(2) hydrogen peroxide solution in manganese dioxide catalyzed the generation of water and oxygen, manganese dioxide before and after the reaction of the quality and chemical properties have not changed, but only accelerated the reaction of hydrogen peroxide to produce the rate of oxygen, so the manganese dioxide is a catalyst, so the answer is: Catalytic effect;
(3) After assembling the device A, we should check the gas tightness of the device first, to prevent gas leakage and can not be collected, resulting in a waste of drugs, so the students first to the dispensing funnel to add water, and then one end of the catheter into the water, and open the piston of the dispensing funnel, to observe the catheter mouth whether there is continuous bubbles out of the mouth of the operation is to check the device's gas tightness; Therefore, the answer is: check the tightness of the device;
(4) because oxygen is not easily soluble in water, so can be collected by the drainage method; Therefore, the answer is: oxygen is not easily soluble in water;
(5) because of the density of oxygen is greater than the air, so the collection of oxygen cylinders full of oxygen should be placed squarely on the desktop; Therefore, the answer is: the density of oxygen is greater than the air.