How do you predict the product of a chemical reaction?

Answer 1

Take a look at the reactants and what type of reaction is going on. In a decomposition reaction a single substance will break down into two or more. Example: calcium carbonate heated breaks down into calcium oxide and carbon dioxide. In a synthesis reaction, two or more substances combine to form a single product. Example: sodium and iodine create sodium iodide when combined. In single replacement reactions an element replaces one part of another compound. Example: lithium is added to hydrochloric acid and the result is hydrogen gas and lithium chloride (note that the element has to be more reactive than the thing it is replacing). In a double replacement reaction the cations and anions switch places in two compounds. Example: potassium bromide plus silver nitrate yields silver bromide and potassium nitrate.

Answer 2

Sometimes you can, sometimes you can't. Some of the typical ones are
Acid-Base: The reaction yields a Salt (anion from base, cation from acid) and Water (in aqueous solution)

Solubility: A soluble salt A+ B- is reacted with another salt X+Y-. However, A+Y- is insoluble and will ppt. out of solution.

Redox: There are several types, but each involves changes of oxidation number, such as

Single displacement: A more electropositive metal "A" displaces a less electropositive metal in solution "B+". B+ is reduced to the elemental state and A becomes ionized in its place. (formation of hydrogen from water with metals like sodium is an example)

Oxidizing agents: A reduced form of an element increases its oxidation number while an oxidizing agent's oxidation number is reduced. An example would be oxidizing Pb+2 to Pb+4 with permaganate (MnO4-), which is reduced to MnO2. These reactions require either base or acid, since water is formed as a by-product.

Combustion: A hydrocarbon compound is burned in oxygen, and CO2 and H2O are formed.