"Cobalt colors" experiment

Have you ever won­dered how em­ploy­ers at ser­vice cen­ters can tell that your smart­phone has got wet? Our col­or­ful ex­per­i­ment will an­swer your ques­tion.

Step-by-step in­struc­tions

Dis­solve 10 g of cobalt chlo­ride (II) in 100 ml of wa­ter. Pour the ob­tained so­lu­tion into two glass­es. Pour dis­tilled wa­ter into one glass, and con­cen­trat­ed hy­drochlo­ric acid into the sec­ond – the col­or of the so­lu­tion will change to bluish-pur­ple. If you add wa­ter to the bluish-pur­ple so­lu­tion, it will be­come pink again, while if you add con­cen­trat­ed hy­drochlo­ric acid to the sec­ond so­lu­tion, it will be­come bluish-pur­ple.

Pro­cess­es de­scrip­tion

The cobalt chlo­ride changes col­or be­cause the salt in the so­lu­tion dis­so­ci­ates into ions. Cobalt ions are hy­drat­ed in the so­lu­tion and have a pink col­or. Adding con­cen­trat­ed hy­drochlo­ric acid to the so­lu­tion changes the col­or of the so­lu­tion to bluish-pur­ple. This is be­cause the chlo­rine ions force the wa­ter out of the wa­ter com­plex of cobalt.

[Co(H₂O)₆]²⁺ +4Cl⁻ ↔ [Co­Cl4]²⁻ + 6H₂O₅

Safe­ty pre­cau­tions

When car­ry­ing out the ex­per­i­ment, wear gloves and pro­tec­tive glass­es. The cobalt so­lu­tion is car­cino­genic — in the case of con­tact with skin, wash it away with wa­ter.

Warn­ing! Sub­stances of this ex­per­i­ment are tox­ic and high­ly dan­ger­ous for your health. Do not try this at home. Only un­der pro­fes­sion­al su­per­vi­sion.