July 2005

Introduction

Many of the dazzling bursts of color that light up the night sky on the Fourth of July are brought to you compliments of chlorine chemistry. Barium chloride, BaCl2, responsible for the glittering green bursts, is this month's featured chlorine compound. But, barium chloride is much more than a pretty flash in the sky.

Manufacturing Barium Chloride

Barite photo from the Mineral Information Institute, courtesy of the Smithsonian Institution.

The chemical element barium is a heavy, soft, silvery-white metal that, like the element chlorine, always occurs combined in nature. Due to their reactivity, barium and chlorine form compounds naturally. The most common form of barium in nature is the salt barium sulfate, BaSO4, also known as the mineral barite. It is also the chief ore of barium. Barium chloride, another barium salt, can be manufactured by reacting barium sulfate with hydrochloric acid, HCl:

barium_formula

Notice that barium and hydrogen switch "chemical partners" between the reactants side and the products side of the equation. This type of reaction is known as a double-replacement reaction, and it may be represented in a fun, cartoon way as:

barium_bees

Barium Chloride: Sulfate Detective

Barium chloride has several important functions, but one very familiar to chemists is the role of "sulfate detective." Sulfate, SO4-2 is a negatively charged chemical unit consisting of a sulfur atom bonded to four oxygen atoms. These negatively charged units are known as negatively charged ions, or anions.

The key to barium chloride's role in tracking down sulfates is the simple fact that barium chloride dissolves in water while barium sulfate hardly dissolves in water at all. Presented with a water-based liquid of unknown chemistry, a chemist might add barium chloride to the solution and watch for the appearance of a thick, white "precipitate" of barium sulfate, indicating the presence of sulfate.

Although the term "precipitate" sounds as though it might be raining in a beaker, a precipitate is simply a solid that forms in a liquid solution as a result of a chemical reaction. Because it is practically undissolvable in the liquid of the solution, the precipitate becomes visible. A chemical precipitate first turns a liquid cloudy and eventually, due to gravity, the cloudiness clears as the solid settles down to the bottom of the vessel.

Other Roles for Barium Chloride

Besides being an important indicator chemical, barium chloride can be a starting chemical for barium and chlorine production. It is also used in manufacturing aluminum alloys, in pigments and dyes and as a water softener.

Taking Advantage of Barium's Physical Properties

Barium, the name for the element of atomic number 56 on the Periodic Table of the Elements, comes from the Greek word "barys," meaning "heavy." Barium is a relatively heavy element because it has 56 protons in its nucleus (compared to chlorine, for example, which has only 17). The same number of electrons balance the electrical charge.

barium_images Barium's compounds are heavy and dense. Barium sulfate, for example, is used in x-ray examinations to image the human digestive system. The patient drinks a "barium milkshake," a slurry of barium sulfate which fills the digestive tract and provides an obstacle (by virtue of its many electrons) to x-rays. In blocking the path of x-rays, barium outlines the tract, permitting doctors to identify problem areas.

Barium sulfate's high density also plays an important role in oil exploration. It is added to "drilling mud," a slurry added to drill holes to control the pressure on underground oil and gas reserves. Barium sulfate helps reduce the risk of blow-out-an escape of oil and gas from the drill hole. Blow-outs can result in fire and explosion.

Follow-up Activities:

  1. The following chemical reaction is an example of a single replacement reaction involving potassium (K), chlorine (Cl) and iodine (I):
     
    barium_form2

    How does this reaction differ from a double replacement reaction? Make a cartoon representation of a single replacement reaction. 

  2. Make a list of the metal chloride compounds that are used in fireworks and the colors they produce.

For a list of previous "Chlorine Compound of the Month" features, click here.

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