Acids, Bases, and Salts

The relevant principle is the relation between salt hydrolysis and the strengths of the parent acid and base. Sodium chloride is produced by the reaction of strong hydrochloric acid with strong sodium hydroxide: HCl + NaOH → NaCl + H₂O. Neither ion undergoes significant hydrolysis, so its aqueous solution is approximately neutral. Ammonium chloride is acidic, while carbonate and hydrogen carbonate salts are basic. Therefore, option C is correct.

An oxide is classified partly by the element combined with oxygen and partly by its reaction with water or bases. Carbon is a non-metal, so CO₂ is a non-metal oxide. In water it forms carbonic acid: CO₂ + H₂O ⇌ H₂CO₃, and it reacts with bases to form salts and water. It therefore shows acidic oxide behaviour. The litmus and metal-based alternatives contradict these facts, so A is correct.

The defining chemical behaviour of a basic oxide is its reaction with an acid to produce salt and water. Calcium oxide is a metal oxide and reacts with hydrochloric acid, for example: CaO + 2HCl → CaCl₂ + H₂O. It can also react with water to form calcium hydroxide, an alkali. Acid rain and red litmus indicate acidic behaviour, and calcium is a metal, so those distractors are incorrect. Thus A is correct.

The Arrhenius concept explains the observation: an acid produces hydrogen-related ions in aqueous solution. Dry HCl gas has no water medium in which HCl can ionise, so it does not supply hydrogen ions to the litmus and does not show the colour change. In water, HCl ionises essentially completely, producing H₃O⁺ ions, which turn blue litmus red. Therefore, A is correct; the other choices reverse or deny the observation.

pH is defined as −log₁₀[H⁺], so a lower pH means a greater hydrogen-ion concentration and stronger acidity. The difference between pH 3 and pH 5 is two units; therefore the pH 3 solution has 10², or about 100 times, the hydrogen-ion concentration of the pH 5 solution, assuming comparable conditions. It is consequently more acidic, making option C correct.

On the pH scale, values above 7 indicate basic solutions, and a higher pH generally means a lower hydrogen-ion concentration and greater basicity. Since 13 is greater than 10, the pH 13 solution is more basic. The three-unit difference also corresponds to a thousand-fold difference in hydrogen-ion concentration under comparable conditions. Options C and D incorrectly call the solutions acidic or neutral, so option B is correct.

The governing concept is acid–base balance in soil. When organic matter decomposes, microorganisms may produce humus and weak organic acids. These substances can gradually lower excessive alkalinity and make the soil more suitable for plant growth. Therefore, option A is correct. Organic matter does not turn soil into metal, remove all water, or always increase basicity, so options B, C, and D are incorrect.

The governing concept is the use of olfactory indicators. Such indicators provide evidence about a solution through a change in smell rather than a colour change. Vanilla’s odour becomes weaker in a basic medium, so this observation indicates a basic solution and makes option A correct. Metallic shine, boiling point, and water of crystallisation are unrelated properties and cannot be identified by this observation.

The governing concept is neutralisation, in which an acid reacts with a base to form salt and water. If the acid and base are present in suitable, approximately equivalent amounts, their acidic and basic effects largely cancel, giving a nearly neutral solution; therefore A is correct. Only acid or only base leaves an excess, while adding a metal is a different reaction and does not guarantee neutrality.

“Suitable amounts” means the acid and base are present in the stoichiometric ratio needed for neutralisation. The reaction is \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\). Hence, the final solution mainly contains aqueous sodium chloride and water. Options B and C would apply only if HCl or NaOH, respectively, were taken in excess. Hydrogen gas is produced when an acid reacts with a suitable metal, not when it reacts with NaOH. Exam tip: recognise acid + base → salt + water as a neutralisation reaction.

The governing reaction pattern is acid plus metal hydrogen carbonate, which produces a salt, water, and carbon dioxide. For example, hydrochloric acid reacts with sodium hydrogen carbonate as: HCl + NaHCO₃ → NaCl + H₂O + CO₂. Hence option A is correct. Hydrogen gas, oxygen, or a free metal are not the products of this carbonate reaction, so the other options are incorrect.

The governing concept is the formation of a salt from the ions supplied by the reactants. In the reaction NaHCO₃ + HCl → NaCl + H₂O + CO₂, sodium combines with chloride to form sodium chloride. Therefore option B is correct. Sodium sulphate would require sulphate ions, while calcium carbonate and magnesium chloride require ions not present in these reactants.

The governing concept is the litmus test for acidic and basic solutions. An acid changes blue litmus to red, while a base changes red litmus to blue. If neither red nor blue litmus changes, the solution is most likely neutral, so option C is correct. The result is not evidence of strong acidity or basicity, and a solution cannot be concluded to be a gas from this test.

