Chemical precipitates

Chemical precipitates are useful in nearly all human endeavors. Precipitates are materials formed by precipitation—the formation of solids from solution. Uses are numerous, including pigments and water treatment. Additionally, precipitates occur in nature to form rocks such as shale and limestone.

The Solvation Process

A proper understanding of how things dissolve is needed to discuss chemical precipitates and their uses. Chemical bonds are, in general, either ionic or covalent.

Covalent bonds occur where atoms share electrons. These bonds are far stronger than ionic bonds, which occur when electrons are transferred. A covalent bond occurs when there is a large difference in electronegativity.

Electronegativity is the measure of how tightly an atom holds its electrons. Atoms with high electronegativity, such as fluorine and oxygen, hold their electrons tightly, whereas atoms of low electronegativity, such as sodium and boron, hold their electrons loosely. This is the result of properties of electron orbitals.

Atoms with eight electrons in their outermost orbital are the most stable. As a result, atoms with many electrons in their outermost orbital, such as fluorine (with seven electrons) and oxygen (with six electrons), try to gain another electron; atoms with few electrons, such as sodium (with one electron) and calcium (with two electrons), try to lose their electrons. When atoms of different electronegativity bond, the less electronegative atom gives its electrons to the more electronegative atom. Once the electrons are transferred, the atoms become ions and are then held together by electrostatic attraction.

Equally covalent bonds result in sharing; and when one is more electronegative than the other, as in water, the result is a polar molecule. In such a case, the oxygen atoms hold the electrons more tightly than the hydrogen atoms, meaning that the oxygen portion has a net negative charge. The hydrogen portion is positive. Water’s shape is also of note, as it is a bent molecule. However, regardless of the shape of the molecule, the electronegative principles are the same.

Taking the example of salt dissolving in water, one can see the effect of these properties. When table salt (sodium chloride, or NaCl) is placed in water, it dissolves. Sodium chloride is an ionic compound. Because its bonds are weaker than the pull of water, the sodium atoms are separated from the chlorine atoms and align to the water molecule’s poles. The water molecules surround the sodium and chlorine molecules, and the structure of the sodium chloride is subsumed by the water.

Dissolved systems are not static; they are under dynamic equilibrium. The solute is continuously coming in and out of solution. When the rate of precipitation exceeds the rate of dissolution, the solute comes out of solution; if the converse is true, the solute dissolves.

Many factors affect solubility. The behavior is particularly complex because it depends on the interactions of the crystal lattice. Only recently have computer models allowed for the prediction of solubility. The solubility of any given substance is related to temperature and pressure.

Also important to note is that not all soluble substances are soluble in the same solutes. The major division is between organic and inorganic solvents. In general, organic molecules are the best solvents of organic solutes, and inorganic solvents work best on inorganic solutes.

Colloids and Suspension

Solids can be contained in water by means other than through dissolving. Solids also can be contained as suspensions and as colloids.

A suspension is a system in which the particles are larger than single molecules and ions but are still small enough to remain in the solvent. A colloid is a substance in which the particles are distributed homogeneously and stably; without changes to the system, the colloid will not separate. Suspensions, in contrast, will naturally separate over time. This process is called flocculation.

Flocculation is the process wherein colloids come out of suspension in the form of clumps of particles called floc. Flocculation differs from precipitation in that the particles are suspended in a colloid rather than dissolved in a solution. The flocs can then come out of suspension.

Water Treatment

Water treatment is the process by which water is cleaned for human use. The use of precipitates is critical for treatment, but much water treatment does not rely on precipitates.

First, in water treatment, waste is screened out using filters. The chunks of waste are usually disposed of in a landfill after drying. Then, after water filtration, the resulting waste is directed to a settlement tank, where the waste particles are allowed to settle. The result is called sludge, which can be refined into fertilizer. The water is then disinfected by chlorination.

The properties of precipitates can remove solutes from water. This is important because many solutes have negative effects upon water. Some solutes, such as iron, can stain objects, such as water fixtures, and can change the color of the water. Sometimes solutes are clear when drawn from a tap but turn yellow upon standing (in place without movement). Other solutes, such as hydrogen sulfide, smell like rotten eggs. Another common concern is the solute pH (potential hydrogen), which can cause corrosion. These chemical properties are caused by things dissolved in water; precipitating removes the properties from the water.

Aeration, the placing of a solute in contact with air, is a common means of precipitation. In this method, a substance dissolved in water is removed from the water by tiny bubbles passing through the solution. The increase of surface area between the solution and the air bubbles creates a space for the dissolved gases to react with the gases in the air bubbles, in this manner the air bubbles “scrub” the water clean of the solutes.

Another common precipitation process is water softening. Hard water is water with a high dissolved mineral content. Most processes aim to remove calcium, magnesium, and other metal ions from the water. One way of doing this is to add compounds to the hard water that will react with the dissolved metals to form an insoluble compound. Another process known as ion exchange works by exchanging one type of ion in a substance for another. Because the goal in water softening is to remove certain kinds of ions, the exchange of ions works fine. Typically, the process works by passing water through resin beads.

Another method, often used in the American Midwest, Florida, and Texas, is lime softening, which involves adding lime to water. The water then undergoes several reactions that lead to the removal of ions through precipitation. This process is notable because the total dissolved particles decreases (with ion exchange, little to no change occurs).

Reverse osmosis is another water treatment method. Osmosis is a natural process that involves moving water (or any other liquid with different concentrations) through a semipermeable membrane. The liquids tend to move from regions of low solute concentrations to high concentrations to create chemical potential equilibrium. Reverse osmosis involves forcing a liquid with dissolved solute through the semipermeable membrane by applying a pressure greater than the osmotic pressure. This filters out the solutes. The holes in the filters are small enough to prevent the ions from passing through them.

