Acids, Bases, and Salts

The governing concept is the strength of a base, which depends on the extent of ionisation in water and the concentration of hydroxide ions produced. Sodium hydroxide dissociates almost completely: NaOH(aq) → Na⁺(aq) + OH⁻(aq). Therefore it is a strong base, so option A is correct. Hydrogen ions indicate acidity, neutrality means pH 7, and sodium hydroxide is not a gas under ordinary conditions.

The governing concept is acid strength, which is related to the extent to which an acid ionises in water. Hydrochloric acid ionises essentially completely: HCl(aq) → H⁺(aq) + Cl⁻(aq), so it produces a high concentration of hydrogen or hydronium ions and shows strong acidity. Option A is correct. Hydroxide ions characterise bases, while a neutral substance would not show this acidic ionisation.

The governing concept is the identification of acids and bases using indicators and pH. A basic solution changes red litmus to blue, while a pH greater than 7 indicates alkalinity; pH 11 is therefore basic. Both independent observations support option A. An acidic solution has pH below 7 and generally turns blue litmus red, a neutral solution has pH about 7, and an ion-containing solution is not ion-free.

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 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.

Brine is a concentrated aqueous solution of sodium chloride. During electrolysis, chloride ions produce chlorine at the anode, water produces hydrogen at the cathode, and sodium hydroxide remains in solution. Because the products include chlorine and an alkali, this industrial method is called the chlor-alkali process. Therefore option B is correct; acidification, simple water decomposition, and crystallisation do not describe this complete electrolytic process.

Sodium chloride is formed from the strong acid hydrochloric acid, HCl, and the strong base sodium hydroxide, NaOH. Its Na⁺ and Cl⁻ ions undergo negligible hydrolysis in water, so the solution is approximately neutral, with pH near 7. Hence option A is correct. Sodium carbonate and sodium acetate are salts of weak acids with a strong base and give basic solutions, whereas ammonium chloride gives an acidic solution because NH₄⁺ hydrolyses.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical name is calcium hydroxide, Ca(OH)₂. The reaction produces bleaching powder, commonly represented as calcium oxychloride, along with water. Therefore, option A is correct. Sodium chloride and water, sodium carbonate with acid, and gypsum with water are not the standard reactants for this preparation.

Bleaching powder releases chlorine or chlorine-containing species that destroy many harmful microorganisms. For this reason, it is used to disinfect drinking water and is also employed as a bleaching agent in textile and paper industries. Option A is correct. It is not used to sweeten food, melt metals, or harden soil; those choices do not describe its standard chemical applications.

Ammonium chloride, NH₄Cl, is formed by neutralisation of the strong acid HCl and the weak base NH₄OH. In water, NH₄⁺ undergoes hydrolysis and produces H₃O⁺ ions, so the solution is acidic. NaCl is essentially neutral, while CH₃COONa and Na₂CO₃ are salts of weak acids with strong bases and generally give basic solutions. Thus option A is correct.

Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid. In the presence of moisture or heat, the acid reacts with baking soda to release carbon dioxide, which makes cakes and bread rise. Slaked lime is alkaline and is not used for this purpose. Exam tip: Remember: baking soda + mild edible acid = baking powder.

Baking powder contains sodium hydrogen carbonate along with an edible weak acid. On heating or on contact with moisture, they react and release carbon dioxide, CO₂. The gas forms bubbles within the dough or batter, and these bubbles expand during heating, making bread and cakes rise and become porous. Hydrogen, chlorine, and oxygen are not the leavening gas here, so option C is correct.