Acids, Bases, and Salts

An acidic solution contains a relatively high concentration of hydrogen or hydronium ions. Added hydroxide ions react with them in the neutralisation step: H⁺ + OH⁻ → H₂O. This lowers the hydrogen-ion concentration. Since pH = −log[H⁺], a decrease in [H⁺] produces an increase in pH. The ions do not increase acidity or form carbon dioxide, so option A is correct.

A red litmus paper turns blue only when a basic medium is produced. The absence of an effect on dry paper shows that moisture is necessary for the gas to dissolve and generate the basic species or hydroxide ions responsible for the colour change. Ammonia is a familiar example: NH₃ dissolves in water and produces an alkaline solution. Thus option A is the correct conclusion.

A pH of 7 indicates a neutral solution, but neutrality does not mean that no ions are present. Sodium chloride dissociates in water into Na⁺ and Cl⁻ ions, allowing the solution to conduct electricity while remaining approximately neutral. Sugar and alcohol generally dissolve as molecules and do not provide many mobile ions. Pure distilled water has extremely weak conductivity, so option A is the best answer.

Marble consists mainly of calcium carbonate, CaCO₃. Acids in acid rain react with this carbonate to form a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated reactions remove small amounts of the carbonate surface, so the statue gradually loses material and detail. Therefore option A correctly explains the corrosion.

The pH scale is logarithmic, so a rise of one pH unit means that the hydrogen-ion concentration, which represents acidity, decreases by a factor of 10. Here the pH rises from 3 to 5, a change of two units. Therefore, the hydrogen-ion concentration becomes 10 × 10 = 100 times smaller. Thus option A is correct; the other choices ignore the logarithmic nature of pH.

Bacteria in the mouth can break down sugars and produce acids. These acids lower the pH around the teeth and gradually dissolve minerals from enamel, causing decay. A mildly basic toothpaste reacts with and neutralises some of the acid, helping the mouth return toward a safer pH and protecting enamel. Therefore option A is correct; the other options would either damage enamel or describe an incorrect effect.

A metal such as magnesium reacts with a dilute acid by displacing hydrogen: Mg + 2HCl → MgCl₂ + H₂. A carbonate reacts with an acid to produce a salt, water, and carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Hence magnesium gives hydrogen while calcium carbonate gives carbon dioxide, so option B is correct.

A base is a substance that reacts with an acid in a neutralisation reaction to form salt and water. Bases need not be soluble in water; for example, copper(II) oxide is a base but is insoluble in water. An alkali is specifically a base that dissolves in water. Therefore the described substance is a base, making option B correct; an indicator only changes colour and an acid does not fit the reaction described.

Methyl orange is red in a sufficiently acidic medium and yellow in a neutral or alkaline medium, with a transition range between them. Adding a base consumes some hydrogen ions and shifts the solution away from strong acidity. The yellow colour therefore indicates movement toward a less acidic, neutral, or basic condition. Option A is correct; the observation does not prove that a metal formed or that no chemical change occurred.

A pH near 7 indicates that the solution is approximately neutral, but neutrality does not mean that no ions are present. Sodium chloride dissociates in water into mobile Na⁺ and Cl⁻ ions. These charged particles carry electric current through the solution. Thus option A is correct. Options B and C wrongly restrict conductivity to acidic solutions, while option D confuses a neutral salt with an acid.

A salt formed from a strong acid and a strong base, such as sodium chloride from hydrochloric acid and sodium hydroxide, generally gives a solution close to pH 7 because its ions do not significantly hydrolyse water. It still dissociates into mobile ions, so the solution conducts electricity. Therefore option A is correct. Salts in B and C generally make acidic or basic solutions, while D cannot conduct in solution.

Blue litmus turning red shows that the aqueous solution is acidic. Many non-metal oxides, such as carbon dioxide and sulphur dioxide, form acidic solutions in water and are called acidic oxides. An acidic oxide reacts with a base in a neutralisation reaction to produce salt and water; for example, CO₂ + 2NaOH → Na₂CO₃ + H₂O. Hence option A is correct.

The governing concept is neutralisation: an acid reacts with a base to produce salt and water. Many metal oxides, such as copper(II) oxide and magnesium oxide, react with acids in this way, so they show basic character. Therefore option A is correct. Option B reverses the usual rule, while options C and D describe properties that are chemically impossible or irrelevant to this reaction.

