Acids, Bases, and Salts

The general reaction of a suitable metal with a dilute acid is: metal + acid → salt + hydrogen gas. For example, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen. Thus option A is correct. Oxygen and nitrogen are not normally released in this reaction, while carbon dioxide is characteristic of acid–carbonate reactions rather than ordinary acid–metal reactions.

Hydrogen is tested by bringing a lighted splint near the gas, using proper laboratory safety. Hydrogen burns rapidly with oxygen and produces a characteristic ‘pop’ sound, so option A is correct. A bell or whistle is unrelated to this chemical test, and saying that no sound is produced would miss the identifying observation. The test follows hydrogen’s combustion property.

Zinc reacts with dilute hydrochloric acid according to the balanced equation: Zn + 2HCl → ZnCl₂ + H₂. The gaseous product is therefore hydrogen, making option A correct; zinc chloride remains in solution. Oxygen is not released, chlorine is not produced because hydrochloric acid supplies chloride ions, and ammonia is unrelated to this acid–metal reaction.

Acids react with metal carbonates to form a salt, water, and carbon dioxide: acid + carbonate → salt + H₂O + CO₂. For example, hydrochloric acid and calcium carbonate produce calcium chloride, water, and carbon dioxide. Therefore option A is correct. Hydrogen is associated with many acid–metal reactions, whereas oxygen and nitrogen are not products here.

Lime water is a dilute solution of calcium hydroxide. When carbon dioxide is passed through it, the reaction CO₂ + Ca(OH)₂ → CaCO₃ + H₂O forms insoluble calcium carbonate, which makes the solution appear milky. Hence option A is correct. Blue, red, and black are not the characteristic result of the standard carbon-dioxide test.

The reaction between an acid and a base is called neutralisation because the acidic and basic effects oppose and reduce each other. In a typical case, HCl + NaOH → NaCl + H₂O, producing salt and water. Therefore option A is correct. Combustion involves burning, evaporation is a physical change of state, and corrosion is the gradual deterioration of materials.

Neutralisation occurs when an acid reacts with a base. The hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt; for example, HCl + NaOH → NaCl + H₂O. Thus option A is correct. Neutralisation generally gives both salt and water, not only water or the other listed combinations.

The stomach releases hydrochloric acid, or HCl, as part of gastric juice. It creates a strongly acidic medium that helps activate pepsin for protein digestion and also helps destroy many harmful microorganisms entering with food. Citric acid is found mainly in citrus fruits, lactic acid is associated with muscles and fermented foods, and tartaric acid occurs in tamarind and grapes; therefore option A is correct.

Acidity commonly results when excess hydrochloric acid is present in the stomach. An antacid is a mildly basic substance, such as a suitable hydroxide or carbonate preparation, that reacts with and neutralizes some of this excess acid. This reduces irritation and provides relief. Vinegar and lemon juice are acidic, while salt water is not a standard treatment; therefore option A is correct.

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.