Written by Elizabeth Mika
Founder and Maker, Bramble & Bear™ Soap Co.
About this Editorial: This article addresses a common question about soap in general. It is intended as general educational information and does not refer specifically to Bramble & Bear soaps.
Soap scum is not simply unrinsed soap. In hard water, minerals—especially calcium and magnesium—can react with true soap and form insoluble deposits.¹ Those deposits can leave a pale or grayish film on tubs, sinks and shower surfaces, and they can also change the way soap lathers and feels during washing.¹
What Makes Water “Hard”?
Water hardness refers mainly to the amount of dissolved calcium and magnesium in the water. The more of these minerals present, the harder the water is considered to be.¹ Hard water is common in many parts of the United States, and the degree of hardness can vary substantially from one location to another.¹ That matters because those minerals can interact with soap before the soap has a chance to behave the way it would in softer water.
How Does Soap Scum Form?
Traditional soap is made from salts of fatty acids. When those soap molecules encounter calcium and magnesium in hard water, they can form insoluble calcium and magnesium compounds.² Those insoluble materials are what we commonly call soap scum. Because they do not remain dissolved in the water, they can settle onto shower walls, tubs, sinks, glass and other surfaces.¹
Why Does Hard Water Affect Lather?
Some of the soap reacts with calcium and magnesium before it can contribute fully to lather.² That is why soap may foam less readily in hard water and why more soap may be needed to produce the same amount of lather.¹ The water chemistry has changed how the soap behaves.
Is Soap Scum Just Soap That Did Not Rinse Away?
Not exactly. Soap scum is a new insoluble material formed when soap reacts with hard-water minerals.¹ Some of that material can remain on bathroom surfaces or temporarily cling to the skin, which is one reason people using hard water may describe a different after-wash feel.
Why Can Soft Water Feel Different?
In soft water, there is less calcium and magnesium available to react with soap. As a result, soap generally lathers more easily and forms less soap scum.¹ Soft water can also feel more slippery during washing because less soap is being converted into insoluble soap scum.
Is Soap Scum the Same as Mineral Scale?
Soap scum and mineral scale are not the same thing. Soap scum forms when soap reacts with hard-water minerals, while mineral scale comes from minerals in the water depositing on surfaces.¹
Do Synthetic Body Washes Behave the Same Way?
Many synthetic detergent cleansers do not form the same insoluble calcium and magnesium soaps that traditional soap does. That is one reason a body wash or syndet cleanser may behave differently from true soap in hard water. The difference is not simply liquid versus solid. It comes from the type of cleansing ingredients used in the formula.
Does Hard Water Mean You Should Avoid Bar Soap?
No. Hard water simply changes the way true soap behaves. Depending on the mineral content of the water, a bar may lather less readily or leave more visible residue on surrounding surfaces.¹ Knowing that water chemistry is involved can help explain why the same bar may behave differently in two different homes.
The Bottom Line
Soap scum forms when true soap reacts with calcium and magnesium in hard water, creating insoluble compounds that can remain on bathroom surfaces and sometimes on the skin.¹ ² Hard water can also reduce lather because some of the soap reacts with those minerals before contributing fully to foam.¹ The bar has not necessarily changed. The water has.
Related reading
Sources
¹ U.S. Geological Survey, “Hardness of Water” and “Why does it take so long to rinse the soap off my hands? What are hard water and soft water?” USGS explains that water hardness is largely determined by dissolved calcium and magnesium, and that these minerals can react with soap to form insoluble soap scum, reduce lather and leave residue on skin and bathroom surfaces.
² U.S. Geological Survey, National Water Conditions — Water Quality Tables. USGS notes that soaps react with hardness salts to form insoluble calcium and magnesium stearates and oleates, consuming soap before its lathering properties become available.
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