CID 5284359
CaCl2
Calcium chloride
- Ca 1
- Cl 2
- IUPAC name
- calcium dichloride
- Molar mass
- 110.978(24) g/mol
- Atoms
- 3
- SMILES
- [Cl-].[Cl-].[Ca+2]
- InChIKey
- UXVMQQNJUSDDNG-UHFFFAOYSA-L
- PubChem CID
- 5284359
Left in damp air, calcium chloride pulls in water vapour until its crystals dissolve in the moisture they have gathered, a behaviour called deliquescence. The salt joins calcium and chlorine as one Ca2+ ion for every two Cl− ions, and its IUPAC name is calcium dichloride. Dry, it is a white, odourless powder or a mass of crystals. EU food-additive rules call it hygroscopic, meaning that it attracts moisture from its surroundings.
Water dissolves it freely and grows warm as it does, and ethanol takes it up readily as well. Its work runs from melting road ice and drying natural gas to thawing frozen coal, weighting tyres as ballast and making concrete set faster. In food it firms fruit and helps cure pickles. Medicine uses it to restore calcium. In Antarctica, a pond rich in it seldom freezes.
Structure and bonding
Anhydrous calcium chloride, the salt with no water in its crystals, is the form drawn. Its one calcium ion, marked +2, and two chloride ions, marked −1, carry charges that cancel, and no line joins them. Those three ions are one formula unit of an ionic lattice, the ratio of the ions in the solid rather than a particle of it. Calcium chloride crystallises with water as a monohydrate, a dihydrate, a tetrahydrate and a hexahydrate, the Hazardous Substances Data Bank (HSDB) records. US food rules name the dihydrate, CaCl2·2H2O, alongside the anhydrous salt.
A real crystal is an extended lattice of calcium and chloride ions, and the ionic model behind its charges is the subject of chemical bonding. Freeing the ions from that lattice costs energy, the lattice energy, while water molecules binding to the freed ions give energy back, the hydration energy. Chem1 treats the heat of solution as the sum of the two, which can be positive or negative. For calcium chloride the balance is exothermic, meaning heat is given out: hydration returns more than the lattice costs, and the surplus warms the water.
Where it turns up
Don Juan Pond, ankle-deep in the lowest part of Antarctica’s Upper Wright Valley, is rich in calcium chloride. NASA’s Earth Observatory puts its salinity above 40 per cent, beyond most of the world’s other very salty lakes, and its water rarely freezes. Salts in the surrounding soil take moisture from the air by deliquescence, and that moisture helps feed the pond.
About 70 per cent of US production, on HSDB’s figure, is refined from natural brines, which the US Federal Highway Administration (FHWA) places chiefly in deep wells in Michigan. It is also a by-product of the Solvay process for soda ash (sodium carbonate). De-icing took 35 per cent of US use in HSDB’s 1980 breakdown, the largest share. The US Environmental Protection Agency (EPA) puts its cost at three times that of sodium chloride, and it is usually kept for vulnerable areas. New England uses it instead on roads where the water already holds much sodium.
Why it matters
On winter roads, calcium chloride dissolves in the moisture on ice and lowers the freezing point of that water. In Chem1’s account, a solute that does not evaporate lowers the vapour pressure of the water, the pressure of the vapour leaving it. That lowers the temperature at which ice and liquid water have equal vapour pressures. NASA describes dissolved particles getting between water molecules and hindering the ice lattice from forming.
Besides the particle count, explained on the sodium chloride page, Chem1 ties a de-icer’s power to how well it dissolves. HSDB puts calcium chloride’s solubility at 81.3 g in 100 g of water at 25 °C. The EPA describes the salt as fast acting and effective below 0 °F, about −18 °C.
A desiccant dehydrator dries natural gas in a bed of deliquescent salt. The EPA’s Natural Gas STAR programme names calcium chloride the most common and cheapest of these salts. Water from the gas turns the salt into a brine that trickles down through the bed, leaving cool, high-pressure gas dry enough for a pipeline.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Formula (Hill order) | CaCl2 | ||
| Molar mass | 110.978(24) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Calcium atoms | 1 | ||
| Chlorine atoms | 2 | ||
| PubChem molecular weight | 110.98 g/mol | g/mol | PubChem |
Safety
Calcium chloride irritates the eyes, and its dust or strong solutions can also irritate the skin and the upper airways. In 99.7 per cent of the reports gathered by the European Chemicals Agency (ECHA) its hazard statement is “Causes serious eye irritation”. In Japan’s classification it also causes serious eye damage, is harmful if swallowed, may irritate the airways and may damage organs through prolonged or repeated exposure. The PubChem Laboratory Chemical Safety Summary for calcium chloride brings these classifications together.
Questions
Is calcium chloride better for the environment than rock salt?
The US Environmental Protection Agency (EPA) calls it safer for the environment than sodium chloride, and its Safer Choice programme lists the salt as verified to be of low concern. Like other road de-icers, though, it can concentrate in runoff that reaches streams or seeps through soil to groundwater. Chloride gives water a salty taste and corrodes pipes, an EPA bulletin on highway de-icing notes.
Does calcium chloride damage concrete?
It can. The calcium hydroxide in cement paste reacts with calcium chloride to form calcium oxychloride. The Federal Highway Administration describes that product as highly expansive, probably through the pressure of its growing crystals. De-icers also intensify the physical harm that repeated freezing and thawing does to a pavement. In fresh concrete the salt has a different job: it is added to make the mix set faster.
Is calcium chloride safe to eat?
As a food additive, US regulations affirm it as generally recognised as safe (GRAS), in a rule dating from 1982. In food it works as a firming agent, a pickling agent, a flavour enhancer and a nutrient supplement, among other roles. The undiluted salt is another matter: it irritates the eyes, and a Japanese classification lists it as harmful if swallowed.
What is calcium chloride used for in medicine?
It serves to raise calcium levels in the body, counters magnesium poisoning as an antidote, and goes into blood and electrolyte replacement products. MeSH, the Medical Subject Headings index, also lists it as an acid-producing diuretic, a drug that increases urine output. The World Health Organization's drug classification files it among mineral supplements, additives for intravenous electrolyte solutions and urinary acidifiers.
Sources
- PubChem CID 5284359, calcium chloride
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for calcium chloride (CID 5284359)
- Stephen Lower, Chem1 General Chemistry, §7.7 Ionic and Ion-Derived Solids (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §8.2.2D Solutions of Solid Solutes in Liquid Solvents (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §8.3 Colligative Properties: Raoult's Law (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §8.4 Colligative Properties: Boiling Point Elevation and Freezing Point Depression (LibreTexts, CC BY 3.0)
- US Code of Federal Regulations, 21 CFR 184.1193, Calcium chloride (eCFR)
- US Environmental Protection Agency, Winter is Coming! And with it, tons of salt on our roads
- US Environmental Protection Agency, Source Water Protection Practices Bulletin: Managing Highway Deicing to Prevent Contamination of Drinking Water
- US Environmental Protection Agency, Natural Gas STAR Program: Desiccant Dehydrators
- NASA Earth Observatory, Saltiest Pond on Earth
- US Federal Highway Administration, Tech Brief: Chemical Deicers and Concrete Pavement: Impacts and Mitigation (FHWA-HIF-17-008)
- US Federal Highway Administration, Manual of Practice for an Effective Anti-Icing Program (FHWA-RD-95-202), Appendix A