Acids, Bases, and Salts

The governing concept is the ionisation of bases in water. A soluble base such as sodium hydroxide dissociates to produce Na⁺ and OH⁻ ions, and the presence of OH⁻ gives the solution its basic properties, such as turning red litmus blue. Therefore option A is correct. Hydrogen ions are associated with acidity, while hydroxide ions do not form metals or carbon dioxide in this context.

The governing safety principle is that dilution of a concentrated acid is highly exothermic. If a small amount of water is poured into a large quantity of acid, the released heat can rapidly heat and boil the water at the contact surface. Steam and acid droplets may then be thrown out, causing burns. Hence option A is correct. The safe method is to add acid slowly to water with stirring, never water to acid.

The governing concept is an exothermic change during acid dilution. When acid particles become surrounded by water molecules, energy is released as heat. Adding the acid slowly to a larger volume of water allows this heat to spread safely and prevents local overheating or splashing. Therefore option A is correct. The process is not defined by absorption of light, formation of sound, or zero heat change.

The governing reaction is metal–acid displacement. Zinc, being above hydrogen in the reactivity series, displaces hydrogen from hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. The gas released is hydrogen, and a sample gives a characteristic ‘pop’ when tested with a burning splint. Thus option A is correct. Oxygen is not released, chlorine is not produced, and carbon dioxide requires a carbonate or hydrogen carbonate reactant.

The governing rule is that a sufficiently reactive metal displaces hydrogen from an acid. In general form, metal + acid → salt + hydrogen gas. For example, Mg + 2HCl → MgCl₂ + H₂. Therefore option A is correct. Oxygen is not the usual product of this reaction, water is associated with acid–base neutralisation, and carbon dioxide is produced when an acid reacts with a carbonate or hydrogen carbonate.

The governing reaction is acid + metal carbonate → salt + water + carbon dioxide. Carbon dioxide is identified by passing it through limewater; it reacts with calcium hydroxide to form insoluble calcium carbonate, making the limewater milky. Hence option A is correct. A pop sound is the test for hydrogen, while litmus colour changes indicate acidic or basic solutions rather than specifically identifying this gas.

The governing pattern for a metal carbonate reacting with an acid is acid + carbonate → salt + water + carbon dioxide. For example, Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Thus option A lists all three products. Hydrogen belongs to the usual acid–metal reaction, whereas oxygen, a new metal, and a base are not expected products in this reaction.

The governing concept is gas evolution in an acid–hydrogen carbonate reaction. Sodium hydrogen carbonate reacts with an acid to form a salt, water, and carbon dioxide; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. The rapid escape of CO₂ bubbles produces effervescence. Therefore option A is correct. No gas freezes, water does not become solid, and the salt is not burning.

The governing concept is neutralisation and ionic exchange. In water, HCl provides H⁺ and Cl⁻ ions, while NaOH provides Na⁺ and OH⁻ ions. H⁺ combines with OH⁻ to form water, and the remaining Na⁺ and Cl⁻ ions form sodium chloride: HCl + NaOH → NaCl + H₂O. Therefore option A is correct; the other pairings do not describe this reaction.

The governing concept is neutralisation, in which the characteristic effects of an acid and a base are reduced by reaction with each other. Hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt: HCl + NaOH → NaCl + H₂O. Hence option A is correct. The reaction produces definite products; it is not merely a colour change and does not generally form a metal and gas.

An acidic solution has a pH below 7 because it contains a relatively high concentration of hydrogen ions. A base reacts with and neutralises some of this acidity. As base is added gradually, the hydrogen-ion concentration generally falls, so the pH rises toward 7. It may rise above 7 if excess base is added. Therefore, option A is correct; option B gives the opposite trend, while C and D are scientifically incorrect.

A basic solution normally has a pH greater than 7 because its hydroxide-ion concentration is relatively high. When acid is added, hydrogen ions react with hydroxide ions and reduce the solution’s basicity. Consequently, the pH generally decreases, first moving toward 7; with enough acid it can become less than 7. Thus option A is correct. The other options either reverse the trend or make unsupported absolute claims.

Excess hydrochloric acid in the stomach can produce a burning sensation and discomfort. An antacid contains a mild base, such as a suitable hydroxide, carbonate, or related basic compound. It reacts with some excess acid and neutralises it, reducing acidity without adding more acid. Therefore option A correctly states the main chemical action. The other choices describe gas control, water removal, or acid increase, none of which explains antacid neutralisation.

Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)2, which behaves as a weak or mildly basic substance in this context. Stomach acidity is associated with excess hydrochloric acid. Magnesium hydroxide reacts with and neutralises part of that acid, thereby reducing irritation. Hence option A identifies the relevant property. It is not a strong acid, and the statements about being merely a metal or not being a salt do not explain its antacid action.

A pH below 5.5 indicates that the mouth is sufficiently acidic to attack the mineral structure of tooth enamel. Acids can dissolve or weaken enamel, the hard protective outer covering of a tooth. Once enamel is damaged, decay can progress more easily. Therefore option A is correct. The distractors incorrectly assign the effect to a base, salt, or water and do not describe acid erosion of enamel.

Bacteria acting on food particles in the mouth can produce acids. These acids lower mouth pH and may gradually weaken tooth enamel. Mildly basic substances in toothpaste can neutralise or reduce some of this acidity, helping restore a less acidic environment during cleaning. Thus option A states the purpose correctly. Toothpaste is not intended to acidify teeth, convert them into salt, or stop saliva, so B, C, and D are unsuitable.

An ant sting commonly introduces formic acid into the skin, which contributes to irritation and a burning sensation. Baking soda, sodium hydrogen carbonate, is mildly basic. When applied appropriately as a dilute solution, it can react with and partially neutralise the acid, helping reduce irritation. Therefore option A is correct. The other choices either reverse the acid-base effect or mention unrelated changes involving skin acidity or carbon dioxide.

Acidic soil contains excess acidity, which can make conditions unsuitable for the healthy growth of some crops. Slaked lime is calcium hydroxide, Ca(OH)2, a basic substance. It reacts with acidic components in the soil and helps neutralise them, raising the soil pH toward a more suitable range. Therefore option A is correct. The other options describe changes that are unrelated to the agricultural use of lime.

Basic soil has an excess of alkaline character and therefore a pH higher than the desired range for some plants. During decomposition, organic matter can produce humic and other weak acidic substances. These substances can help lower excessive alkalinity and improve the soil’s chemical condition. Hence option A is the best answer. The remaining choices either increase basicity or describe unrelated changes involving metal formation or water removal.

In the chlor-alkali process, an aqueous solution of sodium chloride, called brine, is electrolysed. The products include sodium hydroxide in solution, chlorine gas at the anode, and hydrogen gas at the cathode. Thus option A names the correct industrial process. Rusting is corrosion, photosynthesis is a biological food-making process, and combustion is reaction with oxygen; none of these produces sodium hydroxide from brine.