“Chemical garden” experiment

How to grow colorful seaweed from salts

In this ex­per­i­ment, you will learn how to turn a few pinch­es of salt into a chem­i­cal gar­den!

A sim­i­lar ex­per­i­ment is in­clud­ed in the “Ar­ti­fi­cial sea” set from the MEL Chem­istry sub­scrip­tion.

Safe­ty pre­cau­tions

  • Put on pro­tec­tive gloves and eye­wear
  • Con­duct the ex­per­i­ment on the tray.

Reagents and equip­ment:

  • sodi­um sil­i­cate so­lu­tion (1 L);
  • cop­per(II) sul­fate pen­tahy­drate (1 g);
  • cop­per(II) chlo­ride di­hy­drate (1 g);
  • man­ganese(II) sul­fate (1 g);
  • cobalt (II) ni­trate hex­ahy­drate (1 g);
  • iron(III) chlo­ride, a small ves­sel (1 g).

Step-by-step in­struc­tions

Add a pinch of each salt to the sodi­um sil­i­cate so­lu­tion: cop­per(II) sul­fate pen­tahy­drate, cop­per(II) chlo­ride di­hy­drate, man­ganese(II) sul­fate, cobalt (II) ni­trate hex­ahy­drate, and iron(III) chlo­ride. Try to pour the salts in dif­fer­ent ar­eas of the cup so as to keep them from mix­ing. Watch as beau­ti­ful mul­ti­col­ored “sea­weed” grad­u­al­ly grows from the salt crys­tals.

Pro­cess­es de­scrip­tion

The sodi­um sil­i­cate so­lu­tion can be pre­pared by com­bin­ing or­di­nary sil­i­cate glue and wa­ter in a 1:1 ra­tio. When the met­al salts are in­tro­duced to the so­lu­tion, an ex­change re­ac­tion be­gins, and a film of in­sol­u­ble sil­i­cates forms around the salt crys­tals. This film is not 100% im­per­me­able–it does al­low wa­ter through.

The high­er con­cen­tra­tion of salt with­in the film than out­side it caus­es a dif­fer­ence in os­mot­ic pres­sure and draws wa­ter into the sil­i­cate shell. This caus­es the shell to widen and break. When this hap­pens, the salts are again brought into con­tact with the sil­i­cate so­lu­tion and a new sil­i­cate film forms. This process cre­ates the chem­i­cal sea­weed we ob­serve. The sea­weed’s col­or de­pends on its met­al foun­da­tion: cop­per is light blue and green, man­ganese is pink, cobalt is dark blue, and iron is brown.