Acids, Bases, and Salts

Sodium chloride is produced by neutralisation of hydrochloric acid, a strong acid, with sodium hydroxide, a strong base. Both ions remain substantially unhydrolysed in water, so neither an excess of hydrogen ions nor hydroxide ions is generated. The solution is therefore generally neutral. Option B gives an incomplete and incorrect origin, option C is chemically meaningless, and option D is false because sodium chloride dissociates into Na+ and Cl− ions.

The governing concept is the reactivity series and displacement of hydrogen from acids. A metal can liberate hydrogen from a dilute non-oxidising acid only when it is more reactive than hydrogen. Copper lies below hydrogen in the reactivity series, so it cannot displace hydrogen ions from dilute hydrochloric acid under ordinary conditions. The other statements are chemically false: copper is a solid metal, the dilute acid contains water, and hydrogen is a non-metal.

Two independent observations identify the solution as acidic. First, an acid changes blue litmus to red. Second, an acid reacts with a metal carbonate to produce carbon dioxide: acid + carbonate → salt + water + CO2. The gas turns limewater milky because insoluble calcium carbonate forms. A basic or neutral solution would not normally give both of these characteristic tests, and “without nature” is not a scientifically meaningful classification.

The governing relation is pH = −log[H+]. A tenfold decrease in hydrogen-ion concentration increases pH by one unit. Becoming one hundred times less acidic means the hydrogen-ion concentration falls by 100 = 10², so the pH rises by two units. Starting from pH 2, the approximate new pH is 2 + 2 = 4. Dilution does not make the solution strongly basic, so six and eight are unsuitable.

The governing concept is the indicator colour scale and dilution. Orange indicates an acidic solution, while yellow represents a less strongly acidic region on the universal-indicator scale. Adding water lowers the concentration of hydrogen ions, so the pH increases and the acidity decreases. However, a yellow colour does not prove that all acid has disappeared or that the solution has become basic. Ions remain present after dilution, so option A is correct.

Blue litmus turning red indicates that the salt solution is acidic. This generally occurs when a salt is formed from a strong acid and a weak base, because the cation from the weak base undergoes hydrolysis and produces excess hydronium ions. Therefore option A is correct. A weak-acid/strong-base salt is usually basic, a strong-acid/strong-base salt is nearly neutral, and a weak-acid/weak-base salt is not always neutral.

Red litmus turning blue shows that the salt solution is basic. A salt formed from a weak acid and a strong base usually gives a basic solution because the anion of the weak acid reacts with water and generates hydroxide ions. Thus option A is correct. A strong-acid/weak-base salt is generally acidic, while a strong-acid/strong-base salt is approximately neutral; water and a metal do not describe a normal salt-forming combination here.

The gas that turns limewater milky is carbon dioxide. Acids react with carbonates and hydrogen carbonates to release carbon dioxide: carbonate or hydrogen carbonate plus acid produces a salt, water, and CO2. The CO2 then reacts with limewater to form insoluble calcium carbonate, causing milkiness. Therefore option A is correct; ordinary chlorides, sulphates, and nitrates do not give this characteristic gas merely with acid.

Magnesium reacts with a dilute acid by displacement, producing a salt and hydrogen gas; for example, Mg + 2HCl → MgCl2 + H2. Calcium carbonate reacts with acid to produce a salt, water, and carbon dioxide; CaCO3 + 2HCl → CaCl2 + H2O + CO2. Therefore, in the stated order, the gases are hydrogen and carbon dioxide, so option A is correct.

Dilution of a concentrated acid is highly exothermic because hydration of ions releases heat. Adding acid slowly to a larger volume of water allows the heat to spread through the water and be dissipated more safely. If water is poured into concentrated acid, the small amount of water may heat suddenly, boil, and eject acid droplets. Hence option A expresses the laboratory safety rule and is correct.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses only part of its water of crystallisation and changes into calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris. If heating is excessive, the material may become anhydrous calcium sulphate and lose its useful setting property. Therefore, option A is correct; the other options describe unrelated changes.

Hydrated copper sulphate, CuSO₄·5H₂O, is blue because water molecules are present in its crystal structure. Heating removes this water and produces white anhydrous CuSO₄. Adding water restores the hydrated compound and its blue colour. This reversible colour change is evidence of water of crystallisation, so option A is correct. The other choices do not explain both observations.

