Acids, Bases and Salts — Important Questions

H⁺ (H₃O⁺) ion

Step 1: All acids, when dissolved in water, produce hydrogen ions (H⁺). Step 2: Since H⁺ ions cannot exist alone, they combine with water molecules to form hydronium ions: H⁺ + H₂O → H₃O⁺. Step 3: It is this H⁺(aq) or H₃O⁺ ion that is responsible for all the characteristic properties of acids — sour taste, turning blue litmus red, reacting with metals, etc. Step 4: OH⁻ ions are produced by bases, not acids. Step 5: Cl⁻ is the anion in HCl specifically; not all acids have chloride ions.

Pink to colourless

Step 1: NaOH is a base. When phenolphthalein is added to a base, it turns pink. Step 2: As HCl (acid) is added drop by drop to the NaOH solution, the acid starts neutralising the base. Step 3: The neutralisation reaction is: NaOH + HCl → NaCl + H₂O. Step 4: When the acid exactly neutralises the base, the solution becomes neutral (pH = 7). Phenolphthalein is colourless in neutral and acidic solutions. Step 5: So the colour changes from pink (basic) to colourless (neutral), indicating the end point of neutralisation.

Sodium hydrogencarbonate

Step 1: Baking soda is a common kitchen ingredient used to make food soft and fluffy. Step 2: Its chemical name is sodium hydrogencarbonate and its chemical formula is NaHCO₃. Step 3: When heated, it decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ released makes the food rise and become spongy. Step 4: Sodium carbonate (Na₂CO₃) is washing soda, a different compound. Sodium hydroxide (NaOH) is caustic soda. Step 5: Sodium chloride (NaCl) is common salt — used for flavour, not for making food fluffy.

Chlor-alkali process

Step 1: When electricity is passed through brine (aqueous NaCl), it decomposes — this is called electrolysis. Step 2: The reaction is: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Step 3: The name 'chlor-alkali' comes from the two products — 'chlor' for chlorine and 'alkali' for sodium hydroxide (NaOH). Step 4: The Haber process makes ammonia, the Contact process makes sulphuric acid, and the Solvay process makes sodium carbonate. Step 5: All three products of the chlor-alkali process (NaOH, Cl₂, H₂) are industrially very important.