Lesson 2 Acid，Base，Salt and pH

Substances that form ions when they are dissolved in water are called electrolytes，which fall into three types: acids，bases，and salts.

The concept of acids and bases has been important in chemistry for a long time. Many acid-base theories have been advanced over these years. While most of these are valid within narrow limits of applicability，they are not based upon a clear understanding of principles of chemical valence. Not until the Lewis theory had been established could a satisfactory general theory of acid-base behavior be formulated.^① It is not surprising，therefore，the most useful acid-base theory is called Lewis theory. Brönsted-Lowry theory was developed at about the same time as the Lewis theory. It is applicable to water and water-like solvents，and is widely used. In addition，the theories in the conventional sense are restricted to the aqueous solution，but they can be also used in some extent up to now，especially，Arrhenius theory. Therefore，in the chapter，these three acid-base theories including Arrhenius theory，Brönsted-Lowry theory and Lewis theory will be discussed.