“Bloody red crystals” experiment

How to grow different crystals from one substance

Se­cret trea­sures are al­ways as­so­ci­at­ed with dis­tant jour­neys. But you can also ac­quire jew­els by a more eco­nom­i­cal method: for ex­am­ple, the red “twin” of the ruby can be ob­tained in the lab­o­ra­to­ry!

A sim­i­lar ex­per­i­ment is in­clud­ed in the “Crys­tals” sci­ence set from the MEL Chem­istry sub­scrip­tion

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves and glass­es.

Reagents and equip­ment:

  • potas­si­um hex­a­cyano­fer­rate(III) (480 g);
  • hot wa­ter (800 ml);
  • cup;
  • fish­ing line;
  • stick;
  • fun­nel;
  • foil;
  • cot­ton wool.

Step-by-step in­struc­tions

Sprin­kle potas­si­um hex­a­cyano­fer­rate(III) into the cup and pour hot wa­ter over it. Stir thor­ough­ly for 10–15 min­utes. We get a sat­u­rat­ed so­lu­tion. Sep­a­rate the so­lu­tion from the re­main­ing crys­tals and dust us­ing a fun­nel with cot­ton wool. Cov­er the so­lu­tion with foil and leave in a dark place. 24 hours lat­er, pour the so­lu­tion into an­oth­er cup. Place the crys­tals that have formed in a sealed con­tain­er, so they do not dis­in­te­grate. Low­er seeds for crys­tals into the sat­u­rat­ed so­lu­tion of potas­si­um hex­a­cyano­fer­rate (III):

  1. at­tach cot­ton wool to the fish­ing line, soak it in the sat­u­rat­ed so­lu­tion of potas­si­um hex­a­cyano­fer­rate(III) and dry;
  2. from the crys­tals ob­tained ear­li­er, se­lect the crys­tal with the most reg­u­lar shape and at­tach it to the fish­ing line;
  3. sim­ply let the third seed–the small crys­tal–sink to the bot­tom.

Cov­er the cups con­tain­ing the seeds with foil and put in a dark place. A month lat­er, crys­tals of dif­fer­ent shapes will grow from the seeds – a “hedge­hog”, a monocrys­tal and a druse!

Un­like pre­cious stones, crys­tals of potas­si­um hex­a­cyano­fer­rate(III) are sol­u­ble in wa­ter. So if you don’t like the shape of the crys­tal, the ex­per­i­ment can eas­i­ly be re­peat­ed.

Pro­cess­es de­scrip­tion

In a sat­u­rat­ed so­lu­tion, a sub­stance is at max­i­mum con­cen­tra­tion and does not dis­solve fur­ther at the giv­en tem­per­a­ture. At room tem­per­a­ture (25 °С, 77 °F), the sol­u­bil­i­ty of potas­si­um hex­a­cyano­fer­rate(III) in wa­ter is around 50 g/100g of wa­ter. When heat­ed to 90 °С (194 °F), sol­u­bil­i­ty in­creas­es to 92–100 g of wa­ter. When the hot so­lu­tion cools down, it be­comes over­sat­u­rat­ed, i.e. more of the sub­stance is dis­solved in it at the giv­en tem­per­a­ture. As a re­sult, the “sur­plus sub­stance”–in our case potas­si­um hex­a­cyano­fer­rate(III)–pre­cip­i­tates in the form of crys­tals, and the so­lu­tion be­comes sat­u­rat­ed once more. This is how the seeds for the crys­tals were formed.

A sat­u­rat­ed so­lu­tion from which wa­ter evap­o­rates be­comes over­sat­u­rat­ed. As a re­sult, the sur­plus of salt pre­cip­i­tates, and the crys­tals grow from it. If seeds are placed in this so­lu­tion, for ex­am­ple, a crys­tal or cot­ton wool with crys­tals, they do not dis­solve, but will be­come cov­ered in ions of the dis­solved salt, thus form­ing crys­tals of a large size.