Exothermic Reactions
An exothermic reaction is a process or reaction that releases energy in the form of heat. One example of an exothermic reaction is making ice cubes. To make ice cubes, you are freezing water so it goes from a liquid to a solid. In order for something to go from a liquid to a solid phase, a substance must lose energy. The energy is released to the surroundings. For example, in the presence of water, a strong acid with dissociate quickly and release heat and because it releases heat, that makes it an exothermic reaction. Without exothermic reactions, we wouldn't be able to make ice cubes, we wouldn't have candle flames, we wouldn't be able to combine atoms to make molecules in a gas phase, we wouldn't be able to burn sugar, iron wouldn't rust, etc...
Picture: http://en.wikipedia.org/wiki/Exothermic_reaction
Sources: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermic-examples.shtml
https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/thermochemistry-6/enthalpy-59/exothermic-and-endothermic-processes-277-3716/