A pH close to 7 indicates that the sodium chloride solution is approximately neutral, not that it is free of ions. In water, NaCl dissociates into mobile Na⁺ and Cl⁻ ions. These charged particles carry electric current through the solution. Thus a neutral solution may conduct electricity, making option A correct. Options B and C confuse acidity with ionic conductivity, while D contradicts the observation.

Electrical conduction in an aqueous solution requires mobile charged particles. Sugar dissolves as neutral molecules and does not ionise appreciably, so its solution has no sufficient ions to carry current. An ionic salt such as sodium chloride dissociates into Na⁺ and Cl⁻, which move toward opposite electrodes and conduct electricity. Therefore option A is correct; B, C, and D are chemically incorrect.

The chlor-alkali process electrolyses concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced and hydrogen gas is released: 2H₂O + 2e⁻ → H₂ + 2OH⁻. At the anode, chloride ions are oxidised to chlorine: 2Cl⁻ → Cl₂ + 2e⁻. Sodium hydroxide remains in solution. Hence option A gives the correct cathode–anode order; option B reverses it.

Electrolysis of brine produces hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in the solution. Hydrogen and chlorine can react explosively in the presence of suitable conditions, while chlorine can also react with sodium hydroxide to form hypochlorite or chlorate products. Separating the electrode products prevents contamination and unwanted reactions, so option A is correct. The other statements do not give the chemical reason.

On heating, sodium hydrogen carbonate decomposes as 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The carbon dioxide turns limewater milky, confirming the gas. The remaining solid, sodium carbonate, is a salt of a strong base and a weak acid; it hydrolyses in water and produces OH⁻ ions. Its solution is therefore basic, so option A is correct, not acidic or neutral.

Baking powder contains sodium hydrogen carbonate and a mild edible acid, often with a dry filler. In a dry container, the reactants remain largely separate and their reaction is very slow. When water is present in moist dough, the acid and bicarbonate dissolve and react, producing carbon dioxide. The gas bubbles expand during heating and make the dough rise. Thus option A is correct.

Dental enamel contains mineral crystals mainly based on calcium hydroxyapatite. Bacteria metabolise sugars and produce acids, lowering the pH in dental plaque. When the pH falls below about 5.5, the acidic environment favours dissolution of enamel minerals faster than repair can occur. This demineralisation increases tooth-decay risk, so option A is correct. The other options do not describe the acid–mineral process.

After food is eaten, bacteria in the mouth can break down sugars and produce acids. These acids lower the local pH and may dissolve minerals from tooth enamel, increasing the risk of decay. A mildly basic toothpaste reacts with and neutralises some of these acids, helping restore a safer pH. Therefore option A is correct. The other choices would damage teeth or describe chemically impossible effects.

The governing concept is acid–base neutralisation. Acidic soil contains excess hydrogen ions and therefore has a pH below 7. Slaked lime, calcium hydroxide [Ca(OH)₂], is a base that supplies hydroxide ions. These react with hydrogen ions to form water, reducing acidity and raising the soil pH. Thus option A is correct; the other options describe chemically impossible effects.

Highly basic soil has a pH above the range preferred by many crops. Organic matter decomposes through microbial activity and may release organic acids and carbon dioxide, producing a gradual moderating effect rather than an instant chemical change. This can improve nutrient availability and soil condition. Therefore option A is the best answer; the remaining choices are chemically incorrect or exaggerated.

Marble is mainly calcium carbonate, CaCO₃. The governing reaction is acid–carbonate neutralisation: an acid reacts with carbonate to produce a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated contact dissolves or erodes the marble surface. Hence option A is correct, while the other statements contradict its composition or the nature of acid rain.

Curd contains lactic acid, and pickles commonly contain acids such as acetic acid or citric acid. The governing principle is that acids can react with reactive metals, including copper, especially during prolonged contact. Such reactions may form soluble copper compounds that contaminate food and can be harmful in excess. Therefore option A is correct; the other options deny acidity or describe impossible transformations.

The pH scale classifies values above 7 as basic, so both pH 12 and pH 10 represent basic solutions. A decrease from 12 to 10 means the solution has become less basic, often because of dilution or addition of an acidic substance. Since pH 10 is still above 7, it has not become neutral or acidic. Therefore option A is correct; hydroxide concentration has decreased rather than greatly increased.