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In chemistry, acids and bases are two groups of substances that usually exhibit certain properties when they are in the aqueous state. Historically, acids are substances that taste sour, can dissolve many metals, and turn blue litmus paper to red. On the other hand, bases taste bitter, feel slippery, and turn red litmus paper to blue.

One of the physical properties that historically led chemists to classify substances as acids is their sour taste, while bases taste bitter. However, please note that tasting chemicals in a laboratory is not a safe practice and should never be done. Nowadays, this method is not used anymore in chemistry labs.

Another property that led chemists to classify substances as acids or bases was their effect on litmus paper. Litmus paper is a pH-sensitive paper that changes color due to its pH. Acids turn blue litmus paper red, while bases turn red litmus paper blue. This property can be attributed to the different pH levels of acidic and basic solutions, with acids typically having a pH value less than 7 and bases having a pH value greater than 7.

Acids generally react with metals, producing hydrogen gas and a salt. This property made it possible for chemists to identify a substance as an acid if it reacted with metals in this way. For example, when hydrochloric acid (HCl) reacts with zinc (Zn), it produces hydrogen gas and zinc chloride (ZnCl2). \[ 2HCl(aq) + Zn(s) \rightarrow H_{2}(g) + ZnCl_{2}(aq) \] Bases, on the other hand, do not exhibit this reactivity with metals.

Bases, particularly in their aqueous form, feel slippery or soapy to the touch. This property is due to the reaction between the base and the natural oils on your skin, forming soap-like substances. However, touching chemicals directly should not be done in modern laboratory practices due to the potential risk of harmful chemical reactions with the skin. In summary, the historical classification of substances as acids and bases was based on their distinct physical properties, such as taste, effect on litmus paper, reactivity with metals, and feel. Note that these properties are not always exclusive to acids or bases, and modern chemistry relies on more advanced and accurate methods to classify and study substances.