What is Calcium Hydroxide?
Calcium hydroxide is commonly referred to as slaked lime and is described by the chemical formula Ca(OH)2. It is also a white inorganic compound that has a powdery appearance in its solid-state. However, in its crystalline form, it has a colorless appearance.
The other names of this compound can be given as slack lime, hydrated lime, caustic lime, and pickling lime. In general, the calcium hydroxide is prepared by mixing calcium oxide (which is also called quicklime) and water.
Calcium Hydroxide Explained
Also, the chemical reaction between the calcium chloride and sodium hydroxide dissolved in water (aqueous CaCl2) yields this compound. The structural representation of a Ca(OH)2 molecule can be illustrated below.
The calcium hydroxide molecules are held together by ionic bonds between the two hydroxide ions (OH–) and calcium ions (Ca2+). Unprotected exposure to this compound may prove dangerous to humans by leading to skin irritation and chemical burns. Exposure to the concentrated Ca(OH)2 can lead to damage to the lungs and even blindness.
A few of the important properties of calcium hydroxide can be tabulated as follows:
Properties of Calcium Hydroxide
Physical Properties
Let us look at a few of the physical properties of calcium hydroxide as listed below:
Ca(OH)2 has a structure of hexagonal crystals.
This compound is not very soluble in water. But, its solubility reduces with an increase in temperature. For suppose, its solubility at solubility is 1.73 g/L at 20℃ and at 0℃ is 1.89 g/L.
This compound also tends to lose water and decompose at temperatures approaching its melting point.
The solubility product (Ksp) of the calcium hydroxide can be given as 5.5 * 10-6.
Chemical Properties
Let us look at a few of the chemical properties of calcium hydroxide, as listed below.
The calcium hydroxide is quite soluble in acids and glycerol, but it is slightly soluble only in water. It yields a solution that acts as a moderate base (called limewater) when it is dissolved in water to a saturation point.
The lime water reacts with acids and forms salts.
The saturated calcium hydroxide solution in water also reacts with and dissolves the metals such as aluminum.
It also reacts with carbon dioxide and forms calcium carbonate (CaCO3). In general, this reaction can be referred to as carbonation.
Why is Calcium Hydroxide less Soluble in Water?
Calcium hydroxide is also known as slaked lime in general is less soluble in water but, when its temperature is increased, it starts reaching its melting point and decomposes its material. The reason for this less solubility is due to a decrease in the lattice energy in hydroxide salts. The solution formed by the reaction of calcium hydroxide and water is called milk of lime. Unlike water, Calcium hydroxide is easily soluble in acids and glycerol hence, it is used as a pH corrector to reduce a solution’s acidity.
Calcium hydroxide is added to water and sprayed on the soil to correct its pH level and recover the soil, it is also used in the construction of roadways and railways. But, adding it to the soil along with other fertilizers containing ammonium phosphate will reduce the nitrogen content of the soil. But, direct contact with Calcium hydroxide is harmful to humans and might cause loss of vision, so the US has banned adding Calcium hydroxide to the soil. Due to its pH value, Calcium hydroxide is also used as an antacid to treat burns and wounds caused by acids. Even the solution obtained from Calcium hydroxide and water also known as milk of lime is used as an antacid to treat burns and acid wounds.
Uses of Calcium Hydroxide
There are many different uses of calcium hydroxide, and a few of them are listed below. Let us have a look at those.
Ca(OH)2 can be used in the paper industry during the Kraft process, which converts wood into wood pulp.
In the sewage treatment process, calcium hydroxide can be used as a flocculant or clarifying agent.
It is an important compound in ammonia preparation.
Generally, the cucumber pickling is done with the help of Ca(OH)2.
Also, this compound is used as a pH modifier because of its basicity.
The production of various plastics involves the use of calcium hydroxide as a primary ingredient.
In root canal procedures, this compound is used in filling the human teeth cavities.
We can use it in pesticides, manufacturing of ebonite, and hair care products.
Calcium hydroxide can be used in the leather industry to separate the fur or hair from the animal hide.
Sugarcane and Sugar beets are processed via carbonation, which involves the use of Ca(OH)2.
Native American Uses
In Spanish, the calcium hydroxide compound is known as cal. Maize cooked with cal (in a nixtamalization process) becomes hominy (nixtamal), which increases the bioavailability of niacin (vitamin B3) significantly, and also it is considered tastier and quite easier to digest.
Calcium hydroxide is usually chewed alongside chewing coca leaves to keep the alkaloid stimulants available chemically for absorption by the body. In the same way, the Native Americans chewed tobacco leaves traditionally with calcium hydroxide derived from the burnt mollusk shells to enhance its effects. Also, it has been used by a few indigenous American tribes as the main ingredient in yopo, which is a psychedelic snuff prepared from the beans of some species of Anadenanthera.
FAQs on Calcium Hydroxide
1. What is Calcium Hydroxide, and what is its chemical formula?
Calcium Hydroxide is an inorganic compound that is a colourless crystal or white powder. It is produced when calcium oxide (quicklime) is mixed, or “slaked,” with water. The chemical formula for Calcium Hydroxide is Ca(OH)₂. It consists of one calcium ion (Ca²⁺) bonded to two hydroxide ions (OH⁻). For more details on its structure, you can refer to the properties of Slaked Lime.
