The composition of (1) substances is often described by elements, in which Ca, K, Mg and Na refer to elements; In life, soapy water can be used to distinguish hard water from soft water. Soft water has the most foam and hard water has the least foam.

(2) According to the order from top to bottom, the rearrangement should be: removing larger particles with pebbles, removing smaller particles with quartz sand, removing pigments and odor with activated carbon, and finally using fluffy cotton as an auxiliary.

Polluted water is harmful, so we should prevent water pollution. Domestic sewage is discharged after treatment, and phosphorus-containing washing powder is prohibited. Rational use of chemical fertilizers and pesticides will not cause water pollution; The direct discharge of industrial wastewater will cause water pollution.

(4) The experiment of electrolytic water can prove that water is composed of hydrogen and oxygen.

So the answer is: (1) element; Soapy water;

(2)BADC； ?

(3) a;

(4) Hydrogen and oxygen.