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Sodium Hydroxide - Formula, Structure, Preparation, Physical and Chemical Properties, Applications & Uses

These handmade soaps look beautiful, isn’t it? But what do you think exactly goes into making them? It needs a combination of two major ingredients! Oils and sodium hydroxide. Apart from these, there are always infinite scopes of customising soaps in terms of colour additives, fragrances and shaping them into various moulds.

Sodium hydroxide in particular has endless possibilities for usage in multi-dimensional industries. It is like a fatherly figure in chemistry, responsible for the formation of so many compounds of immense importance and through its varied properties, fulfilling various kinds of chemical needs!


  • Introduction to Sodium Hydroxide
  • Structure of Sodium Hydroxide
  • Preparation of Sodium Hydroxide
  • Physical properties of Sodium Hydroxide
  • Chemical properties of Sodium Hydroxide
  • Applications of Sodium Hydroxide
  • Health Hazards related to Sodium Hydroxide
  • Practice problems
  • Frequently Asked Questions-FAQs

Introduction to Sodium Hydroxide

Sodium hydroxide, is an inorganic chemical compound that is normally found as a white translucent solid at ambient temperature. It is a strongly alkaline, ionic compound which is also hygroscopic in nature. It is formed by an ionic bond between and ions. 

It is also commonly known as caustic soda or Lye and is widely used in the manufacturing of soap, detergents, pulp and paper, explosives, liquid drains, oven cleaners, etc.

Structure of Sodium Hydroxide

The structure of the sodium hydroxide molecule is made from an ionic bond formed between and when the cation transfers one electron to the anion. Overall sodium hydroxide is neutral. and its monohydrate form orthorhombic crystals. Each sodium atom is surrounded by three hydroxyl anions and 3 water molecules. 

The hydrogen atoms of the hydroxyl group form a strong bond with oxygen atoms within each oxygen layer. Nearby oxygen layers are held together by hydrogen bonds between water molecules.

Preparation of Sodium Hydroxide

One of the most powerful bases is sodium hydroxide (). It is also one of the most commonly utilised alkali hydroxides in laboratories and in industry. It is most commonly made by electrolysis of salt solution in a diaphragm or Mercury cell. 

Preparation of Sodium Hydroxide by Chloralkali Process

The chloralkali process produces sodium hydroxide, including chlorine and hydrogen. This is done by electrolysis of a sodium chloride solution [] . 

At the cathode, where water is converted to hydrogen gas as well as hydroxide ions. 

At cathode: 

At anode:

Hence in Chlor-alkali process, during electrolysis:

  • Hydrogen gas is liberated at the cathode.
  • Chlorine gas is liberated at the anode.
  • Sodium ions (, from the dissolved sodium chloride) and hydroxide ions (, from the water) stay back and combine to produce sodium hydroxide solution,

It is necessary to prevent the from reacting with the in order to produce . This is usually accomplished in one of three ways, with the membrane cell technique being the most cost-effective.

Three production methods are used as mentioned below:

  • Diaphragm cell process- In the diaphragm cell process, there are two compartments separated by a permeable diaphragm, often made of asbestos fibres. Brine is introduced into the anode compartment and flows into the cathode compartment. Chloride ions are oxidized at the anode to produce chlorine, and at the cathode, water is split into caustic soda and hydrogen. The diaphragm prevents the reaction of the caustic soda with chlorine by utilising a steel cathode. [1] [2] 
  • Membrane cell process- [3] [4] The most common chloralkali process involves the electrolysis of brine in a membrane cell. This membrane is basically used to prevent the reaction between chlorine and hydroxide ions.

A Nafion membrane is used to segregate the cathode and anode processes. This process is quite identical to the diaphragm cell method. The membrane allows just sodium ions and a small amount of water to pass through. It generates of greater purity.[5] [6]

  • Mercury cell process- A mercury cathode produces sodium metal as an amalgam. After that, the sodium is allowed to react to generate .[7] [8] This mercury cell process is alternatively termed as Castner-Kellner Cell. Let's see it more deeply, as it is an important commercially applied method.

