(1) Activated carbon has adsorbability and can be used to adsorb the odor in the refrigerator;

(2) Oxygen isolation can put out the fire, and the oil pan can be covered when it is on fire;

(3) Rainwater will react with carbon dioxide in the air during falling, so that the pH value of rainwater is about 5.6, and the chemical equation of the reaction is CO2+H2O = H2CO3;

(4) Soap water has less foam in hard water and more foam in soft water, so it can be used to test hard water and soft water.

So the answer is (1) adsorption.

(2) Cover the lid.

(3) carbon dioxide+water = carbon dioxide

(4) Soapy water