Acids, Bases, and Salts

The governing concept is the exothermic nature of dilution. Mixing concentrated acid with water releases considerable heat. If water is poured quickly into a small quantity of acid, the water may heat suddenly, boil and throw corrosive acid droplets out of the container. Therefore the safe procedure is to add acid slowly to a larger volume of water while stirring and using protection, so option A is correct. The other choices are unsafe and violate laboratory practice.

Mixing an acid with water, especially when diluting a concentrated acid, releases heat because hydration of the ions is exothermic. A process that gives out heat to its surroundings is called an exothermic process, so option A is correct. Endothermic processes absorb heat, photolytic processes involve decomposition by light, and precipitation produces an insoluble solid; none of these describes ordinary acid dilution.

Sodium chloride is produced from the strong acid hydrochloric acid, HCl, and the strong base sodium hydroxide, NaOH. Its ions do not significantly hydrolyse in water, so its aqueous solution is approximately neutral and option A is correct. Sodium carbonate and sodium acetate come from weak acids with a strong base and give basic solutions, while ammonium chloride gives an acidic solution.

Hydrochloric acid, HCl, ionises almost completely in aqueous solution to form hydrogen or hydronium ions and chloride ions. Because its ionisation is nearly complete, it is classified as a strong acid, making option A correct. A strong base is an alkali such as sodium hydroxide, a neutral salt is a different chemical category, and neither weak-base option describes HCl.

Pure water undergoes slight self-ionisation, represented by H₂O ⇌ H⁺ + OH⁻, but the concentrations of these mobile ions are extremely small. Therefore distilled water conducts electricity very poorly, so option A correctly revises the student’s statement. Dissolved salts increase conductivity by supplying more ions; water does not contain only hydroxide ions, and its ions do not disappear completely.

Diluting concentrated sulphuric acid is highly exothermic. The safe method is to place water in the vessel and add the acid slowly in small portions while continuously stirring, so the released heat is distributed through the larger water volume. Thus option B is correct. Adding water directly to concentrated acid can make the small water layer boil suddenly and eject hot corrosive acid.

Antacids are mild bases that react with and neutralise excess hydrochloric acid in the stomach. For example, magnesium hydroxide follows Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. This reduces excessive acidity and supports relief from indigestion, so option B is correct. Antacids do not produce HCl, increase its concentration, or convert acid into a metal.

Bleaching powder is commonly represented as calcium oxychloride, CaOCl₂, and it releases chlorine or active chlorine in suitable conditions. This gives it bleaching and disinfecting properties. Therefore option A is correct: it is used for bleaching materials and disinfecting drinking water or surfaces. It is not a food ingredient, a toothpaste, or a substance intended to increase stomach acidity.

Sodium carbonate decahydrate, Na₂CO₃·10H₂O, is washing soda. It removes permanent hardness by supplying CO₃²⁻ ions, which react with dissolved Ca²⁺ and Mg²⁺ ions to form insoluble carbonates that can be separated. Hence option A is correct. Calcium sulfate contributes to hardness, sodium bicarbonate is baking soda, and calcium oxychloride is bleaching powder, so the other choices do not perform this softening role.

Slaked lime, Ca(OH)₂, is a base. It reacts with acidic substances in the soil and neutralises some of their acidity, thereby increasing the soil pH toward a more suitable range. Therefore option A is correct. Vinegar and lemon juice contain acids, while dilute hydrochloric acid is also acidic; adding any of these would generally lower pH rather than correct acidic soil.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it decomposes according to 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, so carbon dioxide gas is released and option A is correct. The carbon dioxide forms bubbles and can help cakes or bread rise when the gas is trapped in the mixture. Hydrogen, oxygen, and nitrogen are not products of this decomposition.

The governing concept is the litmus test for acids and bases. When blue litmus turns red, the aqueous solution contains acidic substances, so the white powder is most likely an acid or an acidic compound. A base would produce the opposite change, turning red litmus blue, while a neutral salt would normally cause no colour change. An indicator is a testing substance, not the likely identity of the powder in this observation.

The governing concept is salt hydrolysis and the strength of the parent acid and base. Sodium chloride comes from the strong acid HCl and the strong base NaOH; neither ion hydrolyses appreciably, so its aqueous solution is generally neutral, with pH close to 7. Hydrochloric acid is acidic, sodium hydroxide is basic, and ammonium chloride is acidic because the ammonium ion hydrolyses in water.

The governing concept is the colour response of litmus in basic and acidic media. Sodium hydroxide dissociates in water and produces hydroxide ions, so its solution is basic and changes red litmus to blue. Lemon juice contains citric acid, vinegar contains acetic acid, and hydrochloric acid is also acidic; these substances would turn blue litmus red rather than changing red litmus to blue. Therefore, option C is the only suitable answer.

The governing concept is the chlor-alkali process. Common salt provides sodium chloride, and its concentrated aqueous solution, called brine, is electrolysed. The overall process produces sodium hydroxide in solution along with chlorine gas and hydrogen gas: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Thus sodium hydroxide is the industrial base obtained from common salt; the other listed hydroxides are not the product of this process.

The governing concept is the relationship between a salt and the strengths of its parent acid and base. Sodium carbonate is formed from the strong base NaOH and the weak acid carbonic acid, H₂CO₃. In water, carbonate ions hydrolyse and produce hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. Therefore its solution is basic. Ammonium chloride is acidic, while sodium chloride and potassium nitrate are approximately neutral salts.

The governing concept is pH, acidity, and neutralisation. Tooth enamel can begin to dissolve when the mouth becomes sufficiently acidic, commonly below about pH 5.5. A mildly basic toothpaste helps neutralise acids and removes food residues, reducing the harmful acidic condition. Lemon juice and carbonated drinks add more acids, while retaining more sugar encourages bacterial acid production. Thus brushing with basic toothpaste is the best option.

The governing concept is electrolysis of brine in the chlor-alkali process. Brine contains sodium chloride and water. During electrolysis, chlorine gas is released at the anode and hydrogen gas is released at the cathode, while sodium hydroxide is formed and remains in the aqueous solution. The overall reaction is 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. The other choices are unrelated substances and are not the product left in the solution.

The governing concept is the pH scale and the natural acidity of rainwater. Carbon dioxide dissolves in rain and gives ordinary clean rain a slightly acidic pH of about 5.6. When atmospheric pollutants such as sulfur dioxide and nitrogen oxides form stronger acids and lower the rainwater pH below about 5.6, the precipitation is classified as acid rain. A pH of 7 is neutral, while 10 and 14 indicate alkaline solutions.

The governing concept is neutralisation of excess soil acidity. Slaked lime, Ca(OH)₂, is a basic substance, so it reacts with and reduces acids present in acidic soil. Farmers commonly add suitable lime compounds to raise the soil pH toward a better range for plant growth. Vinegar, lemon juice, and hydrochloric acid are acidic and would increase rather than correct the acidity. Therefore slaked lime is the most suitable answer.