Preparation of Sodium Hydroxide from Brine solution: Castner-Kellner cell

Sodium hydroxide is commercially prepared by the electrolysis of aqueous sodium chloride solution (brine solution) in a Castner-Kellner cell which applies the same principle as chlor-alkali process.

A mercury cathode is used here. Brine solution is electrolysed using a mercury cathode and a carbon (graphite) anode. and will dissociate into its constituent ions. 

In this cell, the mercury flows along the bottom of the cell and acts as a cathode. The brine solution flows in the same direction and the anode consists of several graphite blocks. Sodium metal discharged at the cathode combines with mercury to form sodium amalgam. gas is released at the anode. This sodium amalgam is taken to another container where it will react with water to form

The reactions happening on the electrodes in this process are given as follows:

At Graphite anode (positively charged electrode): Chlorine gas is evolved at anode.[9] [10] value of oxygen is higher than chlorine gas due to the overpotential of . This is why hydroxide ions are not oxidised at anode and chloride ions get oxidised owing to its overall lower electrode potential.

At the Mercury cathode (negatively charged electrode): Sodium metal is discharged at the cathode and it combines with mercury to form sodium amalgam.[11] [12] Since the cathode is itself mercury, it readily forms an amalgam with sodium. This layer at the cathode will therefore prevent the reduction of hydrogen ions and prevent the liberation of hydrogen gas at the cathode.


This sodium amalgam reacts with water and will give sodium hydroxide and hydrogen gas. 

Mercury is then re-circulated in the cell. and are two major by-products obtained in this process. 

Preparation of Sodium Hydroxide from Sodium Carbonate solution 

Commercially, sodium hydroxide can also be prepared by an aqueous solution of sodium carbonate 

() which is treated with hot milk of lime i.e in a tank made up of iron.


The precipitate of calcium carbonate ( ) is removed by filtration and the solution is used for paper making, soap and detergents.

Preparation of pure Sodium Hydroxide

Commercial is purified with the help of alcohol. dissolves in alcohol while the impurities will remain insoluble. The alcoholic filtrate is distilled and then the alcohol distils off white pure solid sodium hydroxide. 

Recommended Video: [13] [14] 

Physical Properties of Sodium Hydroxide

  • is a white crystalline solid with a soapy touch.
  • It has a density of 2.13 and the melting point is .
  • is highly soluble in water and will give a strongly alkaline solution. When solid sodium hydroxide is combined with water, an exothermic reaction occurs.
  • It is corrosive in nature.
  • Crystals of are deliquescent.
  • It has the ability to quickly absorb water and carbon dioxide from the atmosphere.
  • Glycerol, water, and ethanol are all ideal solvents for NaOH. 
  • It has the ability to produce a variety of hydrates. Heptahydrate (), Pentahydrate (), Tetrahydrate (), Trihydrate (), Dihydrate () and Monohydrate ().
  • has a higher viscosity than water.
  • Molar mass is 39.997
  • Density of Sodium hydroxide-
  • Melting point is and boiling point is

Chemical Properties of Sodium Hydroxide[15] [16] 

  • The alkalinity of the solution is pretty high.
  • It is held together by an ionic connection.
  • Carboxylic acids and sodium hydroxide react quickly to create salts.[17] [18] 

  • Base-catalysed process for the hydrolysis of esters can also be done with .[19] [20] 

  • Sodium hydroxide reacts with protic acids to produce water and the corresponding salts. 

  • Sodium hydroxide reacts with acidic oxides such as carbon dioxide and sulfur dioxide as well as amphoteric oxides like aluminium oxides.

[21] [22] 

  • Sodium hydroxide reacts with non-metals like phosphorus to produce phosphine [23] [24] and with sulphur to form sodium thiosulphate and sodium sulphide.

[Disproportionation Reaction]

  • Sodium hydroxide in the molten state also reacts with metals like iron and aluminium.

