SAT考试是美国高中生进入美国大学的标准入学考试，包括SAT1和SAT2。SAT2是专项测验，有数学、物理、化学、生物、外语等，大部分为选择题，是可以选科的。今天我们来学习SATII化学专题分析 Balancing Chemical Equations，希望对大家的复习有所帮助。

　　You may remember that the law of conservation of mass says that matter is neither created nor destroyed during a chemical reaction. This means that all chemical reactions must be balanced—the number of atoms, moles, and ultimately the total mass must be conserved during a chemical process. Here are the rules to follow when balancing equations:

　　Determine the correct formulas for all the reactants and products in the reaction.

　　Begin balancing with the most complicated-looking group. A polyatomic ion that appears unchanged on both sides of the equation can be counted as a single unit.

　　Save the elemental (single elements) reactant and products for last, especially if it is hydrogen or oxygen. Keep your eye out for diatomic molecules such as oxygen, hydrogen, and the halogens.

　　If you get stuck, double the most complicated-looking group and try again.

　　Finally, make sure that all coficients are in the lowest-possible ratio.

　　Know when to quit! None of the reactions you will encounter will be that difficult. If the coficients are getting wild, double-check what you’ve done since you may have a simple mistake.

　　When balancing reactions, keep your hands off the subscripts! Use only coficients to balance chemical equations. Now let’s try an example. When you solve it yourself, make sure to follow the steps!

　　Example

　　Write the balanced equation for the reaction between chlorine and sodium bromide, which produces bromine and sodium chloride.

　　Explanation

　　First write the chemical formulas—be on the lookout for the diatomic elements (such as Cl2):

　　Cl2 + NaBrBr2 + NaCl

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