How crystal water develops in salts

Crystal water or Water of hydration is the name for water that is bound in crystalline solids. Substances containing water of crystallization are often called Hydrates designated. So there are z. B. anhydrous sodium sulfate Na2SO4 and the decahydrate (Glauber's salt) with 10 (old gr .: deca = ten) water molecules per formula unit Na2SO4. The bond is symbolized by a superscript point (sometimes also an x), but this notation does not provide any information about the type of bond: Well2SO4 · 10 H.2O.

The water molecules can be coordinatively bound to ions (coordination water, e.g. in the case of copper sulfate), be present as structural water bound to molecules via hydrogen bonds, or - as with the minerals of the zeolite group - not be involved in the crystal lattice. The binding relationships of water in protein crystals are most diverse.

The crystal water is usually only loosely bound and escapes when heated (technical expression: Calcining, Burn), which in the case of copper sulphate can lead to discoloration, in the case of other substances it can even lead to dissolution in the water of crystallization.

The anhydrous salts are Anhydrates called. (e.g. sodium sulfate anhydrate, copper sulfate anhydrate, not to be confused with anhydrite and anhydride). Anhydrates are mostly highly hygroscopic substances and are therefore used to dry solvents and gases.

During the formation of the hydrates there is energy in the form of Heat of hydration free.


The hydrates get their names from the Greek numerals.

Differentiation from gas hydrates

Gas hydrates (like methane ice) are actually not hydrates, but inclusion compounds (clathrates). Here the gases are stored in the structural cavities of crystalline water, one does not speak of crystal water, because the water molecules themselves create the structure. The trapped atoms or molecules are trapped in the structure like in a cage, hence the name Cage connections. When the water melts, the gases are then released again.

Exploitation in technology

Technically, the ability of gypsum is used to absorb the crystal water that has been partially or completely lost through heating (burning) when mixed with water and to harden it in the process. When heating the dihydrate CaSO4 · 2 H2O is created at around 110 ° C baked plaster (Hemihydrate or Hemihydrate called, CaSO4 1/2 H2O), at 130 to 160 ° C Plaster of paris (Mixture of a lot of hemihydrate and little Anhydrite).

Anhydrite also comes as mineral in salt deposits and consists of anhydrous calcium sulfate (CaSO4). Another technical application is the addition of cobalt chloride to the desiccant silica gel, which is blue when free of water of crystallization and pink when it contains water of crystallization. The silica gel then has to be regenerated by heating.

Salt hydrates can serve as heat stores. The hydrate is melted and can be stored in this way. If necessary, it reacts again to form a hydrate, giving off heat. Work according to this principle:

  • Heat pad
  • Central heating with solar panels
  • Use of waste heat in the car

Categories: Water | Chemical bond