在初中化学“溶液的浓度”一节教学中,我们不但要求学生深刻理解浓度的概念,而且还要求学生熟悉掌握溶液的稀释和混和的有关计算。对于这类计算题,其一般方法是根据溶质的质量在溶液的稀释、混和过程中前后保持不变这一特征,用代数关系式求解。在教学中,我们先重点讲清溶液混和时的有关计算,再由一般到特殊,把溶液的稀释视为溶液混和的特例。让学生首先掌握用代数法求解这类问题。在此基础上,对代数关系式做适当变换,推出简便易行的“杠杆规则法”和“十字交叉法”。 为便于介绍和推证,溶液混和前后的质量和浓度用下表给出。 In the junior high school chemistry “solution concentration” section, we not only require students to understand the concept of concentration, but also require the students to be familiar with the calculation of dilution and mixing of solutions. For this type of calculation, the general method is based on the characteristic that the mass of the solute remains constant before and after the dilution and mixing of the solution, and is solved by the algebraic relation. In teaching, we will first focus on the calculations when the solution is mixed. From general to special, the dilution of the solution is regarded as a special case of solution mixing. Let students first master algebraic methods to solve such problems. On this basis, the algebraic relational equations are appropriately transformed and simple and easy-to-use “leverage rule method” and “crucifix method” are introduced. For ease of introduction and inference, the mass and concentration before and after solution mixing are given in the table below.