Crystal water or hydration water describes water which is bound by crystalline solid bodies as in certain salts which crystallized from certain aqueous solutions. Substances which contain crystal water are also referred to as hydrates. In most cases, crystal water is bound only loosely and escapes during heating processes (technical term: calcination, burning). This means that hydrates disassociate crystal water during heating processes and transform into water-free salt.
Some salts such as sodium carbonate Na2CO3 develop several hydrates. The binding is symbolized by a superscript dot (sometimes it is an x), however, this way of spelling does not contained information on the binding type. Sodium carbonate as an example:
|Sodium carbonate monohydrate||Na2CO3 · H2O|
|Sodium carbonate heptahydrate||Na2CO3 · 7 H2O|
|Sodium carbonate decahydrate||Na2CO3 · 10 H2O|
Salts which do not contain crystal water are called anhydrous. In most cases, anhydrous matters are hygroscopic and are used for drying purposes. The name “hydrates” comes from Greek numerals (Table 1):
Uses in the foundry industry: Plaster model production, plaster mold process and/or silica gel
Technically they use the plaster capability to re-absorb crystal water which was partially or totally disassociated due to heating (burning) when mixed with water again and to harden it. If the CaSO4 · 2 H2O dihydrate is heated up to approx. 110 °C burnt plaster develops (called hemihydrate, CaSO4 · 1/2 H2O), stucco (compound of large amounts of hemihydrate and low amounts of anhydrous matter) develops at temperatures of 130 to 160 °C.
Another technical use is to add cobalt chloride to the desiccant silica gel, which has a blue color when crystal water-free and a pink color when containing crystal water (used in the desiccator). If silica gel was treated like this, the pink color indicates that it can absorb no more humidity and for this reason needs to be regenerated by heating.