What Happens when Soap is Added to HCl?

Is Soap acid or base? How does soap react if it is added to a base or an acid? This article explores these questions in detail. Let’s read all of that below:  

Introduction:

Soap is one of those necessities which always finds its way into our houses. They have been with humans for thousands of years, the earliest history of soap dates back to 2800 BC in the ancient Babylonian era. Before we attempt to answer the question “Is Soap acid or base”, it is relevant to explain first the very basics of soap.

What is Soap Made of?

Soaps are made from a process called saponification in which potassium hydroxide or sodium hydroxide (alkalis) are allowed to react with fats or oils (triglycerides) in the presence of water. To be specific, these alkalis are allowed to react with carboxylic acid (RC(=O) OH) which comes from these fats and oils.

These are usually animal or plant fast including tallow, lard, olive oil, coconut oil, palm oil, etc. This process also produces glycerin as a byproduct which has great moisturizing properties and is a great ingredient for soaps. This reaction can be represented as below. Note that RC(=O)ONa is a typical soap molecule.

RC(=O)OH + NaOH → RC(=O)ONa + H₂O

Is Soap Acid or Base?

Commercially available soaps are usually basic as their pH levels fall between 7 to 8. It is also important to note that the final pH of soaps is based on different types of soaps as they are available in a wide variety of ingredients. But is soap acidic or can be acidic? The answer is no, as all of them are usually basic.

No matter whether acidic or basic, the structure of the molecules of the soap has a water-attracting head (hydrophilic) and water-repelling tail (hydrophobic). This gives soap the ability to effectively clean dirt and oil from the human skin.

The Reaction of Soap with HCl:

As we stated above soap is a weak base and if we react it with a strong base like HCl, then it results formation of salt. The sodium in the carboxylic acid exists as a cation in soap which upon reacting with HCl results in the following reaction:

RC(=O)ONa + HCl → RC(=O)OH + NaCl

The reaction results in a sodium chloride salty solution which is water soluble as well. In this state, the soap also loses its surfactant properties as it works better in basic solutions. The resulting diluted salt solution is a sticky, scummy mess because free fatty acid is very weakly soluble in water. Although it's not quite the same as the calcium and magnesium ions of fatty acids that form your regular "soap scum," it's virtually indiscernible to the naked eye and human fingers.

Some types of soap also have stearinic acid (CH₃(CH₂)₁₆CO₂H) and sodium salt in them. Upon exposure, the basic salt will be neutralized by an aqueous solution of hydrochloric acid. This makes them precipitate stearinic acid leaving sodium chloride in the solution.

Stearinic acid forms a protective barrier around soap, preventing it from dissolving when it is allowed to react with HCl. Stearinic acid have a long-chain fatty acid in its molecular structure and is found in many natural sources, such as coconut oil and shea butter. That is why it makes its way into soap.

It is also worth mentioning that soap is typically just a fatty acid salt that has partial solubility in water. In fact, it ionizes into sodium and fatty acid ions upon dissolution. When sodium chloride, or salt, is added to water, it disintegrates into its ions allowing the sodium ions in the soap to mix with the fatty acid ions and separate into solid soap. This aids in the soap's total removal from the reaction mixture.

Wrapping Up:

To conclude the topic “is Soap Acid or Base”, it can be stated they are usually weak basic and when they dissolve in water, they turn into sodium ions and fatty acid ions. When they react with strong acids like hydrochloric acid, they turn into RC(=O)OH and NaCl, resulting in an inorganic chloride salt of the fatty acid ester and a scummy fatty acid mix.