什么是标准溶液制备的直接法和标定法？直接法所使用的基准物须符合哪些条件？
参考答案：
标准溶液的制备主要有两个步骤：一是配制，二是确定其准确浓度它的。制备方法分为直接法和标定法。直接法就是准确称取一定量的基准物质，溶解后，全部转移到容量瓶中，稀释至刻度，配制成一定体积的溶液，直接计算出溶液的准确浓度。标定法也叫间接法，它是将试剂先配制成接近所需浓度的溶液，再用基准物质或其它标准溶液来标定其准确浓度。直接法制备溶液的物质必须是基准物质，应符合下列条件：
（1）纯度较高，杂质含量不大于0.01～0.02％；
（2）物质的组成应与化学式（包括结晶水在内）完全符合；
（3）性质比较稳定（不分解、不吸潮、不吸收CO2等）；
（4）使用时易溶解；
（5）具有较大的当量，以便减小称量时造成的相对误差。
At the beginning of the world, it was Bear who owned Fire. Bear and his people carried Fire with them ________ they went. One day, Bear and his people came to a great forest, where they found many acorns(橡子) ________ on the forest floor. Bear set Fire at the edge(边缘) of the forest, and he and his ________ began eating acorns. They went further and further away from Fire, eating the delicious acorns and looking for________. Fire blazed (发出火焰) up happily for a while ________ it had burned almost all of its wood. It started to smoke. "_______ me! Feed me!" Fire shouted to Bear. But Bear and his people did not hear Fire's ________. At that moment, Man came walking through the forest and saw the small Fire. "What should I feed you?" Man asked, He had________ seen Fire before. "I eat sticks and wood of all kinds," Fire said. Man ________ a stick and put it on Fire. Man got a second stick and then a third one... By this time, Fire's hunger was satisfied. Man warmed himself by the Fire, enjoying the changed colors. Man and Fire were very happy together, and Man fed Fire sticks whenever it got hungry. And now Fire ________ to Man.
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