Acids, Bases, and Salts

On the usual pH scale, values below seven indicate acidity, seven is approximately neutral, and values above seven indicate basicity. As pH rises above the neutral point, hydrogen-ion concentration decreases relative to hydroxide-ion concentration, so basicity increases. Therefore option A is correct. Options B and C reverse the trend, while D applies only to a solution that remains near pH seven.

The pH scale classifies aqueous solutions relative to pH 7: values below 7 are acidic, about 7 are neutral, and values above 7 are basic. Since 4 is less than 7, the solution is acidic, so option A is correct. It cannot be neutral or basic, and “metallic” is not a pH classification for a solution’s acid–base nature.

For aqueous solutions, pH 7 is approximately neutral; a pH below 7 indicates acidity, while a pH above 7 indicates basicity. Because 9 is greater than 7, the solution is basic, so option A is correct. It is not acidic or neutral, and combustibility is a separate physical or chemical property that cannot be decided from pH alone.

The governing concept is the action of indicators: a base changes red litmus to blue, whereas an acid changes blue litmus to red. Since the given solution produces the red-to-blue change, it is basic and option B is correct. A neutral solution normally causes no colour change. Option D is also inconsistent because the question clearly describes a change in colour.

The governing concept is neutralisation. Lime materials such as calcium oxide or calcium hydroxide are basic, so they react with and reduce excess acidity in soil. This moves the soil pH toward a range more suitable for plant growth; therefore option A is correct. Making the soil more acidic is the opposite of the intended treatment, while removing all moisture or permanently raising temperature is not the chemical purpose of lime.

Vanilla essence is an olfactory indicator because its odour remains detectable in an acidic or neutral medium but is not detected in a basic medium. Litmus, phenolphthalein, and methyl orange are colour-change indicators, not olfactory indicators. Exam tip: an indicator identified by a change or loss of smell is an olfactory indicator.

The governing concept is that acidic behaviour in aqueous conditions depends on the formation of hydrogen ions, or hydronium ions, in water. A dry acidic gas has no water available to ionise it, and dry litmus cannot provide the aqueous medium needed for this ion formation. Therefore no colour change occurs and option A is correct. Litmus is an indicator, not a metal, and indicators do work when moisture is present.

The governing safety concept is controlled dilution of acid. Mixing acid with water is exothermic, so heat is released. The safe procedure is to add acid slowly to a larger quantity of water while stirring, allowing the heat to disperse; hence option A is correct. Adding water rapidly to concentrated acid can make a small volume heat suddenly and splatter. Hand mixing and using a hot vessel are unsafe.

The governing concept is the energy change of a process. A process that releases heat to its surroundings is called exothermic, so the described mixing of acid and water is an exothermic process and option A is correct. An endothermic process absorbs heat rather than releasing it. Decomposition means one compound breaks into simpler substances, and precipitation means formation of an insoluble solid; neither definition fits the observation.

The governing reaction pattern is metal plus dilute acid producing a salt and hydrogen gas. For example, zinc reacts with dilute hydrochloric acid as Zn + 2HCl → ZnCl₂ + H₂. Therefore hydrogen is released and option A is correct; it can be tested by bringing a burning splint near the gas, which gives a characteristic pop. Oxygen, chlorine, and nitrogen are not the general products of this reaction.

The governing reaction rule is that a sufficiently active metal displaces hydrogen from a dilute acid, forming a salt and releasing hydrogen gas. For example, zinc with hydrochloric acid forms zinc chloride and hydrogen: Zn + 2HCl → ZnCl₂ + H₂. Thus option A is correct. Oxygen is not the usual gas produced, carbon dioxide belongs to acid-carbonate reactions, and base plus water describes neutralisation rather than metal-acid reaction.

The governing reaction pattern is acid plus carbonate producing salt, water, and carbon dioxide. For example, a carbonate reacts with hydrochloric acid according to carbonate + acid → salt + H₂O + CO₂. Therefore option A is correct. The released carbon dioxide can be passed through limewater, which turns milky. Hydrogen is associated with many metal-acid reactions, while oxygen and ammonia are not general products here.

The governing concept is the reaction of a hydrogen carbonate, or bicarbonate, with an acid. It forms a salt, water, and carbon dioxide; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Hence option A is correct. The gas may be confirmed because it turns limewater milky. Hydrogen is generally produced when a reactive metal reacts with an acid, while chlorine and oxygen are not products of this reaction.

The governing concept is the limewater test for carbon dioxide. When carbon dioxide is bubbled through limewater, it reacts with calcium hydroxide to form insoluble calcium carbonate, making the solution appear milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Therefore option A is correct. Hydrogen, nitrogen, and oxygen do not normally produce this milky calcium carbonate precipitate under the test conditions.

The governing concept is neutralisation. When an acid reacts with a base, the acidic and basic properties reduce each other, and salt and water are generally formed. For example, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water. Therefore, option A is correct; combustion involves burning, displacement replaces an element, and melting is only a change of state.

According to the aqueous definition of a base, a substance that produces hydroxide ions in water shows basic behaviour. Sodium hydroxide dissociates as NaOH → Na+ + OH−, so option B is correct. Hydrochloric acid produces hydrogen ions, carbon dioxide forms carbonic acid in water, and sodium chloride separates into sodium and chloride ions without directly providing hydroxide ions.

An ant sting commonly introduces methanoic acid into the skin. Baking soda, sodium hydrogen carbonate, is a mild base; its alkaline nature can neutralise some of the acid and reduce irritation, so option C is the best choice. Lemon juice and vinegar are acidic and could increase irritation. Curd is not the standard neutralising treatment described by this concept.

The pH of acidic soil is increased by adding a basic substance that neutralises excess acidity. Calcium oxide is a basic oxide and reacts with acidic components to reduce their effect, so option A is correct. Sodium chloride is essentially a neutral salt, while ethanoic acid would add acidity. Carbon dioxide can form carbonic acid in moist soil and is therefore not the suitable choice.

An antacid is used to relieve excess acidity, especially the burning sensation caused by excess hydrochloric acid in the stomach. Most antacids contain mild bases, which react with or neutralise some of this acid and provide relief. Thus option A is correct. Tooth cleaning, rusting, and water purification are different processes and are not the primary purpose of an antacid.

Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)2. Magnesium hydroxide is a weak or mildly basic substance, so it can neutralise some excess hydrochloric acid in the stomach. Therefore option A is correct. It is not a strong acid, a neutral salt, or metallic magnesium; the name refers to the hydroxide suspension rather than the pure metal.