Types of reactions
Combustion
Combustion is an exothermic chemical reaction that requires oxygen and a fuel substance as reactants and an ignition source as the trigger. A word equation for the reaction, would look like this:
fuel + oxygen 
combustion product + heat
Fuel includes solid, liquid and gas substances, with gases such as methane, hydrogen and propane, the most combustible as they easily form mixtures with the oxygen in the air. Some examples of solid fuels are wood, paper and coal, while liquid fuels include petrol, alcohol and kerosene. See image 1.
The word 'ignition' refers to the act of starting a combustion reaction. Ignition occurs when the energy from one reaction sustains another, adding external energy to the reactants to start the reaction. Earlier, friction energy was mentioned in relation to matches. In this case, the fuel is the potassium chlorate on the match head, the phosphorous on the striking surface and the wood of the match, the oxygen comes from the air surrounding the match and the ignition source is the friction created when the match head is struck against the striking surface. Other ignition sources include electrical sparks from electrical equipment or lightning.
To stop or prevent a combustion reaction, you need to remove the fuel, the oxygen or the ignition source.
Corrosion
While combustion is a fast reaction, other reactions often occur over a long period. Corrosion is the decay of a metal that occurs when a metallic substance oxidises after exposure to gases or liquids. In simple terms, oxidation occurs when a compound gains oxygen atoms.
Rust is a form of corrosion where iron interacts with moist air to form one type of iron oxide (Fe2O3). This product is flaky and allows more moist air to react with the deeper layers of iron, turning the iron item into rust. See image 2.
Some types of corrosion occur at a faster rate, such as the corrosion of sodium or potassium, which can be explosive. Aluminium oxidises but instead of decaying, aluminium oxide (Al2O3) forms a protective layer, which prevents degradation of the metal.
Precipitation
Aqueous products result when reactants combine to create a substance that comes dissolved in water. In this case, the substance and the water are separate compounds, and it would be possible extract the substance if desired. However, in a precipitation reaction, the product is insoluble (unable to dissolve in water) and thus forms a solid. This solid product, called the precipitate, results from reactions between two liquids or gases.
For example, a silver nitrate solution reacts with a sodium chloride solution to form solid silver chloride and sodium nitrate solution, represented by the chemical equation:
AgNO3 (aq) + NaCl(aq) 
AgCl(s) + NaNO3 (aq)
See animation 1.
Precipitation reactions reveal a lot about the behaviour of the reactants as they form products. When these two reactants combine, it can be assumed that the cations attract any of the anions in the reaction and repel other cations, e.g. silver, a cation, attracts the anions in the reaction - the nitrate and the chlorine - and repels the other cation, the sodium. When the silver meets the chlorine, it forms an insoluble solid that 'removes' itself from further reaction. The soluble sodium nitrate, on the other hand, remains dissolved in the solution, which may contain the soluble reactants that have not formed precipitate yet. Precipitation stops only when all the reactants have 'changed partners'.
Combination and decomposition
A combination reaction involves two or more reactants forming a single product. An example of this is sodium chloride where the separate reactants, sodium and chlorine, react to make only one product - sodium chloride (salt).
Conversely, decomposition is the opposite reaction, where the reactant is a single compound that breaks down into two or more smaller products. Often, the compound will need the input of energy (usually heat) for the reaction to take place. For example, when heated, calcium carbonate will break down to form calcium oxide and carbon dioxide. Decomposition is a chemical reaction because new substances result from the reaction.
Saponification
Surfactants are substances that disturb the surface of a liquid to allow for easier spreading of the liquid. Soap is a surfactant with a hydrophilic (water-loving) end and a hydrophobic (water-fearing) end, which breaks the surface tension of water to create bubbles.
In simple terms, saponification is the process by which fat or oil and an alkaline solution forms soap and glycerol. Recall that alkaline solutions have an excess of hydroxide (hydrogen and oxygen) ions (refer chapter 'Acids and Bases'). Glycerol is a type of sugar alcohol.
A typical saponification reaction involves fat or oil combined and heated with sodium or potassium hydroxide to produce soap and glycerol. The fat or oil is naturally derived from an animal or vegetable. To manufacture the soap used today, crude soap goes through a purification process called finishing, where manufacturers sometimes blend in additives such as perfumes and dye.
