Difference Between Lye and Caustic Soda

The key difference between lye and caustic soda is that the term lye may refer to either sodium hydroxide or potassium hydroxide, whereas the term caustic soda refers only to sodium hydroxide.

Although we use the terms lye and caustic soda interchangeably, they are slightly different from each other, because lye is a general term while caustic soda is a specific name. Lye is a metal hydroxide, but caustic soda is specifically sodium hydroxide.

CONTENTS

1. Overview and Key Difference
2. What is Lye 
3. What is Caustic Soda
4. Side by Side Comparison – Lye vs Caustic Soda in Tabular Form
5. Summary

What is Lye?

Lye is a metal hydroxide that can form a caustic basic solution when dissolved in water. Traditionally, people obtained lye from leaching ashes. Lye is a compound that is usually a strong alkali and highly soluble in water. More importantly, we call sodium hydroxide or potassium hydroxide as “lye”. Historically, potassium hydroxide was “lye”, but this term most commonly refers to sodium hydroxide.

The present manufacturing process for this compound is membrane cell chloralkali process. Here, the end product may come in different forms such as flakes, pellets, microbeads, coarse powder and solutions.

When considering the uses of lye, the fields of applications include the food industry, soap production, cleaning agents, tissue digestion, fungus identification, etc. Lye is useful to cure (a method of preservation of food) different food items. Since it can come in both forms, sodium hydroxide and potassium hydroxide, this compound is useful in soap production as well. Moreover, some cleaning agents such as oven cleaners contain this compound.

What is Caustic Soda?

Caustic soda is the common name for the chemical compound sodium hydroxide. It is an inorganic compound with the chemical formula NaOH. It is an ionic compound and exists as a white solid in standard temperature and pressure.

Caustic soda is highly a caustic base and an alkali that can decompose proteins at normal temperatures. Also, it can cause severe burns on the skin. Furthermore, it is highly water soluble and also readily absorbs water vapour and carbon dioxide from the air. Besides, this compound can form a series of hydrates, but the commercially available form is the monohydrated sodium hydroxide.

The uses of caustic soda include the manufacture of pulp and paper, alumina, soap and detergents, petroleum products, and production of other chemical compounds. Other applications include water treatment, as an additive in the food industry, metal processing, mining, etc.

What is the Difference Between Lye and Caustic Soda?

The terms lye and caustic soda may refer to sodium hydroxide but, historically, the term lye was used to name potassium hydroxide as well. Thus, the key difference between lye and caustic soda is that the term lye may refer to either sodium hydroxide or potassium hydroxide, whereas the term caustic soda refers only to sodium hydroxide. Therefore, the chemical formula of lye can be either NaOH or KOH, but the chemical formula of caustic soda is NaOH.

Summary – Lye vs Caustic Soda

Though we can use these terms interchangeably, there is a slight difference between lye and caustic soda. The key difference between lye and caustic soda is that the term lye may refer to either sodium hydroxide or potassium hydroxide, whereas the term caustic soda refers only to sodium hydroxide.

Reference:

1. Helmenstine, Anne Marie. “How to Make Homemade Lye Using Two Ingredients.” ThoughtCo, Jun. 22, 2018, Available here.

Image Courtesy:

1. “Lye” By Certified Lye (talk) – self-made (CC BY 3.0) via Commons Wikimedia
2. “Caustic Soda Flakes” By jill570641 (CC BY-SA 2.0) via Flickr