2. What are the common names for Calcium Hydroxide?
Calcium Hydroxide is known by several common names depending on its form and usage:
Slaked Lime: This name is used for the solid powder form, created by reacting calcium oxide (quicklime) with water.
Limewater: This refers to a clear, saturated aqueous solution of calcium hydroxide, which is often used in laboratory tests.
Hydrated Lime: This is another common industrial name for Ca(OH)₂.
Milk of Lime: This is a suspension of fine calcium hydroxide particles in water, which gives it a milky appearance.
3. How is Calcium Hydroxide prepared?
Calcium Hydroxide is primarily prepared through a process called slaking. This involves the addition of water to calcium oxide (quicklime, CaO). The reaction is highly exothermic, meaning it releases a significant amount of heat. The chemical equation for this combination reaction is:
CaO + H₂O → Ca(OH)₂ + Heat
It can also be prepared in a laboratory by mixing an aqueous solution of calcium chloride (CaCl₂) with an aqueous solution of sodium hydroxide (NaOH).
4. Is Calcium Hydroxide an acid or a base? Explain why.
Calcium Hydroxide is a strong base. According to the Arrhenius theory, a base is a substance that dissociates in water to produce hydroxide ions (OH⁻). When Ca(OH)₂ dissolves in water, it fully ionises to release calcium ions (Ca²⁺) and two hydroxide ions (OH⁻). The presence of these hydroxide ions gives the solution its alkaline properties. Learn more about the difference between a base and an alkali here.
5. What are the main uses of Calcium Hydroxide in industry and daily life?
Calcium Hydroxide has a wide range of applications due to its basic nature and low cost:
Construction: It is a key ingredient in mortar, plaster, and cement.
Water Treatment: Used to clarify raw water and in sewage treatment to raise the pH and remove impurities.
Agriculture: Farmers use it to treat acidic soil, raising the pH to improve crop growth.
Food Industry: Used in sugar refining (carbonation process), making corn tortillas (nixtamalization), and as a pickling agent.
Chemical Manufacturing: Used to produce other chemicals like soda-lime and bleaching powder.
Dentistry: It is used as a pulp-capping agent in root canals due to its antimicrobial properties.
6. Why is Calcium Hydroxide considered a strong base yet is only sparingly soluble in water?
This is a common point of confusion. The terms 'strong' and 'soluble' refer to two different properties:
Strong Base: This refers to the degree of dissociation. Calcium Hydroxide is a strong base because every molecule that dissolves in water dissociates completely into Ca²⁺ and OH⁻ ions.
Sparingly Soluble: This refers to the amount that can dissolve. Only a small quantity of Ca(OH)₂ can dissolve in a given amount of water. However, the portion that does dissolve is 100% ionised, making the resulting solution strongly alkaline.
Therefore, while the concentration of OH⁻ ions is limited by its low solubility, its classification as a strong base is due to its complete dissociation in solution.
7. What is the chemical difference between quicklime (Calcium Oxide) and slaked lime (Calcium Hydroxide)?
The key difference lies in their chemical composition and formation. Quicklime is Calcium Oxide (CaO), an oxide of calcium. Slaked lime is Calcium Hydroxide (Ca(OH)₂), a hydroxide of calcium. Slaked lime is the product of adding water to quicklime in a combination reaction. While quicklime is more caustic and reactive, slaked lime is the hydrated, more stable form used in applications like mortar and soil treatment.
8. What happens when Carbon Dioxide is passed through limewater, and why is this reaction important?
When carbon dioxide (CO₂) gas is passed through limewater (a solution of Ca(OH)₂), a white precipitate of calcium carbonate (CaCO₃) is formed, making the solution turn milky or cloudy. The reaction is:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
This reaction is highly important as it serves as the standard chemical test for the presence of carbon dioxide. If an unknown gas turns limewater milky, it confirms the gas is CO₂.
9. What are the key physical and chemical properties of Calcium Hydroxide?
Calcium Hydroxide exhibits several distinct properties:
Appearance: It is a soft, white powder or a colourless crystal.
Solubility: It is sparingly soluble in water, and its solubility decreases as the temperature increases.
Basicity: It is a strong base, forming alkaline solutions in water with a pH of around 12.4.
Reaction with Acids: It readily neutralises acids to form calcium salts and water. For example, it reacts with hydrochloric acid to form calcium chloride.
Decomposition: When heated to 512 °C, it decomposes into calcium oxide and water.
10. How does the basic nature of Calcium Hydroxide make it useful for applications like soil treatment and dentistry?
The basic (alkaline) nature of Calcium Hydroxide is central to its main applications:
In Soil Treatment: Acidic soil (low pH) can harm crops. Adding calcium hydroxide, a base, neutralises the excess acid in the soil, raising the pH to a level more suitable for plant growth. This process is known as 'liming' the soil.
In Dentistry: The high pH (alkalinity) of calcium hydroxide makes it an effective antimicrobial agent inside a root canal. It creates an environment where bacteria cannot survive, helping to disinfect the area and promote healing before the tooth is permanently sealed.
