Acids, Bases, and Salts

The standard test for carbon dioxide uses lime water, which is an aqueous solution of calcium hydroxide. When carbon dioxide is bubbled through it, calcium carbonate forms and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Vinegar is acidic, while oil and salt water do not give this characteristic test. Therefore, option A is correct.

Blue litmus turning red is the characteristic indicator response of an acidic solution. Acids provide hydrogen ions in water, which cause this colour change. A basic solution generally turns red litmus blue, while a neutral solution produces no litmus change. The observation does not prove that the liquid contains only salt, because salts may be acidic, basic, or neutral. Thus option A is correct.

Neutralisation is the reaction between an acid and 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. Therefore, salt and water are the general products, making option A correct; the other choices do not describe this reaction pattern.

Excess hydrochloric acid in the stomach can cause irritation and a burning sensation. An antacid contains a mild base, such as magnesium hydroxide, which reacts with and neutralises some of the excess acid. This raises the pH toward a less acidic value and provides relief. A strong acid would worsen acidity, while common salt and a metal are not suitable antacids. Hence option A is correct.

Milk of magnesia is a suspension containing magnesium hydroxide, Mg(OH)₂, a weak base. In the stomach it reacts with excess hydrochloric acid and reduces its acidic effect through neutralisation: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. It does not remove excess water, oxygen, or salt. Therefore, option A is correct.

Tooth enamel is mainly composed of calcium compounds and can be attacked when the mouth becomes sufficiently acidic. A mouth pH below about 5.5 is generally regarded as harmful because acid dissolution of enamel becomes significant. A pH of 7 is neutral, whereas 10 and 14 are alkaline values. Therefore, the accepted threshold in this school-level context is 5.5, so option A is correct.

Bacteria can produce acids from sugars left in the mouth after eating. These acids lower the local pH and may gradually damage tooth enamel. Mildly basic toothpaste helps neutralise or reduce the effect of these acids without being strongly caustic. It is not intended to make teeth acidic, colour the tongue, or convert saliva into gas. Thus option A correctly states its purpose.

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.

Sodium carbonate solution is basic because carbonate ions hydrolyse in water and generate hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. It can therefore turn red litmus blue. Sodium chloride is approximately neutral, ammonium chloride is acidic because it comes from a weak base and strong acid, and sodium hydrogen sulfate is acidic. Hence option C is correct.

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.