The governing concept is that dilution of concentrated sulphuric acid is highly exothermic: considerable heat is released when the acid mixes with water. If water is poured rapidly into acid, the small amount of water may heat suddenly, boil, and throw corrosive acid droplets outward. Therefore, acid is added slowly to a larger volume of water with stirring and suitable protection. The other options describe false changes or incorrectly deny dissolution.

The governing concept is the pH scale: a lower pH indicates a greater hydrogen-ion concentration and stronger acidity. A solution of pH 2 has 1,000 times the hydrogen-ion concentration of a solution of pH 5 because the difference is three pH units. Stronger acidity generally makes pH 2 more likely to be corrosive, though actual corrosiveness also depends on concentration, substance, temperature, and contact time. Therefore A is the best answer.

The governing concept is the reaction of a sufficiently reactive metal with an acid. Zinc can displace hydrogen from many dilute acids, forming a salt and hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Hydrogen burns with a characteristic pop when tested near a flame or burning splint. Therefore the unknown solution could be acidic. Neutral water, salt solution, and sugar solution do not normally produce this test with zinc under these conditions, so B is correct.

The governing concept is the acid–carbonate reaction. An acid reacts with sodium carbonate to form a salt, water, and carbon dioxide: 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. Passing the released gas through limewater produces insoluble calcium carbonate, making the limewater milky. This confirms carbon dioxide. Hydrogen would give a pop sound, while oxygen relights a glowing splint; nitrogen has no such limewater test. Therefore D is correct.

The governing concept is neutralisation and the corresponding movement of pH. An acidic solution initially has a pH below 7. As a base is added gradually, hydroxide ions react with hydrogen ions to form water, so acidity decreases and the pH moves toward 7. The universal-indicator colour can therefore shift from acidic colours, such as red or orange, toward green or another neutral colour. It may later become basic if excess base is added, but A correctly describes the initial direction.

The pH of a salt solution depends on the relative strengths of its parent acid and base. A weak acid and a strong base produce a salt whose anion hydrolyses in water and generates hydroxide ions, so the solution becomes basic. Sodium carbonate, formed from weak carbonic acid and strong sodium hydroxide, is an example. Strong acid–strong base salts are generally neutral, while strong acid–weak base salts are acidic.

A salt formed from a strong acid and a weak base can produce an acidic aqueous solution. The cation from the weak base hydrolyses in water and forms hydronium ions, lowering the pH. Ammonium chloride is a standard example because it comes from hydrochloric acid and ammonium hydroxide. Strong acid–strong base salts are generally neutral, whereas weak acid–strong base salts are generally basic.

A salt made from a strong acid and a strong base generally gives a neutral aqueous solution because its ions do not appreciably hydrolyse to produce excess hydrogen or hydroxide ions. Sodium chloride, formed from hydrochloric acid and sodium hydroxide, illustrates this rule and has a pH close to 7 in dilute solution. Weak acid–strong base salts tend to be basic, while strong acid–weak base salts tend to be acidic.

This is a neutralisation reaction: an acid reacts with a base to form salt and water. The balanced equation is HNO₃ + NaOH → NaNO₃ + H₂O. Nitric acid supplies the nitrate ion, NO₃⁻, and sodium hydroxide supplies Na⁺; these ions combine in a 1:1 ratio to form sodium nitrate. Chloride, sulphate and carbonate are absent, so the other options cannot be formed.

Potassium hydroxide and hydrochloric acid undergo neutralisation. The balanced equation is KOH + HCl → KCl + H₂O. Potassium hydroxide contributes K⁺, while hydrochloric acid contributes Cl⁻; the ions combine to form potassium chloride, and H⁺ combines with OH⁻ to form water. Sodium, sulphate and calcium ions are not present, so options A, B and D do not follow from the reactants.

Magnesium oxide is a basic metal oxide, so it reacts with hydrochloric acid in a neutralisation reaction. The balanced equation is MgO + 2HCl → MgCl₂ + H₂O. Magnesium combines with chloride ions to form magnesium chloride, while the oxide ion combines with hydrogen ions to form water. No sulphate, sodium or carbonate is present, so the other products are not possible.

An oxide that reacts with water to produce an acidic solution is classified as an acidic oxide. Sulphur dioxide is an oxide of the non-metal sulphur; it reacts with water to form sulphurous acid: SO₂ + H₂O → H₂SO₃. This acidic product explains the observation. A basic oxide would produce a basic solution, a neutral oxide would not show this behaviour, and “metallic oxide” is not the correct chemical classification here.