Calcium Chloride Dihydrate
Properties
| State | Solid (white granular, hygroscopic) |
| Color | White |
| Solubility | Very soluble in water (745 g/L anhydrous equivalent at 20°C); exothermic dissolution |
| Melting Point | 176°C (loses water at ~175°C) |
| Boiling Point | Decomposes, loses water |
About Calcium Chloride Dihydrate
Calcium chloride dihydrate is the form of CaCl2 you actually order from a chemical supplier. Anhydrous CaCl2 is so aggressively hygroscopic that it pulls water out of humid air fast enough to liquefy itself, which makes it miserable to weigh. The dihydrate has already done that — two water molecules are locked into the crystal lattice as Ca²⁺(H2O)2 — so it sits stably in a bottle on the shelf and gives you a defined formula weight of 147.014 g/mol to do stoichiometry against. Dissolution is still strongly exothermic, dropping ΔHsoln near −47 kJ/mol, enough to noticeably warm a beaker when you mix up a 1 M solution. The dihydrate is the one that shows up in molecular biology procedures. The Mandel–Higa CaCl2 transformation method, which has been a foundation of recombinant DNA work since 1970, uses ice-cold 50–100 mM CaCl2 to neutralize the phosphate backbone of plasmid DNA and the lipopolysaccharides of the E. coli outer membrane simultaneously, allowing DNA to stick to the cell surface; a brief 42 °C heat shock then drives uptake. CaCl2·2H2O is also a Solvay-process byproduct, which is why it is cheap enough to spread on roads by the truckload.
Where you'll encounter it
Walk into any molecular biology lab and you will find a bottle of CaCl2·2H2O on the shelf, used weekly to make competent cells. Brewers keep a sack of food-grade dihydrate to harden soft water for British-style pale ales, where the calcium ion supports yeast flocculation and α-amylase activity in the mash. In a cheese plant it is added to pasteurized milk at around 0.02% to restore the calcium that pasteurization knocked out of soluble equilibrium, helping rennet form a firm curd. On winter highways, the granular dihydrate is the de-icer trucks spray below about −10 °C where rock salt has stopped working — it pulls water out of the air and goes into solution exothermically, cutting through ice mechanically and thermally.
Common Uses
- Source of Ca²⁺ for the heat-shock transformation of competent E. coli at 50–100 mM
- Calcium addition (about 0.02% w/v) in cheesemaking to firm rennet curd
- Brewing salt to harden soft water for traditional pale-ale recipes
- Liquid de-icer and dust suppressant for winter and unpaved roads
- Endothermic-to-exothermic phase-change material in some self-heating packs
- Source for preparing anhydrous CaCl2 desiccant by oven dehydration at 200 °C
- Calcium standard for AAS and ICP analytical work after gravimetric drying
Safety Information
GHS H319 (causes serious eye irritation) is the headline hazard — splash-tight goggles when making concentrated solutions. Skin contact with concentrated brine can dry and crack tissue. Dissolving large amounts quickly releases enough heat to scald (a saturated solution can hit 60 °C); add salt to water in portions, not water to salt. Not classified as toxic; oral LD50 in rats is around 1.0–1.4 g/kg. Avoid contact with zinc — slow hydrogen evolution can occur over time.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.