ex­am­ine the fi­nal stage of the re­ac­tion in the fol­low­ing equa­tion:

R-CH=O + 2[Ag(NH₃)₂]OH → 2Ag↓ + R-COON­H₄ + 3NH₃ + H₂O (re­ac­tion of an alde­hyde R-CH=O and sil­ver di­amine hy­drox­ide so­lu­tion pro­duce the sil­ver sed­i­ment, an car­bon­ic acid amide, and am­mo­nia so­lu­tion; where­in, R is an or­gan­ic func­tion­al group)

It is bet­ter to car­ry out the sec­ond stage of the re­ac­tion by care­ful­ly heat­ing the flask on a burn­er, then the process should pro­ceed with­out any prob­lems.

What can the sil­ver mir­ror re­ac­tion show

This in­ter­est­ing chem­i­cal re­ac­tion not only al­lows us to ex­am­ine cer­tain states of a sub­stance, but can also give an ex­act def­i­ni­tion of alde­hy­des. This re­ac­tion an­swers the ques­tion: is there an alde­hyde group in the so­lu­tion or not?

An aldehyde group formula [Wikipedia]

For ex­am­ple, due to 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 pre­cip­i­tate. To make an anal­y­sis, you should add a 10% glu­cose so­lu­tion in­stead of a formalde­hyde one. Let’s have a look at the equa­tion to see why and how the dis­solved sil­ver turns into a sol­id pre­cip­i­tate:

2[Ag(NH₃)₂]OH + 3H₂O + C₆H₁₂O₆ (glu­cose) = 2Ag↓+ 4NH₃∙H₂O + C₆H₁₂O₇ (glu­con­ic acid forms).