[25] [26] 

[27] [28] 

  • Sodium hydroxide is used as a precipitant in qualitative analysis. Sodium hydroxide is soluble in water,[29] [30] but the hydroxides of most of the transition metals are insoluble, and therefore sodium hydroxide can be used to precipitate transition metal hydroxides which are generally coloured, except for zinc and lead. 

Aluminium hydroxide forms a gelatinous precipitate which can also be identified easily.

  • undergoes Saponification to form soaps. In this process, base-catalysed hydrolysis of long chain fatty acids occurs, producing soaps (sodium salts of fatty acids).

Applications of Sodium Hydroxide

  • It is an active ingredient in soap-making i.e., saponification. Hence it is of huge importance in the soap and detergent industry.
  • It is utilised as a component in drain cleaners.
  • It is widely followed in the food preserving and canning process.
  • It reacts with chlorine to make chlorine bleach that is similar to bleach.
  • It is utilised by the municipal wastewater treatment plant to remove heavy metals from the water.
  • It is utilised as a preservative in foods to keep germs and mould at bay.
  • It is utilised in the manufacturing and recycling of paper.
  • It is used in the textile industry.
  • It is used in the making of glass.
  • It is used to precipitate transition metal hydroxides.
  • It is also used for the base-driven hydrolysis of esters (as in saponification), amides and alkyl halides.

Health Hazards of Sodium Hydroxide

  • Contact with excessively high levels of sodium hydroxide can cause serious burns to the digestive system, lungs, eyes, and skin, resulting in lifelong damage or death.
  • Sodium hydroxide solvation is highly exothermic and can cause splashing to burn. Also, the respiratory system can become inflamed and it also causes irritation of the nasal mucosal surfaces.
  • Even little dosages can have serious consequences including damage to mucous membranes in the throat, nose, bronchial system and lungs. 

Practice Problems:

Q.1. Sodium hydroxide is soluble in:

  1. Benzene
  2. Toluene
  3. Water
  4. Aniline

Answer: (C) 

Solution: is ionic and polar in nature consisting of two oppositely charged ions and . So it is soluble in polar solvents like water. So, option (C) is the correct answer.

Q.2. What is the of an aqueous solution of ?[31] [32] 

  1. 7
  2. 10
  3. 12
  4. 13

Answer: (D)


Hence, = 0.1 M




Sodium hydroxide is strongly basic and hence it has a very high value of 13. 

Q.3. What is the observation when a sample of ferrous sulphate salt is treated with sodium hydroxide?[33] [34] 

  1. Red fumes
  2. Green precipitate
  3. Green gas
  4. White precipitate

Answer: (B)


Green coloured precipitates of ferrous hydroxide is observed, which is an insoluble hydroxide. 

Q.4. What is meant by causticizing? Express in terms of a chemical equation.

Answer: The formation of sodium hydroxide by treating sodium carbonate with calcium hydroxide in a double displacement reaction is called causticizing. Where out of the products formed sodium hydroxide is soluble, while calcium carbonate is not. 

Frequently Asked Question-FAQs

Q.1. What is sodium hydroxide used for?

Answer: Sodium hydroxide is used in making soaps, and detergents used in homes and business applications. By combining chlorine and sodium hydroxide, chlorine bleach is made. Also, it is used as cleansing agents, dishwashing agents etc.

Q.2. Is sodium hydroxide soluble in alcohol?

Answer: Since it is polar and ionic in nature, it has very low solubility in organic solvents like methanol and ethanol. It is highly soluble in water.

Q.3. What is the major point of difference between and ?

Answer: resembles in almost all reactions & its properties are also similar to . Only difference is that is more soluble in alcohol. Hence, we use alcoholic for specific organic reactions and not alcoholic.

Q.4. Is sodium hydroxide good for the skin?

Answer: Sodium hydroxide causes strong irritation and is corrosive to the skin, hair, breathing tract and digestive system. Soap instead which is prepared by the process of saponification of triglycerides (long-chain fatty acids) and sodium hydroxide is safer on the skin.[35] [36] 

Related Topics

Calcium carbonate Oxygen
Potassium Sodium Chloride
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