* Calcium: Calcium is a mineral that is found in many water sources. When it reacts with soap, it forms a scum that can be difficult to remove.

* Magnesium: Magnesium is another mineral that can cause scum. It reacts with soap to form a similar scum as calcium.

* Iron: Iron is a metal that can also cause scum. It reacts with soap to form a brownish-red scum.

* Manganese: Manganese is a metal that can cause a black scum. It reacts with soap to form a compound called manganese dioxide, which is insoluble in water.

* Soap: Soap is a surfactant that helps to remove dirt and grime from surfaces. However, it can also react with minerals in water to form scum.