Acid, Base and Salts RRB PYQ

When litmus is added to a solution of borax, it turns blue. Borax is a basic substance, and it has an alkaline pH. In alkaline solutions, blue litmus paper remains blue, while red litmus paper turns blue, indicating the basic nature of the solution.

NaOH is the odd one out because sodium hydroxide is a strong base and dissociates almost completely in aqueous solution. In comparison, Cu(OH)2 and Zn(OH)2 are sparingly soluble hydroxides, while NH4OH (aqueous ammonia) behaves as a weak base. Thus, among the given options, NaOH is distinguished by its strongly basic character.

Litmus is a natural indicator (from lichens) and is widely used for quick acid-base identification. Litmus shows its main colour-change range roughly around pH ~4.5 (red) to ~8.2 (blue). An acid turns blue litmus red, and a base turns red litmus blue.

When Plaster of Paris (CaSO4·0.5H2O) is mixed with water, it undergoes an exothermic reaction, meaning it releases heat. The chemical formula for the reaction is: 2CaSO4·0.5H2O + 3H2O → 2CaSO4·2H2O. Mixing Plaster of Paris with water gives off heat because the reaction is exothermic. The water changes Plaster of Paris back into gypsum (CaSO4·2H2O), its original form before heating. As gypsum forms, its crystals lock together, creating a hard, solid structure. This quick-setting property makes gypsum useful in construction, sculpture, and medical casts.

When sulphuric acid is poured over zinc, hydrogen gas is produced. The chemical reaction involved is: Zn + H2SO4 → ZnSO4 + H2. This is an example of a single displacement reaction where zinc displaces hydrogen from sulphuric acid.

All acids produce Hydrogen gas when reacting with metals. This reaction typically involves acids like hydrochloric acid or sulfuric acid reacting with metals such as zinc or magnesium, resulting in the release of hydrogen gas and the formation of a metal salt.

The solution with a pH value of 0 is the most acidic among the four options. The pH scale is a way to measure how acidic or basic a solution is. It ranges from 0 to 14, with 0-6: acidic solutions (lower the pH, stronger the acid); 7: neutral solution; 8-14: basic solutions (higher the pH, stronger the base). A pH of 0 indicates a highly acidic solution.

Mixing an acid or a base with water is an exothermic process because heat is released during dilution. Acids ionise in water to produce H3O+ ions, while bases produce OH- ions in aqueous solution. Dilution decreases the concentration of these ions per unit volume. Acids generally have a sour taste and pH below 7, whereas bases are bitter, soapy to touch, and have pH above 7. Therefore, bitterness and pH below 7 are not common properties of both. Use as preservatives is also not a general similarity between acids and bases.

Chlorine gas reacts with dry slaked lime (calcium hydroxide) to produce bleaching powder (calcium hypochlorite). The chemical equation for the reaction is: 2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O. Bleaching powder is used extensively for disinfecting drinking water and as a bleaching agent for cotton and linen in the textile industry. It is also used in the paper industry and for manufacturing various chemicals.

Baking soda, which is sodium bicarbonate, does not have any water of crystallisation. In contrast, washing soda, known as sodium carbonate decahydrate, contains ten molecules of water of crystallisation. Blue vitriol, or copper sulfate pentahydrate, contains five molecules of water of crystallisation. Gypsum, which is calcium sulfate dihydrate, has two molecules of water of crystallisation.