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

A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. Generally paper impregnated with the universal indicator is used for measuring pH.

Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta. When the litmus solution is neither acidic nor basic, its colour is purple. There are many other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution.

Two types of litmus paper - Red litmus and Blue litmus. Acidic solutions turn blue litmus red and Basic or alkaline conditions turn Red litmus paper blue. As the pHs, less than 7 are acidic while pHs greater than 7 are alkaline.

Formic acid or Methanoic acid (HCOOH, also found in bee stings). Uses: As a preservative and antibacterial agent. Malic acid (C₄H₆O₅) is a dicarboxylic acid. Found in Apple, grapes, watermelons, cherries, and vegetables such as carrots and broccoli. Used as a food additive.
Tannic acid (C₇₆H₅₂O₄₆) is found in the nutgalls formed by insects on the twigs of certain oak trees. It has numerous phenol groups and hence is a weak acid. Uses: In the dyeing process for cellulose fibres, to stop bleeding.
Oxalic acid (C₂H₂O₄) is a dicarboxylic acid also known as Ethanedioic acid. Found in Tomato, Spinach, cocoa, nuts, and seeds. Uses - Cleaning Agent, Industrial uses and Medicinal uses.

When Sodium (Na) reacts with hydrochloric acid (HCl) then sodium chloride (NaCl) and hydrogen gas (H₂) forms. Example (Double displacement reaction) - 2 Na (s) + 2 HCl (aq) → 2 NaCl (s) + H₂ (g). Zn (Zinc) + 2HCl (Hydrochloric acid) → ZnCl₂ (Zinc Chloride) + H₂ (Hydrogen). Non-metals do not react with acids because non-metals are themselves acceptors of electrons.

Properties of Acids: Acids have sour taste, have Ability to conduct electricity, pH less than 7, corrosive in nature, mostly present in liquid or gaseous forms, Most acids can be diluted with water to reduce the intensity of their acidity, react with active metals to yield hydrogen gas.

pH tells the concentration of hydrogen ions - lower the pH, higher is the hydrogen ion concentration. Acids have pH ranging from 1-6, pH 7 denotes neutral solution, and pH in the range of 8-14 denotes bases.

Na₂CO₃ (Sodium carbonate) + 2HCl (Hydrochloric acid) → Sodium chloride (2NaCl) + Water (H₂O) + Carbon dioxide (CO₂). Chlorine (Cl₂) - Heavy green-yellow gas with a strong odor. Hydrogen Chloride (HCl) - Colorless to slightly yellow gas with a pungent odor; It can irritate the skin, nose, eyes, throat and larynx.

Nature of dry HCl gas is not acidic due to lack of water for dissociation of HCl in H⁺ and Cl⁻ . Hence, Blue or red litmus paper does not change its colour. Litmus paper is a filter paper that has been treated with a lichen-provided natural-soluble dye.
It produces a result which can be used as a pH indicator. Blue litmus paper turning red under acidic conditions and red litmus paper turning blue under basic conditions. The material is neutral if neither the red nor the blue paper changes colour.

Aqueous solution of copper sulfate (CuSO₄) turns blue litmus red as it is acidic in nature. Litmus paper is made by an extract of a lichen plant. When CuSO₄ is dissolved in water, it forms Cu²⁺ and So₄²⁻. In this case H⁺ ions are generated, that is the reason why the solution is slightly acidic.