题目

Most people feel lonely sometimes, but it usually only lasts between a few minutes and a few hours. This kind of loneliness is not serious. In fact, it is quite normal. For some people, though, loneliness can last for years. Now researchers say there are three different types of loneliness. The first kind of loneliness is temporary(暂时的). This is the most common type. It usually disappears quickly and does not require any special attention. The second kind, situational loneliness, is a natural result or a particular situation---- for example, a family problem, the death of a loved one, or现有NaCl样品（混有少量杂质离子：Mg2+，Ca2+，SO42-），某同学设计了以下实验方案对该NaCl样品进行提纯，并配制溶质质量分数为a%的NaCl溶液．请回答实验中的有关问题．（1）实验过程中，所加物质的化学式，，（2）写出相关实验方法：操作①；操作②．（3）加入过量A溶液的目的是，，可以确定A已经过量．（4）沉淀物N的成分为．