(2) The medicines provided above are only medicines for making oxygen, and the instruments are only instruments needed for decomposing hydrogen peroxide to make oxygen; Hydrogen peroxide uses manganese dioxide as catalyst to generate water and oxygen, so the answer is: ④; 2H2O2？ MnO2？ . ? 2H2O+O2↑
(3) The bottle stopper in operation b should be inverted to avoid drug contamination; D operation should be to hold the recycling bottle in the left hand and the test tube in the right hand. When pouring, the test tube mouth should be next to the recycling bottle mouth. So the answer is BD.
(4) The operation steps of preparing sodium chloride solution with sodium chloride solid are: calculation, weighing, metering and dissolution. The operation steps of preparing dilute hydrochloric acid from concentrated hydrochloric acid are: calculation, measurement and dissolution. The former needs to be weighed with a pallet balance, while the latter does not. So the answer is: tray balance.