(2) The density of oxygen is higher than that of air, and it is not easy to dissolve in water, so it can be collected by drainage or upward exhaust; In order to prevent potassium permanganate powder from entering the catheter when heating, a ball of cotton should be stuffed in the mouth of the test tube; It is necessary to check the air tightness of the equipment before making gas in the laboratory. If the device leaks, the gas will escape or the collected gas will be impure. Potassium permanganate is heated to produce potassium permanganate, manganese dioxide and oxygen.

So the answer is: C or D, a ball of cotton? Air tightness? Potassium permanganate heating potassium permanganate+manganese dioxide+oxygen

(3) The catalyst can change the chemical reaction rate of other substances, but its quality and chemical properties have not changed before and after the reaction. Manganese dioxide is a catalyst for hydrogen peroxide to produce oxygen; Hydrogen peroxide uses manganese dioxide as a catalyst to produce water and oxygen. So the answer is: chemical properties? Catalyst hydrogen peroxide manganese dioxide water+oxygen

(4) Hydrogen peroxide is used to produce oxygen, the reaction condition is normal temperature, no heating is needed, the operation is simple, energy is saved, and the products are water and oxygen, and there is no pollution.

So the answer is: easy to operate? Energy-saving products have no pollution to the environment (choose two items)

(5) Sulfur and oxygen are ignited to generate sulfur dioxide, which is toxic and will pollute the air. Sulfur dioxide dissolves in water and reacts with water, so adding water to the bottom of the bottle can absorb sulfur dioxide and prevent air pollution; The thin iron wire burns in oxygen, and the product temperature is relatively high, which will cause the bottom of the bottle to burst. Adding water to the bottom of the bottle can prevent the combustion products from splashing and cracking the bottom of the bottle. So the answer is: absorb sulfur dioxide, prevent air pollution, prevent combustion products from splashing down and crack the bottom of the bottle.

(6) It can be exhausted downward and collected, indicating that the density of gas is less than that of air; Hydrogen is insoluble in water and can be collected by drainage. So the answer is: little D.