Sour foods commonly contain acids such as acetic, citric, or lactic acid. These acids may react slowly with the copper or zinc components of brass, especially during prolonged storage, producing dissolved metal salts. Therefore option A identifies the relevant food-safety risk. Options B and C have no chemical basis, and option D is unsafe because contact and storage time can permit reaction.

The pH relationship is pH = −log[H⁺], so every increase of one pH unit represents a tenfold decrease in hydrogen-ion concentration. The change from pH 4 to pH 7 is three units; therefore the concentration changes by 10³ = 1000 in the decreasing direction. Hence option A is correct, not B, which treats the pH difference as an ordinary multiplier.

When oral bacteria metabolise sugars, they produce acids that lower the pH near the teeth. Tooth enamel, which is mainly a mineral containing calcium compounds, begins to lose minerals when the surrounding pH falls below its critical value of about 5.5. Therefore option A is correct. The other choices do not describe acid attack or the chemistry of tooth decay.

A carbonate reacts with an acid to form a salt, water, and carbon dioxide: carbonate + acid → salt + H₂O + CO₂. Passing carbon dioxide through limewater produces insoluble calcium carbonate, which makes the liquid milky. Therefore option B is correct. Hydrogen, oxygen, and nitrogen do not give this characteristic limewater test under these conditions.

Phenolphthalein is colourless in acidic or neutral conditions but turns pink in a sufficiently basic solution. If adding acid makes the pink colour disappear, the solution must initially have contained enough base to produce that colour. The acid lowers the alkalinity through neutralisation. Thus option B is correct; the other choices cannot explain the initial pink indicator colour.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, calcium hydroxide. In simplified form, the process is represented as calcium hydroxide reacting with chlorine to produce bleaching powder along with other products. Thus option C is correct. Hydrogen, carbon dioxide, and oxygen are not the characteristic reactant gases used for this preparation, so they cannot explain the manufacture of bleaching powder.

The governing concept is the distinction between a base and an alkali. A base is a substance that can neutralise an acid, but some bases are insoluble in water. Only a base that dissolves in water is called an alkali; for example, sodium hydroxide is an alkali. Therefore option B correctly corrects the student. The other statements confuse solubility, acidity, or neutralisation.

The governing concept is acid-base neutralisation. A basic solution initially has a pH above 7 because hydroxide ions are present in greater effective concentration. As acid is added, hydrogen ions react with hydroxide ions to form water, so the basicity decreases. Near the neutralisation point, the pH approaches 7, making option A correct. The other choices contradict the direction of the pH change or describe an impossible transformation.

The governing concept is the removal of hardness by precipitation. Hard water contains dissolved calcium and magnesium ions. Washing soda, sodium carbonate, supplies carbonate ions that react with these ions and form insoluble calcium carbonate and magnesium carbonate. The precipitates can then be separated, reducing hardness. Thus option A is correct. Sodium ions are not destroyed, and the process neither makes water acidic nor produces chlorine.

The governing concept is the relationship between acidity and pH. Acid rain forms when atmospheric acidic oxides produce acids in rainwater. The dissolved acids increase hydrogen-ion concentration and lower the pH. Such water can react more readily with metals, carbonates, and some building materials, increasing corrosion or damage. Therefore option A is correct. Sugar, absence of ions, and pH 14 do not describe acid rain.

The governing concept is neutralisation of a basic solution. A pH of 11 and pink phenolphthalein both indicate an alkaline medium. When acid is added, its hydrogen ions react with hydroxide ions to form water: H+ + OH− → H2O. Consequently, the effective hydroxide-ion concentration or basic effect decreases first. Option A is correct; chloride ions and sodium mass are not the primary quantity reduced, and water does not lose its chemical identity.

The governing concept is recognising characteristic acid reactions. Zinc, a reactive metal, reacts with a dilute acid to form a salt and hydrogen gas, for example Zn + 2HCl → ZnCl2 + H2. Limestone is mainly calcium carbonate; with acid it produces a salt, water, and carbon dioxide, for example CaCO3 + 2HCl → CaCl2 + H2O + CO2. Thus option A correctly identifies the gases. The reversed pair and chlorine or oxygen alternatives do not fit these reactions.