Acids, Bases, and Salts

The governing concept is the indicator test for acids and bases. An acidic solution contains hydrogen ions in water and changes blue litmus to red. Therefore, option A is correct. Option B describes the usual action of a base on red litmus, while options C and D do not represent the standard litmus response to an acid.

The governing concept is the use of litmus as an acid-base indicator. A basic solution produces hydroxide ions in water and changes red litmus to blue, so option B is correct. Option C is the characteristic response of blue litmus to an acid. Options A and D do not describe the normal result of testing a basic solution with red litmus.

The governing concept is the behaviour of a natural indicator. Turmeric remains yellow in an acidic or neutral medium but becomes reddish brown in a basic medium. Soap solution is generally basic because of its alkaline components, so option C correctly explains the colour change. Options A and B would not produce this characteristic reddish-brown turmeric colour.

An ant sting may cause irritation due to formic acid. Baking soda solution contains sodium hydrogen carbonate, a mild base that neutralises the acid and reduces its effect; therefore, option C is correct. Lemon juice and vinegar are acids themselves, while sodium chloride solution is generally neutral, so neither neutralises the acid. Exam tip: To reduce an acidic effect, select a suitable mild base.

A dilute acid reacts with a reactive metal, such as zinc, to form a salt and hydrogen gas. For example: \(Zn + 2HCl \rightarrow ZnCl_2 + H_2\). Therefore, hydrogen is the correct answer. Carbon dioxide is produced when an acid reacts with a carbonate or hydrogen carbonate, not with a metal. Exam tip: Hydrogen burns with a ‘pop’ sound when tested with a burning splint.

The governing concept is salt hydrolysis. Ammonium chloride is formed from strong acid HCl and weak base NH₄OH. In water, NH₄⁺ hydrolyses and increases hydronium-ion concentration, making the solution acidic with pH below 7; therefore option B disproves the statement. NaCl is nearly neutral, distilled water is not a salt, and NaOH is a base rather than a salt.

The governing concept is a precipitation reaction used to test carbon dioxide. Lime water contains calcium hydroxide, Ca(OH)₂. When CO₂ is passed through it, insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The suspended CaCO₃ makes the liquid milky, so option A is correct; the other products are not formed in this reaction.

The governing safety concept is controlled dilution of a concentrated acid. Dissolving acid in water releases heat, so adding acid slowly to a larger quantity of water while stirring allows the heat to disperse and reduces sudden boiling or splashing. Therefore option B is correct. Adding water to concentrated acid can cause the small water layer to heat rapidly and eject corrosive acid.

The governing concept is electrical conduction through mobile charged particles. When an acid dissolves in water, it ionises or dissociates and produces hydrated hydrogen ions, along with corresponding negative ions. These ions move through the solution and carry current, so option C is correct. Acids do not conduct because they cool water, change colour, or simply produce free electrons.

The governing concept is the Arrhenius description of bases. A base produces hydroxide ions, OH⁻, in aqueous solution; these ions account for its alkaline properties. Hence option B is correct. Hydrogen ions are associated with acids, while chloride and carbonate ions may be present in particular salts but are not the defining ions produced by every base. The answer refers to the general school-level definition.

The governing concept is the pH scale, which indicates the hydrogen-ion concentration of an aqueous solution. At ordinary school-level conditions, pH 7 is neutral; values below 7 indicate acidic solutions, while values above 7 indicate basic solutions. Therefore option A, seven, is correct. Ten and fourteen are alkaline-side values, and zero is an acidic pH value rather than the boundary asked for.

The governing concept is the relation between a salt and the strengths of its parent acid and base. Sodium chloride is formed from strong acid HCl and strong base NaOH, so its ions undergo negligible hydrolysis and its aqueous solution is approximately neutral; option A is correct. Sodium carbonate is basic, ammonium chloride is acidic, and copper sulfate is generally acidic in water.

The pH scale indicates the acidic or basic nature of an aqueous solution. At 25°C, pH 7 is neutral, values below 7 are acidic, and values above 7 are basic because hydroxide-ion concentration is relatively greater than hydrogen-ion concentration. Therefore, option C is correct. Option A describes an acidic solution, option B describes neutrality, and option D is only one possible acidic value, not the basic range.

Tooth enamel is mainly a mineral containing calcium compounds. Acids produced by bacteria from sugars lower the pH in dental plaque. When the pH falls below approximately 5.5, enamel begins to dissolve more readily, increasing the risk of tooth decay. Thus option C, 5.5, is correct. A pH of 7 is neutral, while 10.5 and 14 are alkaline and do not represent the acidic condition that damages enamel.

Acid rain forms when atmospheric pollutants such as sulfur oxides and nitrogen oxides react with water and produce acids. Consequently, its pH is lower than that of ordinary rain, so it can be more corrosive. It may damage limestone buildings, disturb soil chemistry, and harm aquatic organisms. Therefore option A is correct. Normal rain is not always exactly neutral, but the defining issue here is the greater acidity, not the absence of ions or alkalinity.

Indigestion accompanied by heartburn can result from excess hydrochloric acid in the stomach. An antacid contains a mild base or another acid-neutralising substance, so it reacts with and reduces excess acid. Hence option B is correct. An antibiotic acts against certain bacteria, a vitamin supplies nutrients, and a disinfectant is used on non-living surfaces; none of these directly neutralises stomach acid for this purpose.

Excessively acidic soil can prevent plants from absorbing nutrients properly and may reduce crop growth. Lime, commonly containing calcium oxide, calcium hydroxide, or calcium carbonate, is basic and is added in a suitable quantity to neutralise some of the excess acidity. Therefore option A is correct. Vinegar and lemon juice are acidic, while sulfuric acid would increase acidity and damage the soil further rather than improve it.

Excessively basic soil may make several nutrients less available to plants. Organic matter such as compost or manure decomposes through microbial activity and can produce mildly acidic substances, helping lower the soil’s excessive basicity over time. Thus option C is the best answer. Slaked lime and sodium hydroxide are bases, so adding them would not correct alkaline soil; adding more lime would generally intensify the problem.

Ammonium chloride is formed from the strong acid HCl and the weak base ammonia. In water, the \(NH_4^+\) ion reacts with water and increases the concentration of \(H_3O^+\) ions, so the solution is acidic. Sodium carbonate and sodium acetate are salts of a weak acid and a strong base, so they give basic solutions. Sodium chloride, a salt of a strong acid and a strong base, is nearly neutral. Exam tip: salts formed from a strong acid and a weak base are generally acidic.

This is a neutralisation reaction: \(\mathrm{NaOH + HCl \rightarrow NaCl + H_2O}\). The \(\mathrm{Na^+}\) ion from NaOH combines with the \(\mathrm{Cl^-}\) ion from HCl to form sodium chloride, so option A is correct. Sodium carbonate would require a carbonate ion, \(\mathrm{CO_3^{2-}}\), which is not present in the reactants. Exam tip: in neutralisation, identify the salt by combining the cation of the base with the anion of the acid.