Soap vs Detergent

The words soap and detergent are used interchangeably all the time, but they are actually two quite different things, made differently, from different ingredients, and with different strengths and limitations.

Understanding the difference helps you choose the right product for the right job, and gives you a much clearer picture of what you're actually using at home.

What is soap?

Soap is one of the oldest cleaning agents we know of. Archaeological evidence from Babylon dates soap-like materials to around 2800 BC, with clay tablets describing mixtures of fats and ashes used for cleaning textiles and treating skin conditions. The basic chemistry has not changed much since then.

For something to be a true soap, it must be made through a process called saponification: a chemical reaction between fats or oils and a strong alkali, commonly known as lye. Without this reaction, you don't have soap. You might have a cleanser, a detergent bar, or a synthetic alternative — but it's not technically soap.

Lye comes in two main forms: Sodium hydroxide (NaOH) used to make hard bar soaps. Potassium hydroxide (KOH) used to make soft or liquid soaps. Both are highly caustic and need to be handled carefully. 

A useful bit of reassurance: when soap is made properly, the lye is fully consumed in the saponification reaction. There should be no residual lye in a finished, well-made soap. In fact, most handcrafted soap makers deliberately use a slight excess of oil (called a "superfat") to ensure this.

Glycerol (Glycerine) is also naturally produced during saponification, which is one reason why handmade soaps often feel different to commercial bars.

What soap does well Made from natural fats and oils Biodegradable and gentler on waterways and aquatic life Generally milder for skin Kinder to septic systems Has a natural moisturising by-product (glycerol) Is a surfactant which reduces surface tension to lift and suspend dirt Where soap has limitations Less effective in hard water, the minerals (calcium and magnesium) in hard water react with soap's fatty acid salts to form insoluble compounds, which is what creates that familiar white soap scum Can leave residue on surfaces, fabrics, and in pipes over time Generally more expensive to produce using quality oils and fats Offers mild to moderate cleaning power, appropriate for many household tasks but not always suited to heavy-duty applications

What is a detergent?

Detergents are synthetic cleaning agents. Unlike soap, they are not produced through saponification but through industrial chemical processes using petroleum-derived or, increasingly, plant-derived raw materials.

Modern detergent formulations typically contain multiple ingredients beyond the surfactant itself: builders to improve cleaning performance, preservatives, fragrance, colour, pH adjusters, and more. The complexity varies considerably depending on the product and its intended purpose.

What detergents do well Effective in hard water as synthetic surfactants do not react with calcium or magnesium ions, so they don't produce soap scum Wide range of cleaning power, from gentle to very strong Generally less expensive to produce at scale Where detergents have limitations Many synthetic surfactants are more persistent in the environment and more toxic to aquatic organisms than natural soap  Stronger formulations tend to be harsher on skin and surfaces More complex supply chains and manufacturing processes The environmental impact of individual detergent products varies widely depending on specific ingredients, concentrations, and packaging

 

How to think about the choice

Neither soap nor detergents are universally better. They are different tools with different strengths and limitations, and the right choice depends on your water type, what you're cleaning, and what matters to you.

If you live in a soft water area and are making your own products for general household cleaning, natural soap can be an excellent, low-impact option. If you have hard water, you'll likely find synthetic surfactants more practical for tasks like dishwashing.

Understanding what you're using, how it works, and what tradeoffs are involved puts you in a much better position to make decisions that actually align with your values.