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Acids, Bases and pH
Chemistry has several definitions of an acid and base, but one of the most easily understood is the Bronsted-Lowry definition, which says that an acid can donate a positive hydrogen ion (H+), while a base can accept H+. Strong acids, such as HCl, are therefore chemicals that release a large amount of H+ when dissolved in water. Strong bases generally release hydroxide ion (OH-), which can combine with H+. The pH of a solution is a measure of acidity and is related to the H+ concentration; a high level of H+ means an acidic solution and a low pH value.
Neutralization
Pure water has only a very small amount of free H+ and a pH of 7, which is considered neutral. When a strong acid such as HCl is added, the acid greatly increases the H+ concentration of the solution and the resulting mixture is acidic. If an equivalent amount of a strong base such as NaOH is then mixed into this acidic solution, the base will counteract that of the acid and the pH will return to neutral. Similarly, adding a strong acid to a basic solution will neutralize it.
Chemical Products of Neutralization
When a strong acid and base combine in a neutralization reaction, the H+ of the acid combines with the OH- of the base and forms H2O, or water. This is an essential component of the neutralization, since it ties up both the H+ and OH- to produce neutral water. The remaining portions of the acid and base molecules also combine to form a compound known generically as a salt. In the case of HCl and NaOH, the sodium ion (Na+) and chloride ion (Cl-) combine to form common table salt, NaCl. Since the reaction produces water, and salts are highly soluble, what is formed is actually a solution of dissolved salt.
Heat Production
The other product that typically results from a neutralization reaction is heat. Most reactions between strong acids and bases are "exothermic," which means that they produce heat as they occur. This means that a mixture of a strong acid and a strong base will heat up as they react. Therefore, the final product of a neutralization reaction will be a warm solution of a salt in water. In terms of a chemical reaction, you can write out the example of hydrochloric acid and sodium hydroxide as HCl + NaOH --> H2O + NaCl + heat.