One Sunday, Mark decided to go sailing in his boat with his friend Dan, but Dan happened to be away. Dan's brother, John offered to go instead though he did not know anything about sailing. Mark agreed and they set out to sea．
Soon they found themselves in a thick fog. Mark was sure that they would be hit by a big ship. Fortunately he saw a large buoy（浮标）through the fog and decided to tie the boat to it for safety. As he was getting onto the buoy, however, he dropped the wet rope. The boat, moved away in the fog carrying John, who did not know how to use the radio. He drifted（漂流）away and was not seen until twelve hours later.
Mark spent the night on the buoy. In the early morning he fell asleep, and was having a bad dream when a shout woke him up. A ship, the Good Hope, came up and he climbed onto it and thanked the captain. The captain told him that John had been picked up by another ship and the ship's captain had sent out a message. "Without the message I would not have found you on the buoy," he said.
What made it possible for Mark to be found on the buoy?（ ）
根据最后两句话得知是约翰告诉人们去哪找马克的，才使他最终得救。
把10gMgCl2和MgSO4的混合物溶于适量水中，滴加2mol／L的BaCl2溶液至沉淀完全，共消耗BaCl2溶液25mL。求混合物中MgCl2和MgSO4的质量各是多少克
固定在匀强磁场中的正方形导线框abcd，各边长为1．其中ab是一段电阻为R的均匀电阻丝，其余3边均为电阻可忽略的铜线。磁场的磁感应强度为B，方向垂直纸面向里。现有一与ab段的材料、粗细、长度都相同的电阻丝PQ架在导体框上，如图所示。PQ以恒定速度方向如何
一小物块以速度v0=10m／s沿光滑地面滑行，然后沿光滑曲面上升到顶部水平的高台上，并由高台上飞出，如图所示。问当高台的高度h多大时，小物块飞行的水平距离最大这个距离是多少(g=10m／s2)
某晶体盐的水溶液呈浅绿色，在此溶液中通入氯气，溶液变为棕黄色，把所得棕黄色溶液分成两份：一份加入KSCN溶液，溶液变深红色；另一份加入BaCl2溶液，产生不溶于稀HNO3的白色沉淀，则该晶体盐的化学式是_______。
欲除去乙烷中的乙烯，可将混合气体通过盛有_______的洗气瓶；欲除去乙炔中的硫化氢，可将混合气体通过盛有_______的洗气瓶。
浓硝酸常盛装在棕色瓶中，且贮放在冷暗处，原因是_______，
碳酸氢钠溶液和烧碱溶液反应的离子方程式为_______。
常温下，氨水中存在的离子有_______。
在酸或碱存在的条件下，甲酸甲酯跟水发生水解反应的化学方程式为_______。
有下列几种气体：H2、O2、HCl、NO、NH3、CH4，其中通常情况下能用来和水做“喷泉”实验的是_______。
