Acids, Bases, and Salts

Magnesium hydroxide, Mg(OH)2, is a sparingly soluble and mildly basic compound. Its aqueous suspension is commonly called milk of magnesia and is used as an antacid because it reacts with excess hydrochloric acid in the stomach. Vinegar and lemon juice contain acids, while soda water contains dissolved carbon dioxide; therefore option A is correct.

At about 25°C, the pH scale classifies a solution with pH 7 as neutral; values below 7 indicate an acidic solution, while values above 7 indicate a basic solution. A lower pH generally means a greater concentration of hydrogen ions and stronger acidity, though strength also depends on concentration. Thus the correct classification is acidic, option A; basic and neutral describe the other pH ranges, and metallic is not a pH classification.

The pH scale compares acidity and basicity through hydrogen-ion concentration. Under standard school conditions, pH 7 represents a neutral solution: it is neither acidic nor basic in the intended classification. Pure water is the usual example, although temperature and dissolved substances can affect its exact pH. Acidic solutions have pH below 7, and basic solutions have pH above 7, so option A is correct.

On the usual pH scale, a value greater than 7 means that hydroxide ions are relatively more abundant than hydrogen ions, so the solution is basic or alkaline. Basic solutions can turn red litmus paper blue, and soap solution is a familiar example. Acidic solutions have pH below 7 and neutral solutions have pH about 7; being salty does not by itself determine pH. Therefore option A is correct.

Pure water is conventionally treated as neutral because it contains equal concentrations of hydrogen ions and hydroxide ions in equilibrium. At approximately 25°C, this gives a pH close to 7. A pH of 2 indicates a strongly acidic solution, while 10 indicates a basic solution and 14 represents a very strongly basic value on the common scale. Therefore option A, 7, is correct; the word approximate allows for temperature effects.

Rainwater naturally dissolves some carbon dioxide, so ordinary rain is slightly acidic and commonly has a pH near 5.6. It is classified as acid rain when its pH falls below about 5.6, usually because sulfur dioxide and nitrogen oxides form stronger acids in the atmosphere. Such rain can harm aquatic life, soil, plants, and buildings. Hence option A is correct; 7 is neutral, while 8.5 and 10 are alkaline values.

Soap solution is commonly basic because many soaps contain salts of weak acids and strong bases; in water, their hydrolysis produces an alkaline solution. It can turn red litmus blue, which is a characteristic test for a base. Lemon juice and orange juice contain citric acid, while vinegar contains ethanoic acid, so those are acidic rather than basic. Therefore option A is correct.

Vinegar is a dilute aqueous solution containing ethanoic acid, also called acetic acid, usually produced when ethanol is oxidised during bacterial fermentation. This acid gives vinegar its characteristic sour taste and smell. Citric acid is associated with citrus fruits, lactic acid with curd and fermentation, and methanoic acid with ant stings. Therefore the household acid in vinegar is ethanoic acid, option A.

The governing concept is that many fruits and vegetables contain naturally occurring organic acids. Tomato contains oxalic acid, which contributes to its mildly sour taste; therefore option A is correct. Hydrochloric, sulphuric and nitric acids are mineral acids generally associated with laboratory or industrial contexts, not the characteristic natural acid of tomato in this school-level example.

The governing concept is the Arrhenius description of acids. When an acid dissolves in water, it increases the concentration of hydrogen ions, commonly represented as H⁺; hydronium ions, H₃O⁺, are formed in actual aqueous solution. Thus option A gives the expected school-level answer. Hydroxide ions indicate bases, while sodium and chloride ions may occur in particular salts but do not define all acids.

The governing concept is the Arrhenius definition of a base. A soluble base dissociates in water to produce hydroxide ions, OH⁻, which account for its basic behaviour; therefore option A is correct. Hydrogen ions are associated with acids. Nitrate and carbonate ions can be parts of particular salts or bases, but neither one is the ion that defines every base in water.

The governing concept is that hydrochloric acid shows acidic behaviour only after ionisation in water. Dry hydrogen chloride gas cannot provide hydrogen ions to dry litmus because there is no water to form hydronium ions. Therefore option A is correct. The acid is present, the litmus need not be damaged, and hydrogen chloride is not a basic gas; the missing condition is moisture.

The governing safety principle is controlled dilution. Mixing a concentrated acid with water releases considerable heat. Adding acid slowly to a larger volume of water, while stirring carefully, allows the heat to disperse and reduces sudden boiling or splashing; hence option A is correct. Adding water to acid can make a small amount of water heat rapidly and eject corrosive acid. Heating beforehand is unsafe.

The governing concept is heat change during a process. When concentrated acid is diluted with water, hydration and mixing release heat to the surroundings, so the process is exothermic; option A is correct. An endothermic process absorbs heat, which is the opposite description. “Neutral” describes acid-base character rather than heat transfer, and slow evaporation is a physical change unrelated to acid dilution.

The governing concept is the classification of substances by their aqueous ions. Sodium hydroxide, NaOH, dissociates in water into Na⁺ and OH⁻ ions, so it is a base; because it ionises almost completely, it is commonly described as a strong base. Thus option A is correct. It is not an acid, ordinary salt, or indicator, although it may be used in chemical tests.

The governing concept is the relationship between chemical formulae and common names. Calcium hydroxide, Ca(OH)₂, is commonly called slaked lime, so option A is correct. It is produced when quicklime reacts with water and is also associated with limewater and whitewash. Baking soda is sodium hydrogen carbonate, washing soda is sodium carbonate decahydrate, and common salt is sodium chloride.

The governing concept is identifying familiar substances by their chemical and common names. Sodium chloride, NaCl, is widely known as common salt, so option A is correct. It is an ionic salt made of sodium and chloride ions and is commonly used in food. Baking soda is sodium hydrogen carbonate, bleaching powder is a chlorine-containing calcium compound, and slaked lime is calcium hydroxide.

The governing concept is the identification of common salts by their chemical names and formulae. Baking soda is sodium hydrogen carbonate, NaHCO₃, so option A is correct. It releases carbon dioxide when heated or when it reacts with an acid, which helps baked products rise and also explains some antacid use. Sodium chloride, calcium carbonate and calcium hydroxide are different compounds.

Washing soda is the hydrated salt sodium carbonate decahydrate, represented by Na₂CO₃·10H₂O. The term “decahydrate” means that each formula unit of sodium carbonate is associated with ten molecules of crystallisation water. It is used in laundry and for removing the hardness of water. Sodium hydrogen carbonate is baking soda, sodium chloride is common salt, and calcium sulphate hemihydrate is Plaster of Paris; hence option A is correct.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, calcium hydroxide, Ca(OH)₂. In school-level chemistry this preparation is represented approximately as Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. The product releases chlorine or hypochlorous substances in suitable conditions and therefore helps in bleaching and disinfection. Common salt is a raw material for producing chlorine industrially, but it is not the direct substance used here; therefore option A is correct.