Acids, Bases, and Salts

An ant sting commonly introduces formic acid into the skin. Baking soda, sodium hydrogen carbonate (NaHCO₃), is mildly basic, so a suitable mild solution can help neutralise some of the acid and reduce irritation. Lemon juice and vinegar are acidic and would not neutralise the sting acid, while hydrochloric acid would add more acid. Therefore, option A is correct; practical treatment should still follow appropriate medical advice.

Sodium hydroxide solution contains hydroxide ions and is strongly basic. A base changes red litmus paper to blue, so option C gives the stated observation. Lemon juice contains citric acid, vinegar contains acetic acid, and hydrochloric acid is also acidic; these substances do not turn red litmus blue. They generally turn blue litmus red instead. Therefore, sodium hydroxide solution is the unambiguous correct answer.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, and is a basic salt. Its alkaline nature helps remove grease and dirt, so it is widely used as a cleaning agent. It also softens hard water by removing calcium and magnesium ions. Making food sour, increasing stomach acid, and damaging teeth are not its proper uses; therefore option A is correct.

Hydrochloric acid, HCl, ionises in water and produces hydrogen ions, commonly represented as H⁺ or hydronium ions, H₃O⁺. These ions give the solution an acidic nature and change blue litmus to red. NaOH and Na₂CO₃ produce basic solutions, while NaCl is essentially neutral in this comparison. Therefore HCl, option A, is the only suitable answer.

Sodium hydrogen carbonate, NaHCO₃, also called baking soda, is a mild basic salt. It neutralises excess hydrochloric acid in the stomach according to the reaction NaHCO₃ + HCl → NaCl + H₂O + CO₂. NaCl is neutral, CaSO₄ is not the usual antacid, and sodium carbonate is too strongly alkaline for this common use. Hence option B is correct.

Washing soda is sodium carbonate decahydrate, whose chemical formula is Na₂CO₃·10H₂O. The dot and the coefficient 10 show that each formula unit contains ten molecules of water of crystallisation; “deca” means ten. NaHCO₃ is baking soda, CaOCl₂ is bleaching powder, and CaSO₄·½H₂O is Plaster of Paris. Therefore option B is correct.

Plaster of Paris is prepared by gently heating gypsum, which is hydrated calcium sulphate, CaSO₄·2H₂O. Heating removes part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, known as Plaster of Paris. Common salt, baking soda, and vinegar do not undergo this preparation process. Hence option A is correct.

Slaked lime is calcium hydroxide, Ca(OH)₂, a base that neutralises excess acids in soil. This neutralisation reduces acidity and raises the soil pH toward a more suitable range for plant growth. Hydrochloric acid, vinegar, and lemon juice are acidic substances, so adding them would increase rather than reduce acidity. Therefore the recommended substance is slaked lime, option A.

Gypsum has the formula \(\mathrm{CaSO_4\cdot 2H_2O}\). Its two water molecules occur in a fixed proportion as part of the crystal structure, so this water is called water of crystallisation. Water of constitution is bonded differently within a compound’s chemical structure, whereas \(\cdot 2H_2O\) in gypsum denotes water of crystallisation. Exam tip: \(\cdot nH_2O\) in a salt formula generally indicates water of crystallisation.

Hydrochloric acid dissolves and ionises in water, producing H⁺ or H₃O⁺ ions. These ions are responsible for acidic behaviour and cause blue litmus paper to turn red. Sodium hydroxide is a base, calcium oxide forms a basic solution with water, and sodium chloride is approximately neutral. Thus only hydrochloric acid satisfies the stated test, making option B correct.

Anhydrous copper sulphate is white because it lacks water of crystallisation. When water is added, it becomes hydrated copper sulphate, CuSO₄·5H₂O, which has a characteristic blue colour. This hydration change explains the observation. The substance does not become red, yellow, or black under the stated conditions, so the correct observation is blue, option A.

Sodium carbonate, Na₂CO₃, is formed from a strong base, NaOH, and a weak acid, H₂CO₃. In water, carbonate ions react partly with water and produce hydroxide ions, OH⁻, making the solution basic. Sodium chloride is neutral, whereas ammonium chloride and copper sulphate generally give acidic solutions. Therefore sodium carbonate, option C, is correct.

On heating, sodium hydrogen carbonate undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The gas released is therefore carbon dioxide. It can be identified because it turns limewater milky, although this test is not required to select the answer. Oxygen, hydrogen, and chlorine are not products of this decomposition, so option B is correct.

