Calcium Hydroxide

OVERVIEW

Calcium hydroxide (KAL-see-um hye-DROK-side) is a soft, white odorless solid that occurs as granules or a powder. It has a slightly bitter, alkaline taste. Calcium hydroxide absorbs carbon dioxide readily from the air, changing to calcium carbonate (CaCO₃). For this reason, the compound often is contaminated with the carbonate unless it is kept in tightly sealed containers. Calcium hydroxide is a relatively inexpensive chemical and, for that reason, is used in production processes where a base is needed. Although calcium hydroxide is not very soluble in water, a suspension of the finely divided powder in water can be made, a suspension known as limewater or milk of lime.

KEY FACTS

OTHER NAMES:

Calcium hydrate; caustic lime; hydated lime; slaked lime

FORMULA:

Ca(OH)₂

ELEMENTS:

Calcium, oxygen, hydrogen

COMPOUND TYPE:

Inorganic base

STATE:

Solid

MOLECULAR WEIGHT:

74.09 g/mol

MELTING POINT:

Not applicable; loses water when heated to become calcium oxide (CaO)

BOILING POINT:

Not applicable

SOLUBILITY:

Slightly soluble in water; soluble in glycerol

HOW IT IS MADE

Calcium hydroxide is produced by adding water to calcium oxide (CaO). This reaction is highly exothermic and must be carried out with some care to avoid loss of the product through spattering.

COMMON USES AND POTENTIAL HAZARDS

Calcium hydroxide has a number of uses in building and paving materials. It is found in mortar, plasters, and cements. When used in mortar, calcium hydroxide is mixed with sand and water to make a paste. The paste is put between bricks to hold them together. When water in the paste evaporates, the remaining sand-calcium hydroxide mixture becomes a hard, strong adhesive holding the bricks together.

Calcium hydroxide is also used extensively as a neutralizing agent. As a base, it reacts with acids formed in industrial operations, in the environment, and in other situations. For example, soil that is too acidic for plants to grow properly can be treated with calcium hydroxide. The hydroxide ion (OH⁻)in the calcium hydroxide reacts with the hydrogen ion (H⁺) in the acidic soil to form water, resulting in a more neutral soil. A similar application involves the treatment of exhaust gases from factories with calcium hydroxide. Nitrogen and sulfur oxides in the gases, which are the primary cause of acid rain, are neutralized when passed through "scrubbers" containing calcium hydroxide.

Among the many other uses of calcium hydroxide are:

• In the leather and hide tanning industry, where calcium hydroxide is used to remove hair from hides and then to neutralize tanning solutions;
• For the purification of sugar solutions, such as those used in making fruit juices;
• As a component of certain types of pesticides;
• As an additive to lubricants and oil drilling fluids, to improve the viscosity of the fluids;
• In the manufacture of paints and waterproof coatings;
• As a food additive, used for the purpose of preventing foods from becoming too acidic;
• As an accelerant (a compound that increases the rate of a chemical reaction) in processes for the production of synthetic rubber and plastics; and
• As a food additive in poultry feed, to improve the strength of egg shells.

Interesting Facts

• One of the oldest methods of coating a surface is with whitewash, a mixture made of calcium hydroxide, water, and chalk (calcium carbonate). Whitewash is used to whiten the walls of a building, a fence, or some other structure.

Exposure to calcium hydroxide can cause irritation of the skin, eyes, and respiratory system. These conditions are treated satisfactorily by flushing the contaminated area with ample amounts of water. People who work directly with calcium hydroxide in the workplace are more likely to be at risk for health problems because of their exposure to larger amounts of the chemical for longer periods of time. In such cases, more serious health problems, such as burns and blistering of the skin, damage to the retina of the eye, and injury to the gastrointestinal system may develop.

Words to Know

EXOTHERMIC REACTION

A reaction in which heat is produced.

FOR FURTHER INFORMATION

"Calcium Hydroxide or Hydrated Lime." Peters Chemical Company. http://www.peterschemical.com/calcium_hydroxide_or_hydrated_li.htm (accessed on September 26, 2005).

"Material Safety Data Sheet: Calcium Hydroxide Products." Pulpdent Corporation. http://www.pulpdent.com/c_hydro/CHMSDS.pdf (accessed on September 26, 2005).

See AlsoCalcium Oxide