Barium is one of the most reactive metals on the periodic table. It reacts with water, air, acids, and a wide range of nonmetals, often vigorously. As an alkaline earth metal in Group 2, barium gives up its two outer electrons easily (its first ionization energy is just 5.21 eV), which makes it eager to form compounds with almost anything it contacts.
Barium and Water
Barium reacts with cold water readily, producing barium hydroxide and hydrogen gas:
Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)
This reaction is noticeably more vigorous than the same reaction with calcium or magnesium, which sit higher in Group 2. The trend holds across all alkaline earth metals: reactivity with water increases as you move down the group. Barium’s reaction produces enough hydrogen gas to generate visible bubbling, and the resulting barium hydroxide solution is strongly basic.
Barium and Air
Left exposed to air, barium tarnishes quickly. If ignited, it burns to produce a mixture of white barium oxide (BaO) and barium nitride (Ba₃N₂). A barium superoxide (BaO₂) also forms during this combustion. The fact that barium reacts with both oxygen and nitrogen in air sets it apart from many metals, which typically react with only one or the other.
This broad atmospheric reactivity is actually useful. In vacuum tubes and other sealed electronic devices, barium serves as a “getter,” a material that scavenges leftover gas molecules after the tube has been pumped down. Barium reacts with oxygen, nitrogen, carbon dioxide, hydrogen, and carbon monoxide at room temperature, making it the most efficient getter metal tested, outperforming aluminum, magnesium, thorium, and uranium. The barium is flash-evaporated inside the tube, and the vapor reacts instantly with residual gases, forming a thin metallic “mirror” on the glass walls.
Barium and Acids
Barium dissolves in dilute acids like hydrochloric acid and sulfuric acid. With hydrochloric acid, the reaction produces barium chloride and hydrogen gas:
Ba(s) + 2HCl(aq) → BaCl₂(aq) + H₂(g)
One important distinction: barium chloride dissolved in hydrochloric acid does not react further, because barium chloride is already soluble. In other words, if you add barium chloride (the salt) to hydrochloric acid rather than barium metal, nothing visible happens.
With sulfuric acid, the reaction is more interesting. Barium sulfate is extremely insoluble, so a white precipitate forms immediately. This can actually slow or stop the reaction by coating the barium metal surface, preventing further contact with the acid.
Barium and Halogens
Barium reacts with all the halogens (fluorine, chlorine, bromine, and iodine) to form barium halides. These reactions are exothermic. For example, barium combines with chlorine gas to produce barium chloride:
Ba(s) + Cl₂(g) → BaCl₂(s)
The resulting barium halides are all soluble ionic compounds, which is one reason soluble barium salts pose a toxicity risk compared to insoluble ones.
Precipitation Reactions
In solution, barium ions (Ba²⁺) react with several common anions to form insoluble precipitates. This is some of the most practically relevant barium chemistry, because it shows up in water treatment, analytical chemistry, and medical applications. The key insoluble barium compounds and their solubility product constants at 25°C are:
- Barium sulfate (BaSO₄): Ksp = 1.1 × 10⁻¹⁰
- Barium carbonate (BaCO₃): Ksp = 5.0 × 10⁻⁹
- Barium chromate (BaCrO₄): Ksp = 2.1 × 10⁻¹⁰
- Barium phosphate (Ba₃(PO₄)₂): Ksp = 3.4 × 10⁻²³
The lower the Ksp, the less soluble the compound. Barium phosphate is essentially impossible to dissolve, and barium sulfate is nearly as stubborn. This means that whenever barium ions encounter sulfate, carbonate, chromate, or phosphate ions in solution, a solid precipitate crashes out almost immediately. Adding barium chloride solution to a sample is, in fact, a classic lab test for the presence of sulfate ions: if a white precipitate forms, sulfate is present.
Why Barium Sulfate Is Safe to Swallow
Barium sulfate’s extreme insolubility is the reason it can be used as a contrast agent for X-rays and CT scans of the digestive tract. Patients drink a chalky barium sulfate suspension before imaging. Because BaSO₄ does not dissolve in stomach acid and is not absorbed through the intestinal lining, it passes through the body without entering the bloodstream. It coats the walls of the stomach and intestines, making them visible on imaging, then exits naturally. It is considered safe even during pregnancy for the same reason: no barium ions reach the fetus.
The danger arises only when barium sulfate is contaminated with soluble barium compounds. Soluble forms like barium chloride or barium carbonate can release free Ba²⁺ ions into the body, and those ions are toxic.
How Soluble Barium Affects the Body
Soluble barium compounds react with biological systems in a very specific way. Barium ions target potassium channels in cell membranes, specifically the inward rectifier channels found in muscle and heart tissue. The Ba²⁺ ion enters the channel’s selectivity filter (the narrow part that normally lets only potassium through) and binds tightly, plugging the pore. This blocks potassium from flowing in or out of the cell.
Barium is a potent, nonspecific blocker of these channels and affects all types of muscle at concentrations as low as micromolar levels. The result is a shift of potassium from the bloodstream into cells, causing dangerously low blood potassium levels. This potassium imbalance, combined with the direct channel blockade, explains the two hallmark symptoms of barium poisoning: muscle weakness and abnormal heart rhythms. Poisoning cases have occurred from accidental ingestion of soluble barium salts or from contaminated batches of medical-grade barium sulfate.
Summary of Key Reactions
- Water: Produces barium hydroxide and hydrogen gas
- Oxygen: Burns to form barium oxide (BaO) and barium superoxide (BaO₂)
- Nitrogen: Forms barium nitride (Ba₃N₂)
- Acids: Dissolves to form barium salts and hydrogen gas
- Halogens: Reacts to form barium halides (fluoride, chloride, bromide, iodide)
- Sulfate ions: Precipitates as barium sulfate
- Carbonate ions: Precipitates as barium carbonate
- Carbon dioxide and carbon monoxide: Reacts at room temperature, useful in vacuum gettering