If a solution forms permanent lather when shaked, also when adding solution of calcium ions or magnesium ions (hard water), white solid (scum) is formed.

• 若一溶液在摇动时产生恒久的泡沫；当加入钙盐溶液或镁盐溶液(硬水)后，有白色的固体(浮渣)生成。