A man was traveling abroad in a small red car. One day he left the car and went shopping. When he came back, its roof was badly damaged.

    Some boys told him that an elephant had damaged it. The man did not believe them, but they took him to a circus(马戏团) which was near there. The owner of the elephant said, “I am very sorry! My elephant has a big, round, red chair. He thought that your car was his chair, and he sat on it!"

    Then he gave the man a letter, in which he said that he was sorry and that he would pay for all the damage.

    When the man got back to his own country, the customs（海关） officers wouldn't believe his story. They said, “You sold your new car while you were abroad（国外） and bought this old one!"

    It was only when the man showed them the letter from the circus man that believed him.

现有下列物质：①氯酸钾 ②硫酸铜 ③二氧化硅 ④镁 ⑤氧气 ⑥氢氧化钠⑦硫酸　⑧甲烷   

请将上述物质按下列要求分类，并将其序号填入空白处：

（1）按组成分类，属于有机物的是　 ____　（填序号）．

（2）属于电解质的有　 ____　（填序号），依次写出其电离方程式：　 ____　

（3）从上述物质中选出1种或2种为反应物，按下列反应类型各写1个化学方程式：

①化合反应 ____　；

②分解反应　 ____　；

③置换反应　 ____　；

④复分解反应 ____