When did you last see a polar bear? On a trip to a zoo, perhaps? If you had attended a winter activity in New York a few years ago, you would have seen a whole polar bear club. These " Polar Bears" are people who meet frequently in winter to swim in freezing cold water. That day, the air temperature was 3℃, and the water temperature was a bit higher. The members of the Polar Bear Club at Coney Island, New York are usually about the age of 60. Members must satisfy two require-ments. First, they must get along well with everyone else in the group. This is very important because there are so many different kinds of people in the club. Polar Bears must also agree to swim outdoors at least twice a month from November through February.
Doctors do not agree about the medical effects of cold-water swimming. Some are worried about the dangers of a condition in which the body′ s temperature drops so low that finally the heart stops.
Other doctors, however, point out that there is more danger of a heart attack during summer swimming because the difference between the air temperature and water temperature is much greater in summer than in winter.
The Polar Bears themselves are satisfied with the benefits (or advantages)of cold-water swim-ming. They say that their favorite form of exercise is very good for the circulatory system(循环系统)because it forces the blood to move fast to keep the body warm. Cold-water swimmers usually turn bright red after a few minutes in the water. A person who turns blue probably has a very poor circula-tory system and should not try cold-water swimming.
The main benefits ( or advantages) of cold-water swimming are probably mental. The Polar Bears love to swim year-round; they find it fun and relaxing. As one 70-year-old woman says, "When I go into the water, I pour my troubles into the ocean and let them float away. "
Doctors .
从第二段可知，医生对冬泳的医疗作用有不同的观点。一些医生认为冬泳时体温降得太低，容易使心脏停止跳动，另一些医生认为在夏季游泳更容易导致心脏病的危险。
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_______。