Sodium hydroxide is a base. In water, it dissociates into \(Na^+\) and \(OH^-\) ions, so option B is correct. Hydrochloric acid forms \(H_3O^+\) ions in water, whereas sodium chloride mainly gives \(Na^+\) and \(Cl^-\) ions. Exam tip: a substance that gives \(OH^-\) ions in aqueous solution is a base.

The governing concept is salt hydrolysis and the strengths of the parent acid and base. Sodium chloride is produced from the strong acid HCl and the strong base NaOH. Neither ion hydrolyses water appreciably, so the solution is approximately neutral, with pH near 7. Sodium carbonate and sodium acetate are basic salts, while ammonium chloride is acidic because it comes from a weak base and a strong acid. Therefore, option A is correct.

The governing concept is neutralisation: an acidic substance is treated with a suitable base. Slaked lime, Ca(OH)₂, supplies hydroxide ions that react with excess hydrogen ions in acidic soil, reducing its acidity. In simplified form, H⁺ + OH⁻ → H₂O. Dilute hydrochloric acid and carbon dioxide would add to acidity, while sodium chloride is essentially neutral and is not the appropriate corrective material. Thus option A is correct.

The governing concept is the colour scale of a universal indicator. Around pH 7, the indicator appears green, showing a neutral solution. Acidic solutions have pH values below 7 and generally produce colours ranging from yellow to red, whereas basic solutions have pH above 7 and tend toward blue or violet. The colour does not merely prove that a salt is present. Therefore, the student’s statement should be corrected as in option A.

In the chlor-alkali process, brine (aqueous sodium chloride solution) is electrolysed. At the cathode, water is reduced to form hydrogen gas: \(2H_2O + 2e^- \rightarrow H_2 + 2OH^-\). Chlorine is produced at the anode, so option B is incorrect. Exam tip: reduction always takes place at the cathode during electrolysis.

The governing concept is acid-base neutralisation in an antacid. Magnesium hydroxide, Mg(OH)₂, is a mild base and reacts with excess stomach hydrochloric acid: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. This consumes acid and provides relief when used appropriately. Sodium chloride does not neutralise the acid, while ethanoic acid and lemon juice are acidic and would not be suitable antacids. Hence option A is correct.

The governing concept is the pH scale and the natural acidity of rainwater. Unpolluted rain is mildly acidic, with pH about 5.6, mainly because atmospheric carbon dioxide dissolves in water to form weak carbonic acid. Rain with pH below approximately 5.6 is commonly classified as acid rain, often due to stronger acids formed from sulfur and nitrogen oxides. Values 7, 10, and 14 do not represent this threshold, so option A is correct.

The governing concept is the relation between a salt’s solution and the strengths of the acid and base from which it is formed. Sodium chloride comes from strong HCl and strong NaOH. Its Na⁺ and Cl⁻ ions do not significantly react with water, so the solution remains nearly neutral, with pH close to 7. Sodium carbonate and sodium acetate hydrolyse to give basic solutions, while ammonium chloride gives an acidic solution. Therefore, option A is correct.

The governing concept is neutralisation or reduction of excess alkalinity in soil. During decomposition, organic matter can produce acidic substances and organic acids. These react with some basic components of alkaline soil and help lower its excessive basicity, improving soil conditions. Organic matter does not turn soil into metal, stop gas movement, or generally make an already basic soil more alkaline. Therefore, the scientifically appropriate answer is option A.

The governing concept is that hydrogen chloride shows acidic behaviour only after dissolving in water. Moisture on the litmus paper allows HCl gas to dissolve and ionise, producing hydrochloric acid and hydrogen ions: HCl(aq) → H⁺ + Cl⁻. These ions turn blue litmus red. Dry HCl gas cannot ionise effectively without water, so the word “moist” is essential. Hence option A is correct.

The governing concept is the colour change of an acid-base indicator. Methyl orange is red in an acidic medium and yellow in a neutral or basic medium. Therefore, when it is added to a basic solution, the observed colour is yellow. Pink and black are not the characteristic methyl-orange colours, and red indicates an acidic rather than a basic condition. Thus option B is the only correct answer.

The governing concept is safe handling of an exothermic dilution. Concentrated sulphuric acid releases a large amount of heat when mixed with water. The safe method is to add the acid slowly to a larger volume of water while stirring continuously, allowing heat to disperse and reducing the chance of local boiling and splashing. Adding water directly to concentrated acid can make the small water layer boil suddenly. Therefore, option A is correct.