Question

1. what is an acid?
2. why do we say vinegar is acidic?

Explanation:

Sub - question 1: What is an Acid?

An acid is a substance that, in aqueous solution, donates a proton ($H^+$ ion) (according to the Brønsted - Lowry theory) or accepts an electron pair (Lewis theory). In the Arrhenius theory, an acid is a substance that dissociates in water to produce hydrogen ions ($H^+$). For example, hydrochloric acid ($HCl$) dissociates in water as $HCl
ightarrow H^++Cl^-$, and sulfuric acid ($H_2SO_4$) dissociates to give $H^+$ ions. Acids generally have a sour taste (like citric acid in lemons), turn blue litmus paper red, and can react with bases to form salts and water (neutralization reaction).

Vinegar contains acetic acid ($CH_3COOH$). Acetic acid, when dissolved in water (as vinegar is an aqueous solution of acetic acid), dissociates partially to produce hydrogen ions ($H^+$) and acetate ions ($CH_3COO^-$) according to the equation $CH_3COOH
ightleftharpoons H^++CH_3COO^-$. The presence of $H^+$ ions in the vinegar solution gives it acidic properties. For example, vinegar can turn blue litmus paper red, and it shows typical acid - like behavior such as reacting with bases (like when vinegar reacts with baking soda, a base, to produce carbon dioxide, water, and a salt).

Answer:

An acid is a substance that (by Arrhenius: dissociates in water to give $H^+$ ions; by Brønsted - Lowry: donates $H^+$ ions in solution; by Lewis: accepts an electron pair). It has properties like sour taste, turning blue litmus red, and reacting with bases.

Sub - question 2: Why do we say vinegar is acidic?