MgCl₂ is the chemical formula for magnesium chloride, a naturally occurring salt made of one magnesium atom bonded to two chlorine atoms. It dissolves readily in water, shows up in seawater and underground mineral deposits, and has an unusually wide range of practical uses, from keeping roads ice-free in winter to setting tofu in Japanese kitchens. If you’ve ever driven on a treated highway or taken a magnesium supplement, you’ve likely encountered it.
Chemical Basics
In its pure, anhydrous form, magnesium chloride is a white crystalline solid. But the version you’ll actually find on store shelves or in industrial supply catalogs almost always contains water molecules locked into its crystal structure. The most common natural form is the hexahydrate (six water molecules per unit), a mineral called bischofite. These hydrated forms dissolve easily: about 54 grams will dissolve in 100 mL of water at room temperature, and that number climbs to nearly 73 grams at boiling point. That extreme solubility is central to nearly everything magnesium chloride is used for.
De-Icing and Dust Control
Magnesium chloride is one of the most widely used road treatment chemicals in cold climates. It’s hygroscopic, meaning it actively pulls moisture out of the air. When spread on a road before a storm, it forms a liquid brine layer that lowers the freezing point of water and prevents snow and ice from bonding to the pavement. Its pellet form is designed to bore down through existing ice, creating a melting layer underneath that makes plowing and scraping far more effective.
The same moisture-attracting property makes it useful for dust control on unpaved roads. When magnesium chloride settles onto a gravel surface, it draws enough humidity from the air to keep the surface slightly damp. This binds fine dust particles into a hard film, reducing airborne dust and extending the time between reapplications. Road crews sometimes use it as a pre-wetting agent for rock salt too, coating the salt so it sticks to the road surface instead of bouncing and scattering during spreading. This cuts the total amount of de-icer needed.
Environmental Effects on Roadside Plants
Heavy, repeated use of magnesium chloride does take a toll on nearby vegetation. A study of two Colorado counties found that roadside trees along treated unpaved roads had a larger proportion of severe damage than trees along untreated roads. The symptoms look similar to damage caused by ordinary road salt: leaf scorching, brown edges on leaves, and burned needle tips on conifers. Aspen, Engelmann spruce, and lodgepole and ponderosa pines all showed increasing damage at higher application rates.
The damage pattern tells a clear story about how the chemical moves through the environment. Vegetation growing downslope from treated roads suffered significantly more than plants upslope, because chloride salts travel through the soil with rainwater and snowmelt. Plants absorb the chloride through their roots, and at high enough concentrations, it causes visible tissue damage. That said, the majority of roadside vegetation in the Colorado study (72% to 79%, depending on position) remained healthy, so the effects concentrate in areas closest to heavy application zones.
How It’s Used in Food
In Japanese cooking, magnesium chloride goes by the name nigari. It’s the traditional coagulant for making tofu. A small amount of nigari dissolved in water is added to warm, fresh soy milk, where the magnesium ions cause the soy proteins to clump together and form curds. The result is a tofu with a smooth, slightly sweet flavor that differs noticeably from tofu set with calcium sulfate (gypsum). If you’ve ever seen “magnesium chloride” on a tofu ingredient label, that’s nigari.
Magnesium Supplements and Health
Magnesium chloride is one of several forms of magnesium sold as a dietary supplement, primarily used to treat or prevent low magnesium levels. Adults need between 310 and 420 mg of magnesium daily depending on age and sex. The tolerable upper limit for supplemental magnesium (from pills, powders, or liquids, not food) is 350 mg per day for anyone 9 and older. Going above that threshold commonly causes diarrhea, and magnesium chloride is specifically one of the forms most likely to produce that side effect.
Magnesium chloride also comes in spray form, marketed for absorption through the skin. The evidence here is thin but not entirely dismissive. A small pilot study tested topical magnesium chloride spray on patients with chronically low magnesium levels, applying 150 mg per day for six weeks. Of the six patients who completed the study, two showed meaningful increases in blood magnesium, and one was able to skip a scheduled intravenous magnesium infusion. None got worse. That’s a far cry from proving the sprays work reliably for the general population, but it suggests some absorption does occur through the skin.
Magnesium Chloride vs. Other Salts
People often wonder how magnesium chloride compares to more familiar salts. Ordinary table salt (sodium chloride) shares the chloride component but pairs it with sodium instead of magnesium. For de-icing, magnesium chloride works at lower temperatures and is less corrosive to concrete and metal than sodium chloride, though it costs more. Calcium chloride is another common de-icer that works at even colder temperatures but tends to leave a slippery residue.
As a supplement, magnesium chloride competes with magnesium oxide, citrate, and glycinate. Oxide packs more elemental magnesium per pill but is poorly absorbed. Citrate absorbs well and is gentler on the stomach. Chloride falls somewhere in the middle: reasonably well absorbed but more likely to cause digestive upset than citrate or glycinate forms. Your choice depends largely on whether you’re prioritizing absorption, convenience, or cost.