物质分类是化学知识的重要内容，下列各组物质中：

①洁净的食盐水   ②液氧   ③铁粉   ④冰水混合物   ⑤氯酸钾   ⑥四氧化三铁

属于单质的是（填序号，下同）____，属于化合物的是____，属于氧化物的是____属于纯净物的是____．

    The world renowned British Physicist Stephen Hawking who tried to explain the mysteries of the universe to millions of people passed away on March 14th, 2018 at the age of 76. His family said he died peacefully at his home in Cambridge. Professor Hawking used wheelchair and was able to speak only through a computer synthesizer(合成器), having been suffering from a rare form of motor neuron disease as a young man. Despite being given only a few years to live, he wrote popular science books for several decades. A film about his life The Theory of Everything won an Oscar for the actor Eddie Redmayne who portrayed him.

    Stephen's daughter Lucy spoke about her father's love of the limelight in 2012. She said, “Actually my father is also a showman. He loves a big show, a big stage and bright light, whether it's him on stage or whether it's watching the spectacle.”

    Stephen Hawking's death was announced in the statement by his children who said they will miss him forever. They said his courage and brilliance inspired people across the world. Many leaders from different countries paid tribute(致敬) to him including the Indian Prime Minister Narendra Modi, who said Professor Hawking had made the world a better place.