Key Points

A chemical reaction is a process where substances undergo a chemical change. Key indicators include a change in state, change in color, evolution of a gas, or a change in temperature.

A chemical equation is a symbolic representation of a chemical reaction. Reactants are on the left-hand side (LHS) and products are on the right-hand side (RHS), separated by an arrow.

A chemical equation must be balanced to obey the Law of Conservation of Mass, which states that mass is neither created nor destroyed. This means the number of atoms of each element must be equal on both sides of the equation.

To make equations more informative, physical states are shown using symbols. These are (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution.

In a combination reaction, two or more substances (elements or compounds) combine to form a single product. For example, the formation of slaked lime: $CaO(s) + H_2O(l) \\rightarrow Ca(OH)_2(aq)$.

A decomposition reaction is when a single compound breaks down into two or more simpler substances. It is the opposite of a combination reaction. Example: heating limestone, $CaCO_3(s) \\rightarrow CaO(s) + CO_2(g)$.

Decomposition reactions require energy. Thermal decomposition uses heat, electrolytic decomposition uses electricity ($2H_2O(l) \\rightarrow 2H_2(g) + O_2(g)$), and photolytic decomposition uses light ($2AgCl(s) \\rightarrow 2Ag(s) + Cl_2(g)$).

In a displacement reaction, a more reactive element displaces a less reactive element from its compound. For example, iron displaces copper: $Fe(s) + CuSO_4(aq) \\rightarrow FeSO_4(aq) + Cu(s)$.

A double displacement reaction involves an exchange of ions between two compounds. Example: $Na_2SO_4(aq) + BaCl_2(aq) \\rightarrow BaSO_4(s) + 2NaCl(aq)$.

Any reaction that produces an insoluble solid, called a precipitate, is a precipitation reaction. The formation of barium sulphate ($BaSO_4$) in a double displacement reaction is an example.

Exothermic reactions release heat into the surroundings, like respiration ($C_6H_{12}O_6 + 6O_2 \\rightarrow 6CO_2 + 6H_2O$). Endothermic reactions absorb heat from the surroundings, like most decomposition reactions.

Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. Reactions where both occur simultaneously are called redox reactions.

When copper oxide is heated with hydrogen, copper oxide is reduced to copper and hydrogen is oxidized to water. The equation is $CuO + H_2 \\rightarrow Cu + H_2O$.

Corrosion is the process where metals are slowly eaten away by the action of air, moisture, or chemicals. The rusting of iron (formation of a reddish-brown coating) is a common example of corrosion.

Rancidity is the oxidation of fats and oils in food items, resulting in an unpleasant smell and taste. It can be prevented by using antioxidants or flushing food packages with nitrogen gas.