In a basic double replacement equation you will have 2 cations and 2 anions. The general equation takes the form of AB + CD → AD + CB, where A and C are cations and B and D are anions. You also want to determine the charges of each ion.  For example: AgNO3 + NaCl → ? The cations are Ag+1 and Na+1. The anions are NO31- and Cl1-. Once you have identified all of the ions and their charges, rearrange them so that the first cation is now paired with the second anion, and the second cation is now paired with the first anion. Remember the equation: AB + CD → AD + CB.  Remember to balance the charges when forming new compounds. For example: AgNO3 + NaCl → ? Ag+1 now pairs with Cl1- to form AgCl. Na+1 now pairs with NO31- to form NaNO3. After writing the products that will form in the equation, you can write the whole equation with both products and reactants. Keep the reactants on the left side of the equation and write the new products on the right side with a plus sign between them.  For example: AgNO3 + NaCl --> ? AgNO3 + NaCl --> AgCl + NaNO3 It’s important to indicate the states of matter for both the reactants and the products. There is a designated letter for each state of matter which goes in parentheses. Put this information after the formula of the substance it is describing.Use “(g)” to indicate a gas, “(s)” to indicate a solid, “(l)” to indicate a liquid, and “(aq)” to indicate a substance dissolved in water. The only way to get better at writing chemical equations is to actually do it. Work your way through these examples to make sure you really understand the process.  NiCl2 + (NH4)2S → ? Cations: Ni2+ and NH4+  Anions: Cl1- and S2-  Recombine ions to make new products: NiS + NH4Cl Write the equation: NiCl2 + (NH4)2S → NiS + NH4Cl Balance the equation: NiCl2 + (NH4)2S → NiS + 2NH4Cl

How would you rephrase that briefly using Chinese?
确定反应物中所有的正离子和负离子。 交换离子形成生成物。 写出完整的反应式。 标注物质的状态。 通过一些例子来练习。