Acids, Bases, and Salts

Laboratory safety requires that chemicals must never be tasted because even a small quantity of an acid may be corrosive, poisonous, or contaminated. The correct approach is to use an indicator, pH paper, or an instrument under proper supervision. Therefore option A is correct. Acids may have tastes, but taste cannot safely or accurately identify them; indicators can be used for both acidic and basic solutions.

Blue litmus changing to red is the standard indicator test for an acidic medium. Passage of current through an aqueous solution requires mobile charged particles, usually ions, to carry charge between the electrodes. Combining both observations gives the strongest conclusion in option A: the solution is acidic and contains ions. A neutral solution can also contain ions, but options B, C, and D do not fit the stated observations.

A pH near 7 describes the acid-base character of the sodium chloride solution, not the absence of charged particles. Sodium chloride dissolves and dissociates into mobile Na+ and Cl− ions, which carry electric current. Therefore option A is correct: neutrality and ionic conductivity can coexist. Options B and C incorrectly restrict conduction, while option D is false because sodium chloride is soluble in water.

A soil pH of 5 indicates appreciable acidity, which can hinder the availability of nutrients and affect crop growth. Slaked lime, calcium hydroxide, is a base; in a carefully measured amount it neutralises part of the excess acidity and raises the soil pH toward a more suitable range. Hence option A is correct. Vinegar, lemon juice, and dilute acid would add acidity and generally worsen the stated problem.

Adding water causes dilution: the concentration of hydrogen ions per unit volume decreases, so the pH usually moves upward toward 7. However, dilution does not remove every hydrogen ion or automatically create a basic solution. If hydrogen-ion concentration is still greater than hydroxide-ion concentration, the solution remains acidic. Therefore option A correctly explains the observation; the other choices contradict dilution and acid-base behaviour.

Sodium hydrogen carbonate, NaHCO3, is the chemical name of baking soda, so option A is correctly matched. Plaster of Paris is calcium sulphate hemihydrate, not calcium carbonate; washing soda is sodium carbonate decahydrate, not calcium sulphate hemihydrate; and bleaching powder is mainly calcium oxychloride rather than sodium chloride. Thus only the first pair is correct.

Zinc can react with an acidic solution and displace hydrogen, producing hydrogen gas; in a simplified equation, zinc plus acid forms a salt and H2. Carbon dioxide, however, appears when an acid reacts with carbonate or hydrogen carbonate. Since acid addition produced no CO2, carbonate is not indicated, while hydrogen from zinc supports an acidic nature. Therefore option A is the best conclusion among the choices.

The governing concept is the distinction between a base and an alkali. A base is a substance that can neutralise an acid, but bases do not all dissolve in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Therefore option A gives the correct correction; the other statements wrongly reverse acid-base properties or confuse solubility with acidity.

The governing reaction pattern is acid plus carbonate or hydrogencarbonate producing a salt, water, and carbon dioxide. For example, an acid reacting with a carbonate releases CO2, while an acid reacting with a hydrogencarbonate also releases CO2 after the intermediate carbonic acid decomposes. Thus option A correctly states the common products. The other options omit the characteristic gas or name products that are not formed.

The governing concept is the pH scale and dilution. Both pH 2 and pH 4 are below 7, so the solution remains acidic after water is added. Because pH increases, the hydrogen-ion concentration decreases; in fact, a two-unit rise represents a large reduction in hydrogen ions. Therefore acidity has decreased but has not disappeared, making option A correct. The other choices confuse dilution with becoming basic or claim the opposite ion change.

The governing concept is the reaction of acids with certain metals. Curd contains lactic acid and is therefore acidic. If it is stored for some time in copper or brass, the acid may react with the metal surface and produce metal compounds or salts that can contaminate the food. Hence option A gives the correct safety reason. Gold formation is impossible here, copper does not always destroy acids, and the claim that curd has no acid is false.

The governing concept is salt hydrolysis and the relative strengths of the parent acid and base. A salt produced from a strong acid and a strong base, such as sodium chloride from hydrochloric acid and sodium hydroxide, generally does not hydrolyse appreciably in water. Its aqueous solution is therefore approximately neutral, with pH near 7. Salts involving a weak acid or weak base are commonly acidic or basic, so options B and C are not the best answer.

The governing concept is that litmus is only one indicator of acidity, not the complete list of acid properties. Acids can react with active metals to release hydrogen, with carbonates to release carbon dioxide, and with bases in neutralisation reactions to form salt and water. Therefore option A correctly rejects the student’s incomplete claim. Options B and C contradict known reactions and ionisation, while D wrongly says acids are always bases.

The governing concept is acid-base neutralisation. Solution A turns blue litmus red, so it is acidic; solution B turns red litmus blue, so it is basic. When suitable amounts are mixed, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while a salt is produced. The slight temperature rise is consistent with heat released during neutralisation. Therefore option B is correct, not precipitation, evaporation, or sublimation.

The governing concept is the logarithmic pH scale, where pH = −log[H+]. A tenfold decrease in hydrogen-ion concentration raises pH by one unit. A thousandfold decrease is 10^3, so the pH rises by three units: 4 + 3 = 7. Therefore option C is the approximate answer. Option A would represent greater acidity, option B ignores the change, and option D assumes a four-unit rise instead of three.

The governing concept is the relation between salt hydrolysis and the strengths of its parent acid and base. A pH of 8.5 is above 7, and red litmus turning blue confirms that the solution is basic. Salts formed from a weak acid and a strong base, such as sodium carbonate in the usual school-level classification, generally hydrolyse to produce a basic solution. Hence option B is correct; strong-acid/weak-base salts are usually acidic, while strong-acid/strong-base salts are approximately neutral.

The governing concept is the reaction of carbon dioxide with limewater. The gas turns limewater milky because insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. When CO₂ is passed in excess, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble, so the milkiness disappears. Therefore option C is correct; calcium carbonate is only the intermediate product.

Electrical conduction in an acid solution is caused mainly by mobile ions. At the same concentration, an acid that produces fewer ions through ionisation generally conducts electricity less effectively. Thus the most appropriate conclusion is that it ionises less and is likely to be a weak acid. Option B is opposite to the evidence, while C and D do not follow from conductivity. Strictly, ion mobility and experimental conditions can also affect conductivity, so option A is the best school-level conclusion.

Dilution of a concentrated acid with water is usually exothermic, meaning that considerable heat is released. If water is poured into a small amount of concentrated acid, the water may heat suddenly, boil, and eject corrosive acid droplets. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring and using protection. Therefore option B is correct; the other methods increase splash, pressure, or heating hazards.

An acid shows its characteristic effect through hydrogen ions, more accurately hydronium ions, in aqueous moisture. The gas does not change dry blue litmus because there is no sufficient water medium for these ions to form and move. When moisture is present, the gas dissolves and reacts with water to produce an acidic solution, which turns blue litmus red. Hence option A is correct; the other choices do not explain the dependence on moisture.