What is acid? Give an example of two locally available acidic substances.

An acid is a substance that donates hydrogen ions (H⁺) when dissolved in water, producing a solution with a pH below 7.

Properties of acids:

• Sour taste (safe, dilute acids only)
• pH less than 7 (neutral water = pH 7)
• Turns blue litmus paper red
• Reacts with metals to produce hydrogen gas
• Reacts with bases to form salt and water (neutralization)
• Conducts electricity in solution

Strong vs. Weak acids:

• Strong acids (fully ionize): HCl (hydrochloric), H₂SO₄ (sulfuric), HNO₃ (nitric)
• Weak acids (partially ionize): Citric acid, Acetic acid, Carbonic acid

Two locally available acidic substances:

1. Lemon juice: contains citric acid (pH ~2–3); the sharp, sour taste is citric acid at work
2. Vinegar: contains acetic acid (~5% concentration, pH ~2.5–3); found in every kitchen; used in cooking, pickling, and cleaning

Both are weak acids safe in food quantities but capable of mild irritation in concentrated form.