Acids, Bases, and Salts

The governing concept is that indicators are substances used to test the acidic or basic nature of a medium. Litmus and several other indicators show colour changes, while olfactory indicators show changes in smell. Thus option A gives the most complete correct statement. Indicators do not identify only metals, neutralise every solution, or cause a gas-to-solid conversion, so B, C, and D are false.

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 laboratory safety. Acids and bases can irritate or damage body tissues, and concentrated substances may be corrosive or poisonous. Therefore directly tasting them is unsafe, making option A correct. The other choices are scientifically wrong: substances can have tastes, acids are not all sweet, and bases are not all sour. Indicators should be used instead.

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 dilution. Adding water increases the total volume of an acidic solution while the amount of acid already present does not increase. Consequently, the concentration, or number of hydrogen ions per unit volume, decreases. The solution becomes less acidic and its pH generally rises toward 7, although it does not necessarily become neutral. Thus option A is correct; the other choices contradict dilution or describe impossible changes.

The governing concept is dilution of a base. When water is added, the total volume increases, but no extra hydroxide-producing solute is supplied. Therefore, the number of hydroxide ions per unit volume decreases. The solution becomes less basic, and its pH generally moves toward 7 rather than instantly becoming acidic. Option B correctly states the concentration change; the other options claim an increase or impossible chemical transformations.

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 colour scale of a universal indicator. It displays different colours over a range of pH values: acidic solutions are generally red to orange, strongly basic solutions are blue to violet, and green is associated with a pH close to 7. Thus the observed green colour indicates that the solution is approximately neutral. It does not prove that the solution is metallic or strongly acidic/basic, so option B is correct.

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 governing concept is the ionisation of acids in aqueous solution. Hydrogen chloride, HCl, dissolves in water and forms hydronium ions, represented at school level as hydrogen ions: HCl + H₂O → H₃O⁺ + Cl⁻. These aqueous hydrogen or hydronium ions are responsible for acidic properties. Sodium hydroxide supplies hydroxide ions, while calcium oxide and sodium carbonate produce basic or alkaline solutions in water. Therefore option B is correct.

The governing concept is the aqueous behaviour of bases. A base produces hydroxide ions in water, either directly, as sodium hydroxide dissociates into Na⁺ and OH⁻, or through reaction with water. These ions give the solution basic properties; its pH is generally greater than 7. Acids provide hydrogen or hydronium ions, bases turn red litmus blue rather than blue litmus red, and a pH below 7 indicates acidity. Therefore option C is correct.

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.