Carbon and its Compounds

Calcium Hydroxide

Calcium hydroxide, commonly known as slaked lime. Calcium hydroxide is an inorganic compound. It has the chemical formula \(Ca(OH)_2\). Calcium hydroxide has a white powdery appearance in its solid-state and has a colourless appearance in its crystalline form. The other names of calcium hydroxide compound are hydrated lime, slack lime, pickling lime, and caustic lime. Calcium hydroxide is produced by mixing water and calcium oxide (quick lime).

Calcium hydroxide

Introduction to Calcium Hydroxide

Molecules of calcium hydroxide are held together by ionic bonds that are between the calcium ion \(Ca^{2+}\) and two hydroxide ions \(OH^-\). Unprotected exposure to this compound is dangerous to humans, leading to irritation of the skin and chemical burns. Exposure to concentrated Calcium hydroxide can lead to lung damage and even blindness.

The molecular weight or molar mass of calcium hydroxide is 74.093 grams per mole. The density of calcium hydroxide is 2.211 grams per cubic centimetre. The melting point of calcium hydroxide is 853K. Calcium hydroxide adopts a polymeric structure similar to all metal hydroxides. The structure is identical to that of \(Mg(OH)_2\) that is brucite structure, i.e., the cadmium iodide motif.

Physical Properties of Calcium Hydroxide

  • Calcium hydroxide is relatively insoluble in water, with a solubility product \(K_{sp}\) of \(5.5 \times 10^{-6}\). The acid dissociation constant \(K_a\) of calcium hydroxide is large enough that its solutions are basic according to the following reaction:

\(Ca(OH)_2 \rightarrow  Ca^{2+} + 2 OH^-\)

  • At ambient temperature, calcium hydroxide dissolves in pure water to make an alkaline solution with a pH of 12.4. Calcium hydroxide solutions can cause chemical burns.
  • At a high pH value, its solubility drastically decreases. This behaviour is appropriate for cement pastes. Aqueous solutions of calcium hydroxide are limewater.
  • These are medium strength bases that react with acids and can easily attack some metals like aluminium while protecting other metals from corrosion. Limewater turns milky in the presence of carbon dioxide. It is due to the formation of calcium carbonate. This process is carbonation.

For example, lime water

\(Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O\)

  • When heated to temperature 512^oC, the partial pressure of water with calcium hydroxide in equilibrium reaches 101 kPa and decomposes calcium hydroxide into calcium oxide and water.

\(Ca(OH)_2 \rightarrow  CaO + H_2O\)

  • The solubility of calcium hydroxide at \(70^oC\) is about half of its value at \(25^oC\). The phenomenon behind this is that the dissolution of calcium hydroxide in water is an exothermic process. It also adheres to Le Chatelier’s principle.
  • A lowering of temperature thus allows the elimination of the heat by the process of dissolution and increases the equilibrium constant of dissolution of \(Ca(OH)_2\), and so increase its solubility at low temperature. The variably hydrated phases of calcium sulfate also exhibit retrograde solubility for the same reason as their dissolution reactions are exothermic.

Uses of Calcium Hydroxide

  • Calcium hydroxide in the process of sewage treatment is in use as a clarifying agent or as a flocculant.
  • \(Ca(OH)_2\) is in use in the paper industry during the Kraft process of converting wood into wood pulp.
  • Calcium hydroxide is an important compound in the preparation of ammonia.

\(Ca(OH)_2 + 2NH_4Cl \rightarrow 2NH_3 + CaCl_2 + 2H_2O\)

  • Calcium hydroxide is also in use as a pH modifier due to its basicity.
  • The pickling of cucumbers is generally done with the help of Calcium hydroxide
  • The production of many plastics involves the use of calcium hydroxide.
  • Calcium hydroxide is in use in pesticides, hair care products, and the manufacture of ebonite.
  • In root canal procedures, Calcium hydroxide is in use to fill the cavities in the human teeth.
  • Sugar beets and sugarcane are processed through carbonation, which involves the use of calcium hydroxide.
  • Calcium hydroxide is in use in the leather industry to bifurcate the fur/hair from the animal hide.

FAQs on Calcium Hydroxide

Question 1: What are the uses of calcium hydroxide?

Calcium hydroxide is a white powder that does not have any characteristic odour. It is in use in industrial settings like the treatment of waste, the manufacturing of paper, building and processing of food. It has several medical and dental applications of calcium hydroxide. For example, fillings used in root canal treatments often have calcium hydroxide.

Question 2: Is calcium hydroxide acidic or basic?

Calcium hydroxide is also known as slaked lime. It is a source of hydroxide ions when dissolved in aqueous solutions. That’s why this compound is a base. Due to electrolyte dissociation, Calcium hydroxide liberates \(OH^- \) ions.

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