Chelation is often used as well. In chelation, metal ions are dissolved by organic compounds. By dissolving them in the organic compound, they are removed from the water.

By all these processes, salts and metal ions can be removed. Salts are ionic compounds that result from the neutralization of an acid with a base. Acids are acidic because they produce H⁺ ions, whereas bases produce OH^- ions. When acids and bases come into contact, they undergo a special reaction called neutralization. In neutralization, the H⁺ and OH^- ions form water, leaving the anions (negative ions) of the acid and the cations (positive ions) of the base to form an electrically neutral compound. Because it is made of ions in this manner, it is an ionic compound.

To change the pH of the water, one can add either an acid or a base to neutralize the liquid’s excess. (pH is a measure of how acidic or basic something is.) According to the system, pure water is neutral with a pH of 7. Solutions with pH less than 7 are acidic and those with pH greater than 7 are basic or alkaline. pH is a logarithmic scale based on hydrogen ion activity. Improper pH can cause problems, such as pipe corrosion and harm to plants and other vegetation, so managing pH is an important part of water treatment.

Pigments

A pigment is a material that provides color to something as a result of absorbing and reflecting certain wavelengths of light. Pigments are used to make many things, ranging from clothing to paint. Pigments are suspensions, but precipitation is important for manufacturing a specific kind of pigment called a lake pigment.

A lake pigment is made when a dye is precipitated with an inert binder, called a mordant. A dye is a colored solution. Dyes do not always hold well, so they need to be changed chemically to supply color. The mordant is a metallic salt. Metallic salts are compounds like sodium chloride. The colored precipitant of a reaction of a dye and a mordant, such as alumina, is usually more stable and will remain in the object being colored better than the dye alone would.

This process is ancient. In earlier times chalk, white clay, and bones were used for their calcium carbonate and calcium phosphate. Now, however, the most common salts include aluminum hydroxide, alumina (aluminum oxide), calcium sulfate, and barium sulfate. No matter the era, these mordants were all insoluble in dyes and had neutral colors. The neutral colors are needed so that the color of the precipitate is dependent upon the dye.

Rock Formation

Some sedimentary rocks are formed by the process of precipitation as well. Water is a polar molecule and a good solvent. Seawater contains many kinds of solutes, and the average liter has about 35 grams of dissolved salts.

The oceans exhibit a great deal of variance in temperature and pressure across their expanse and depths, so it is only natural that they should also display a wide variance in what and how much they can dissolve. The varying nature of seawater leads to different rock formations.

A good example of this is the formation of limestone, which is made of calcium carbonate (a soluble). In equatorial surface seawater, however, there is so much limestone dissolved in the seawater that it is practically insoluble. As a result, it tends to come out of solution to form rock. The vast majority of limestone laid down, however, comes from sea-life shells, which are composed of calcium carbonate as well. When these creatures die, they fall to the seabed and form layers of calcium carbonate. A different scenario is possible, however.

Should a creature’s shell fall below a certain depth, it will dissolve. This depth is known as the carbonate (or calcite) compensation level. Under conditions that exist below a certain depth, calcium carbonate’s solubility increases dramatically. The calcium compensation level depends upon many factors but is about 42 to 500 meters (138 to 1,640 feet) in the Pacific Ocean, except in the equatorial upwelling zone, where it is 5,000 m (16,404 ft). Below this level, the seabed is mainly made up of silicates; above this level is carbonates.

Additionally, under certain conditions, dissolved substances precipitate from water. Warm water often releases many precipitates, causing the mineral deposits in many salt lakes. Additionally, the deposition of certain minerals reveals to climatologists the temperature of the seas.

Another result of precipitates is manganese nodules. These are metallic spheroids that form on the ocean bottom. They may prove important in future metal production.

Cementation

Sedimentary rocks are formed by minerals coming out of solution and settling on the bottom of a body of water. As this happens over time, the layers begin to compress. Water containing ions then flows between the grains and precipitates them out to form crystalline material between the grains, joining them. This can occur through groundwater or in seafloor sediments.

This process is used by humans in the making of cement. Various chemicals are mixed in an aqueous solution and then are left to dry. As the moisture content changes, the chemicals are left to precipitate out and crystallize. These crystals give concrete its strength.

Principal Terms

covalent bond: a chemical bond characterized by electron sharing

dye: a colored solution

flocculation: the process by which particles are released from a colloid

ionic bond: a chemical bond formed from the transfer of electrons and electromagnetic attraction between the ions

ions: atoms with a net charge through electron addition or loss

sedimentary rock: rocks created by layering of sediment

semipermeable membrane: a membrane with spaces large enough for the molecules, but not the ions, of a liquid to pass through

solute: a substance that is dissolved

solution: a mixture in which a solute is dissolved in a solvent

solvent: a substance that dissolves another

Bibliography

Chang, Raymond. Chemistry. Boston: McGraw-Hill Higher Education, 2007.

Crittenden, John C. Water Treatment Principles and Design. Hoboken, N.J.: Wiley, 2005.

Delamare, Francois, and Bernard Guineau. Colors: The Story of Dyes and Pigments. New York: H. N. Abrams, 2000.

Edzwald, James K. Water Quality and Treatment: A Handbook on Drinking Water. New York: McGraw-Hill, 2011.

Faulkner, Edwin B., and Russell J. Schwartz. High Performance Pigments. Hoboken, N.J.: Wiley, 2009.

"Forming a Precipitate." American Chemical Society, 16 Aug. 2023, www.acs.org/middleschoolchemistry/lessonplans/chapter6/lesson3.html. Accessed 25 July 2024.

Lutgens, Frederick K., and Edward J. Tarbuck. Essentials of Geology. Boston: Prentice Hall, 2012.