(1) When alum is added and stirred, small particles of insoluble solids can condense into large particles. After standing, insoluble solids can be removed by filtration. Activated carbon has adsorption, which can adsorb pigments and odors. No new substances are produced in this process, which belongs to physical changes. Hard water will bring a lot of inconvenience to life. For example, washing clothes with hard water not only wastes soap but also washes them cleanly. Therefore, fill in: filter; Adsorption; Physics; Washing clothes with hard water wastes soap and can't be cleaned;

(2) Soap water is used to test hard water and soft water, in which soft water produces more foam and hard water produces less foam; Hard water can be softened by boiling in daily life; So fill in: soapy water; Boil;

(3) Reasonable suggestions to prevent water pollution include multi-use of one water or the use of water-saving faucets.