化学方程式是化学中最重要的用语,在初中化学中占有十分重要的地位。每年各省市的中考化学试题中,按题目要求写出化学方程式的题是必有的,且所占分值较大。这类题目,有的难度较小,例如,直接给出反应物要求写化学方程式(象煅烧石灰石、碳酸钠与氢氧化钙起反应等);有的难度适中,例如,给出一种反应物和一种生成物(或只给出一种反应物或一种生成物)要求写出化学方程式(象写出你应该知道的Zn参加反应的化学方程式、用化学方程式表示制备MgCl_2的七种方法、写出碳酸与某种物质反应生成碳酸钙的化学方程式等);有的难度较大,例如,没有给出任何一种反应物和生成物的名称(或化学式),要求根据题目提供的信息写出化学方程式(象“一种紫黑色晶体受热分解生 The chemical equation is the most important term in chemistry and plays a very important role in junior chemistry. Each year in the provinces and cities of the chemical examination questions, according to the title requirements to write a chemical equation is a must-have question, and the score is larger. Some of these problems are less difficult. For example, directly giving reactants requires a chemical equation (such as calcined limestone, sodium carbonate and calcium hydroxide to react, etc.); some are moderately difficult, for example, giving a reactant And a product (or just a reactant or a product) requires a chemical equation (like writing chemical formulas for which you should know that Zn participates in the reaction, and chemical equations for the seven methods of preparing MgCl_2) Write chemical equations for the reaction of carbonic acid with certain substances to produce calcium carbonate, etc.); some are more difficult, for example, no names (or chemical formulas) are given for any reactants and products, and information required by the title is required. Write the chemical equation (like “a kind of purple black crystal