中考中的化学除杂质题，既能考查考生对物质性质的灵活运用能力，又能考查考生对有关的实验技能的掌握程度。因此，该类题综合性较强，要对此类题准确作答，主要是要把握有关物质的性质，并能周全地考虑问题中的各有关因素。解“除杂质题”的基本原则是：既不引入新的杂质，又? The problem of chemical impurity removal in the middle school entrance examination can not only examine the candidate’s ability to use materials in a flexible manner, but also examine the candidate’s mastery of relevant experimental skills. Therefore, this category of questions is more comprehensive. To accurately answer such questions, it is mainly necessary to grasp the nature of the relevant substances and to consider all relevant factors in the questions. The basic principle for solving the “contamination problem” is: not introducing new impurities, but also?