Acids and bases are compounds that, when added to an aqueous solution, change the amounts of H3O+ and OH- ions in solution. Acids increase the amount of H3O+ in solution by donating protons (H+) to water (H2O), forming H3O+. Bases increase the amount of OH- by removing protons from water, forming OH-. Acidic solutions have more H3O+ than OH- in them, while the opposite is the case for basic solutions.
Acids in water solutions show the following common properties: they taste sour; turn litmus paper red, and react with certain metals, such as zinc, to yield hydrogen gas. Bases in water solutions show these common properties: they taste bitter, turn litmus paper blue, and feels slippery. When a water solution of acid is mixed with a water solution of base, water and salt are formed, this process, called neutralization, is complete only if the resulting solution has neither acidic nor basic properties.
What is important to your understanding of acids and bases is this one fact: No matter how much acid you put into a solution, there are still some OH- ions floating around due to auto ionization of water that constantly takes place, and no matter how basic you make a solution, there is still some H3O+ present in that solution. A solution containing H3O+ can be basic, because there are more OH- ions, and a solution containing OH- ions can be acidic, there just needs to be more H3O+ present.
Acid-base chemistry is an important part of everyday life. The excess hydronium ions in acids give them interesting properties. Acids can react with metals and other materials. The strong acid HCl is produced in your stomach to help digest food. In dilute concentrations, acids are responsible for the sour taste of lemons, limes, vinegar and other substances. Bases are also very reactive. The strong base NaOH is used in many household-cleaning agents such as oven cleaner and drain clog-remover.