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问题 选择题

进行下列实验后，溶液的PH值变大的是（　　）

A．稀盐酸中加入铁屑

B．食盐溶液中滴入硝酸银溶液

C．NaOH溶液中加入MgCl2溶液

D．Ca（OH）2溶液中通入适量的CO2

答案

A、稀盐酸中加入铁屑，铁与盐酸反应生成氯化亚铁和氢气，溶液由反应前的盐酸溶液变为氯化亚铁溶液，溶液中H+减少，溶液的酸性减小pH值变大；

B、食盐溶液中滴入硝酸银溶液，反应生成氯化银沉淀和硝酸钠，反应前的溶液氯化钠溶液、硝酸钠溶液中H+含量不变pH不变；

C、NaOH溶液中加入MgCl2溶液，反应生成氢氧化镁沉淀和氯化钠，反应前的碱氢氧化钠溶液变成了反应后的氯化钠溶液，溶液中OH-减少碱性减弱，溶液的pH变小；

D、Ca（OH）2溶液中通入适量的CO2，反应生成碳酸钙沉淀和水，反应前的氢氧化钙溶液反应后变成水，溶液中OH-减少碱性减弱，溶液的pH减小．

故选A

单项选择题

Most earthquakes occur within the upper 15 miles of the earth’s surface. But earthquakes can and do occur at all depths to about 460 miles. Their number decreases as the depth increases. At about 460 miles one earthquake occurs only every few years. Near the surface earthquakes may run as high as 100 in a month, but the yearly average does not vary much. In comparison with the total number of earthquakes each year, the number of disastrous earthquakes is very small.
The extent of the disaster in an earthquake depends on many factors. If you carefully build a toy house with an erect set, it will still stand no matter how much you shake the table. But if you build a toy house with a pack of cards, a slight shake of the table will make it fall. An earthquake in Agadir, Morocco, was not p enough to be recorded on distant instruments, but it completely destroyed the city. Many per earthquakes have done comparatively little damage. If a building is well constructed and built on solid ground, it will resist an earthquake. Most deaths in earthquakes have been due to faulty building construction or poor building sites. A third and very serious factor is panic. When people rush out into narrow streets, more deaths will result.
The United Nations has played an important part in reducing the damage done by earthquakes. It has sent a team of experts to all countries known to be affected by earthquakes. Working with local geologists and engineers, the experts have studied the nature of the ground and the type of most practical building code for the local area. If followed, these suggestions will make disastrous earthquakes almost a thing of the past.
There is one type of earthquake disaster that little can be done about. This is the disaster caused by seismic sea waves, or tsunamis. (These are often called tidal waves, but the name is incorrect. They have nothing to do with tides.) In certain areas, earthquakes take place beneath the sea. These submarine earthquakes sometimes give rise to seismic sea waves. The waves are not noticeable out at sea because of their long wave length. But when they roll into harbors, they pile up into walls of water 6 to 60 feet high. The Japanese call them "tsunamis", meaning "harbor waves", because they reach a sizable height only in harbors. Tsunamis travel fairly slowly, at speeds up to 500 miles an hour. An adequate warning system is in use to warn all shores likely to be reached by the waves. But this only enables people to leave the threatened shores for higher ground. There is no way to stop the oncoming wave.

The significance of the slow speed of tsunamis is that people may

A．notice them out at sea．

B．find ways to stop them．

C．be warned early enough．

D．develop warning systems．
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填空题

如图所示，某人用一根轻质木棒挑着为120N的物体站在水平地面上，木棒保持水平，棒AB长为1.2m，重物悬挂处离肩膀距离BO为0.8m，则手在A端对木棒竖直向下的作用力大小为_________N。
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