Railway Science (Chemistry-Acid ,Bases and Salt) (Part-II)

(Aluminum oxide). Amphoteric oxides can react with both acids and as well as bases to form salt and water. It possesses both acidic and basic properties. Examples - Al₂O₃ + 2NaOH (Base) → 2NaAlO₂ + H2O. Al₂O₃ + 6HCl (acid) → 2AlCl₃ + 3H₂O.
CO₂ (Carbon dioxide) - An acidic oxide that dissolves in water to form carbonic acid (H₂CO₃). SO₂(Sulfur dioxide) - It is also non-metallic oxide and is acidic in nature. CuO (Copper oxide) is a basic oxide.

Phenolphthalein (C₂₀H₁₄O₄) : Used as an indicator in acid-base titrations. It turns purple if it meets basic, such as ammonia, it stays colorless if it meets an acid like vinegar or a neutral substance like water. Acid - Base indicators : Methyl orange (red color in acidic medium and yellow color in basic medium), Litmus (in acidic medium - blue litmus paper turns red, while in a basic or alkaline medium, red litmus paper turns blue).

Litmus solution is made up of purple dye which is extracted from Lichens. It is a natural indicator and it is used to indicate the presence of an acid or a base. A litmus paper will turn red in the presence of an acid and it turns blue in the presence of a base.

The color of the pH paper did not change because pH paper shows color depending on the hydrogen ion concentration in solution. Solid sodium bicarbonate does not produce hydrogen ions in its solid state thus does not give any color on pH paper.

Olfactory indicators are substances whose odour changes in acidic or basic medium. These substances are often used in qualitative analysis to detect the presence of acidic or basic solutions based on changes in smell. Other examples : Onion and clove. Example of other Indicators : Natural indicators (Litmus, turmeric), Synthetic indicators (methyl orange, phenolphthalein).

Aqua regia, (Latin for ‘royal water’) is a freshly prepared mixture of nitric acid and concentrated hydrochloric acid in the ratio of 1:3. Aqua regia is a highly corrosive, fuming liquid. It is one of the few reagents that is able to dissolve gold and platinum.

A neutral solution is a solution which is neither acidic (pH < 7) nor basic (pH > 7). Example - A strong acid and a strong base, such as HCl(aq) and NaOH(aq) will react to form a neutral solution. HCl(aq) + NaOH(aq) ⇌ NaCl(aq) + H₂O(l).

In bases its characteristic smell cannot be detected. Olfactory indicators are substances whose smell changes upon contact with an acid or a base.

Tartaric acid is added to it, to neutralize the bitterness produced by sodium bicarbonate (NaHCO₃). Tartaric acid is found in tamarind, grapes, unripe mangoes etc. Other Acids (found in) : Formic acid (Ant’s sting), Oxalic acid (Spinach), Lactic acid (curd).

Carbonic acid (H₂CO₃): Formed when water is mixed with carbon dioxide. Reaction is given as CO₂ + H₂O → H₂CO₃ . Oxalic Acid (C₂H₂O4) - Spinach, Tomato. Tartaric Acid (C₄H₄O₆) - Tamarind, Grapes and Unripe Mangoes. Citric Acid (C₆H₈O₇) - Oranges, lemons.