Acids, Bases, and Salts

Acidity increases as pH decreases because lower pH corresponds to a greater hydrogen-ion concentration. Since 2 is lower than 6, the solution with pH 2 is more acidic, so option A is correct. On the logarithmic pH scale, the difference of four units represents a 10⁴, or 10,000-fold, difference in hydrogen-ion concentration, assuming comparable conditions.

The governing concept is the pH scale: a higher pH indicates a greater basic character, while a lower pH indicates greater acidity. Comparing the two values, 13 is higher than 9, so the pH 13 solution has lower hydrogen-ion concentration and is more basic. Therefore, option B is correct. Option A is less basic, option C incorrectly treats the values as equal, and option D is not justified.

The governing concept is the ionisation of acids in aqueous solution. Acids produce hydrogen ions in water; more precisely, these hydrogen ions associate with water molecules to form hydronium ions, H₃O⁺. This is responsible for acidic behaviour and low pH. Therefore, option A is correct. Hydroxide ions are characteristic of bases, whereas sodium and carbonate ions occur only when particular salts or compounds dissolve.

The governing concept is the behaviour of bases in aqueous solution. A soluble base dissociates or ionises to produce hydroxide ions, OH⁻, in water; these ions are responsible for basic properties and a pH above 7. Therefore, option B is correct. Hydrogen ions are associated with acids, while chloride and nitrate ions are negative ions supplied by particular salts or acids, not by every base.

The governing concept is that hydrogen chloride shows acidic behaviour only after dissolving in water. In moist conditions, HCl ionises in water and produces hydronium ions, which change blue litmus to red. Dry HCl gas and dry litmus contain no water medium, so the necessary ions are not formed and no colour change occurs. Thus option A is correct; the gas is not basic, litmus is not a metal, and colourlessness is irrelevant.

The governing safety principle is controlled dilution of acid. Mixing concentrated acid with water is exothermic, so considerable heat may be released. The safe procedure is to add acid slowly to a larger quantity of water while stirring, allowing heat to disperse. Therefore, option A is correct. Adding water rapidly to concentrated acid can make the small amount of water boil suddenly and eject corrosive acid; hand mixing and hot oil are unsafe.

When an acid is mixed with water (diluted), heat is released to the surroundings; therefore, it is an exothermic process. An endothermic process absorbs heat from the surroundings, so option B is incorrect. Exam tip: always add acid slowly to water, not water to acid.

Gastric glands in the stomach secrete hydrochloric acid. It provides the acidic medium needed for digestive enzymes to work and also helps kill harmful microbes. Citric acid occurs mainly in fruits such as lemons; it is not the acid secreted by the stomach. Exam tip: Remember the stomach acid as HCl.

The governing concept is neutralisation. Excess hydrochloric acid in the stomach can cause symptoms such as burning or indigestion. An antacid is a mild base, or a substance that reacts with and neutralises excess acid without being strongly corrosive. Therefore, option A is correct. A strong acid would increase acidity, while a metal or a precipitate is not a general name for an acidity-relieving medicine.

The governing concept is the basic nature of magnesium hydroxide, Mg(OH)₂. Milk of magnesia is a suspension of this sparingly soluble base in water. It can react with and neutralise excess hydrochloric acid in the stomach, which is why it is used as an antacid. Therefore, option A is correct. It is not a strong acid, a gas, or simply a metallic substance; those descriptions do not represent its chemical nature.

The governing concept is the effect of acidity on tooth enamel. When the pH in the mouth falls below about 5.5, the medium becomes sufficiently acidic to promote demineralisation of enamel, especially if acids remain in contact with the teeth. This increases the risk of tooth decay, so option A is correct. pH 7 is neutral, while pH 10 and pH 14 are alkaline and do not represent the critical acidic threshold.

The governing concept is neutralisation of acids by a base. Bacteria can produce acids from food residues in the mouth, and these acids may attack tooth enamel. Mildly basic toothpaste helps neutralise or reduce the effect of those acids during brushing, supporting oral hygiene. Therefore, option A is correct. It does not make teeth acidic, convert saliva into gas, or turn teeth into salt; those choices describe no relevant chemical purpose.

The governing concept is the indicator test for bases. Soap solution is generally alkaline because soap undergoes hydrolysis in water and produces an alkaline medium. Bases change red litmus paper to blue, so option B is correct. Red would indicate no relevant change, green is associated with some universal-indicator readings rather than ordinary litmus paper, and colourless is not the expected litmus response. The result assumes a normal aqueous soap solution.

The governing concept is neutralisation: an antacid should contain a mild base that reacts safely with excess hydrochloric acid in the stomach. Magnesium hydroxide is suitable because Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Sodium hydroxide is a strong, corrosive alkali, while acetic acid and hydrochloric acid would add acidity rather than reduce it. Therefore, option A is correct.

An olfactory indicator is identified through a change in smell rather than a change in colour. Onion is an olfactory indicator because its characteristic smell is affected by the nature of the medium and may be detected differently in acidic and basic conditions. Turmeric, litmus and phenolphthalein are colour indicators; they show changes through colour. Thus, onion, option B, is the correct answer.

The chemical name of bleaching powder is calcium oxychloride, commonly represented as \(\mathrm{CaOCl_2}\). It is used for bleaching textiles and paper pulp and for disinfecting water. Sodium carbonate is washing soda, not bleaching powder. Exam tip: Remember bleaching powder together with calcium oxychloride and \(\mathrm{CaOCl_2}\).

The pH scale shows that a pH below 7 represents an acidic substance; therefore, soil with pH 5 is acidic. Quicklime, CaO, is a basic oxide. In moist soil it forms calcium hydroxide, which neutralises excess acidity and raises the pH toward a more suitable range. HCl would increase acidity, while ammonium chloride is an acidic salt and sodium chloride is not the appropriate corrective material. Option A is correct.

The governing idea is the litmus test for acidic, basic and neutral solutions. An aqueous solution of sodium chloride, formed from a strong acid and a strong base, is generally neutral and therefore does not change either red or blue litmus. Hydrochloric acid turns blue litmus red, whereas sodium hydroxide and calcium hydroxide turn red litmus blue. Hence, option C is correct.

Plaster of Paris, CaSO₄·½H₂O, reacts with water to form gypsum, CaSO₄·2H₂O. Interlocking gypsum crystals make the material set and become hard. It does not dissolve completely or produce a gas. Exam tip: Remember that Plaster of Paris hardens on adding water, so it is used for casts and moulds.

Water of crystallisation is the definite number of water molecules chemically associated with a salt within its crystal structure. It is not simply moisture or any natural source of water. For example, washing soda is Na₂CO₃·10H₂O, where ten water molecules are associated with each formula unit. River water, rainwater and seawater are external mixtures, not crystal water. Therefore, option A is correct.