Classify a chemical reaction as synthesis, decomposition, single replacement, double replacement, or combustion, and predict the products of simple reactions in each category.
Chemical reactions fall into recognizable patterns. In a synthesis (combination) reaction, two or more simple substances combine into one more complex product: A + B -> AB, as in 2Na + Cl2 -> 2NaCl. In a decomposition reaction, one compound breaks down into two or more simpler products -- the reverse pattern: AB -> A + B, as in the heat-driven breakdown 2H2O -> 2H2 + O2.
In a single replacement reaction, one element takes the place of another element within a compound: A + BC -> AC + B, but only if A is MORE reactive than B. An activity series (a ranked list of metals by reactivity) predicts whether this will actually happen -- zinc is more reactive than hydrogen, so Zn + 2HCl -> ZnCl2 + H2 occurs, but copper is less reactive than hydrogen, so copper metal will NOT react with HCl in the same way.
In a double replacement reaction, the positive ions of two ionic compounds swap partners: AB + CD -> AD + CB. This only actually happens when it produces something that leaves the solution -- a solid precipitate, a gas, or water -- otherwise the ions would simply stay dissolved and mixed with no real reaction occurring. Finally, a combustion reaction is a hydrocarbon (or similar fuel) reacting with oxygen gas to produce carbon dioxide and water, releasing energy: CH4 + 2O2 -> CO2 + 2H2O.
Mastery looks like: They correctly classify a given reaction into one of the five major types and predict reasonable products for a simple synthesis, decomposition, single replacement, double replacement, or combustion reaction.
Common stumbles: Assuming every double replacement written on paper actually occurs, without checking for a precipitate, gas, or water; mixing up which element must be more reactive in a single replacement reaction.