What Is Caustic Soda?

by Jessica Clifton

With the exception perhaps of water, few chemicals have almost ubiquitous industrial applications, but caustic soda is one of those chemicals. In this article, we explore what caustic soda is, what it’s made of, its uses and its chemical structure.

About caustic soda

Caustic soda, which is the same chemical as sodium hydroxide and is also known as lye, is a strong alkaline inorganic compound. It is used either as a reagent or as an ingredient in a wide variety of products.

It is amongst the most widely used chemicals in industrial manufacturing processes, and is ranked in the top five in terms of the amount produced globally per year, at more than 70 million tonnes. One reason for that is because sodium hydroxide is a by-product of the manufacture of chlorine.

Lye has been known about for thousands of years, long before it was chemically identified. Archaeological excavations suggest that as far back as 2,800 BCE, lye-based soaps were being used by Babylonians, who made them by mixing animal fat and ash in water. The ancient Romans used volcanic ash to make lye, which they used as a cleaning agent.

Caustic soda, as its name suggests, is highly caustic, which means that it can easily burn, dissolve or corrode organic materials such as skin. Its caustic property makes it ideal for dissolving the cellulose in plant materials, converting them to pulp for manufacturing paper. 

What is caustic soda made of?

Caustic soda is an inorganic chemical compound that is made of sodium, oxygen, and hydrogen. Its less pure form, lye, is a product of wood ash. 

Lye has many impurities but its active chemical ingredient is the same as the purer caustic soda/sodium hydroxide used in industry. The precise identification of sodium hydroxide as a distinct alkali compound came almost five thousand years after the first use of lye as a soap ingredient. 

The strength of caustic soda as an alkaline (base) compound can be measured in two ways:

  • pH level – the pH level of a sodium hydroxide solution varies depending on its concentration in water. For instance, a one mole solution has a pH value of 13. That means the alkalinity strength of sodium hydroxide solution directly depends on how much of the base is present in the solution.
  • Dissociation constant – unlike pH level, the dissociation constant is a more objective way of measuring the strength of an alkaline compound as it is independent of its concentration. The dissociation constant of sodium hydroxide is Kb = 6.3 X 10-1. This makes it a strong monoprotic. A monoprotic base can only react with one proton per molecule.

The dissociation constant of an acid or a base does not have a unit because it is simply the ratio of the concentration of ions of one mole of that substance in a solution and the molecules of the compound.

Hence, the generalised formula for the dissociation constant of a base can be expressed as:

Kb = [BH+] [OH]/B


  • Kb – the dissociation constant value
  • BH+ – the concentration of positive ions
  • OH – the concentration of the negative ions
  • B – the original concentration of the base 

What’s the common name for caustic soda?

Caustic soda is actually one of the common names of sodium hydroxide. The other common name of the compound is lye.

Caustic soda is a descriptive name based on its industrial applications, in that the compound is caustic enough to dissolve many types of organic materials, like protein and cellulose. The modern English word, lye, on the other hand, can etymologically be traced to the Old English word lēag. This is from Proto-Germanic laugō, which is from Proto-Indo-European lewh, which means “to wash”.

As you can see from the etymology of the word, it implies washing or cleaning, which isn’t surprising when you consider the fact that lye has been used as a cleaning agent since ancient times.

Caustic soda structure

Caustic soda, or sodium hydroxide, has a relatively simple molecular structure (remember, it’s only made up of sodium, oxygen, and hydrogen). At the centre of a molecule of caustic soda is an oxygen atom.

The chemical structure of caustic soda
The chemical structure of caustic soda

The sodium atom has an ionic bond with the oxygen atom, while the hydrogen atom has a covalent bond with the oxygen atom. This makes the sodium atom positively charged, and the oxygen atom negatively charged. Therefore, sodium hydroxide is a polar molecule. which means it can dissolve and dissociate in water, but it cannot do the same in hydrocarbons.

Caustic soda formula

Caustic soda is chemically known as sodium hydroxide, which is expressed in the chemical formula NaOH. The chemical formula of a substance indicates the proportions of atoms in the molecular compound. In this case, there is one sodium, one oxygen and one hydrogen per molecule of sodium hydroxide.

Based on the formula, the molecular weight or the molar mass can also be calculated. Sodium hydroxide has a molar mass of 39.9971 g mol−1. The molar mass is one of the few properties of a chemical that do not need to be experimentally determined. Other properties, like melting point and solubility, are empirically determined. 

Caustic soda chemical name

Caustic soda, otherwise known as lye, is chemically known as sodium hydroxide. You may also hear it called sodium hydrate or soda lye. The systematic IUPAC name of the compound is sodium oxidanide.

What is caustic soda used for?

Caustic soda is a common chemical used in various industrial and household applications. On its own, it is commonly used as a cleaning agent, but it’s also used in the preparations of various products, ranging from detergents to medicines. It is crucial in many steps of various manufacturing processes. Here are some of the most common applications of caustic soda:

  • Cleaning and disinfectant products – detergents, bath soaps, and cleaning agents typically contain sodium hydroxide as an active ingredient.
  • Pharmaceuticals and medicines – manufacturing many types of medicines, like anti-cholesterol drugs and painkillers, wouldn’t be possible without caustic soda. This alkaline substance serves as a reagent to create chemical precursors.
  • Energy production – caustic soda is used in manufacturing fuels cells for storing and producing electrical energy.
  • Water treatment – the acidity of water supply for cities and other large communities is balanced using sodium hydroxide. It is also used to produce sodium hypochlorite, which in turn is used to disinfect water.
  • Food production – caustic soda is used in food processing methods such as curing foods. It is also used to peel the thin skins of fruits, such as tomatoes, for canning. It has preservative properties that prevent the growth of mould and bacteria.
  • Wood and paper products – the caustic property of the substance makes it ideal for dissolving the cellulose polymers in wood, converting them to pulpy substance, which can be processed into paper.

How to use caustic soda

At home, you can use caustic soda as a drain unblocker to dissolve organic materials, such as hair and animal fats, that are blocking your drain. You can also use it to de-clog your toilet and clean your tiles. Make sure you’re safe when you’re handling it, and always use heavy-duty gloves, aprons, goggles, and a face mask when using caustic soda as a cleaning agent.

How to unblock a sink using caustic sodaIf you want to get creative, you could try experimenting with making your own soap using caustic soda, oil, and scents.


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