Acids, Bases, and Salts

Acidic properties in aqueous solution arise from hydronium ions, H₃O⁺, formed when an acid transfers a proton to water. Hydrogen chloride ionises in water: HCl + H₂O → H₃O⁺ + Cl⁻. These mobile ions account for acidic indicators, electrical conduction and reactions with bases. Dry HCl gas does not show the same aqueous acidic behaviour because water is needed for ion formation. Therefore option A expresses the correct reason; the other choices are chemically incorrect.

Electrical conduction in an acid solution depends on mobile ions, not simply on the presence of acid molecules. Water provides the medium in which many acids ionise and in which ions can move through the solution. With very little water, ionisation and ionic mobility may be reduced, so the measured conductance can be low; a very viscous or poorly hydrated medium also limits movement. The acid is not inherently always non-conducting, and colour or container material is not the governing reason. Thus A is correct.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so its crystals contain water of crystallisation. When exposed to dry air, some of this water may be lost through efflorescence. The hydrated crystals then lose their clear crystalline appearance and may become opaque, crumbly, or powder-like. Therefore, option A is correct; no hydrogen gas, copper sulphate, or acid is formed.

Sodium hydroxide is a strong base, while the residue in an acid bottle may contain an acid. If the two come into contact, neutralisation occurs: acid plus base forms salt and water, and the process releases heat. A concentrated mixture may heat rapidly and cause splashing, burns, or damage to the container. Thus option A identifies the real laboratory hazard; the other choices are chemically false.

A very low pH indicates that the soil is acidic. Lime, commonly containing calcium carbonate or calcium oxide, is basic and is added to neutralise excess acidity. However, the amount must be controlled. If too much lime is applied, the neutralisation can go beyond the suitable range and raise the pH excessively, making the soil alkaline. Therefore option A is correct; the other choices do not describe this reaction.

Dental plaque contains bacteria that metabolise sugars left on the teeth. During this process they produce organic acids. The added hydrogen ions lower the pH at the tooth surface. If the pH remains sufficiently low, minerals dissolve from enamel and decay can begin. Hence option A is correct. Teeth do not manufacture bases, water does not become hydrogen gas, and toothpaste is generally used to help reduce acidity rather than increase it.

Carbonate rocks such as limestone contain carbonate ions, which react with acids. In a simplified reaction, an acid and a carbonate produce a salt, water, and carbon dioxide; for example, carbonate minerals can slowly dissolve in acidic rainwater. Repeated dissolution weakens and erodes the rock surface. Therefore option A is correct. Acid rain does not strengthen all carbonates, and its effects are not limited to iron.

At about 25°C, a neutral aqueous solution has pH 7 because hydrogen-ion and hydroxide-ion concentrations are equal. A pH value above 7 indicates that hydroxide ions predominate relative to hydrogen ions, so the solution is basic. A solution of pH 8 is therefore only mildly alkaline, not neutral. Option A states this rule correctly; pH 8 is not a strong acid, and neutral pH is not 14.

Diluting a concentrated acid is exothermic: heat is released when acid mixes with water. If a small amount of water is poured into a larger amount of concentrated acid, that water can heat suddenly, boil locally, and eject acid droplets. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring and wearing protection. Therefore option A identifies the mistake and its hazard.

The governing concept is the characteristic test for hydrogen. When a reactive metal reacts with an acid, hydrogen gas is commonly released, for example: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is highly combustible and, on bringing a flame near the collected gas, burns rapidly with a distinct pop sound. Carbon dioxide would extinguish a flame, while nitrogen is not combustible. Therefore, option A is correct.

Heating baking soda, sodium hydrogen carbonate, produces sodium carbonate, carbon dioxide, and water: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate is a salt of a strong base and a weak acid; its aqueous solution undergoes hydrolysis and contains excess hydroxide ions. Hence the solution, and the substance in the school-level sense asked here, is basic. Option A is correct; it is not acidic, neutral, or a description of metallic nature.

The governing concept is water of crystallisation: a fixed number of water molecules incorporated into the crystal structure of some salts. On heating, this water may be released as droplets or vapour, and removal can alter the crystal structure and colour. For example, hydrated copper sulphate is blue and becomes white on losing water. A salt cannot be defined as ion-free or always gaseous. Thus option A is correct.

Two observations support the same conclusion. Blue litmus turning red indicates an acidic solution. Chalk is mainly calcium carbonate, CaCO₃. Acids react with carbonates to produce a salt, water, and carbon dioxide: acid + CaCO₃ → salt + H₂O + CO₂. The escaping CO₂ causes bubbles. Hydrogen and oxygen are not the expected gases in this reaction, so option A is correct.

Calcium hydroxide, Ca(OH)₂, is a base and can react with an acidic oxide such as carbon dioxide. For example, Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; calcium carbonate is the salt formed, and the limewater may turn milky. Vinegar, hydrochloric acid, and lemon juice are acidic substances, so they do not normally neutralise an acidic gas to produce a salt in this context. Hence option A is correct.

On the usual school pH scale, values above 7 indicate basic solutions, while 7 is neutral. A pH of 11 therefore represents a basic solution. Adding water dilutes the hydroxide ions, reducing their concentration and moving the pH toward 7. Although the pH falls to 9, it remains above 7, so the solution is still basic but less strongly basic. Thus option A is correct; it has not become acidic or neutral.

An acidic solution contains a relatively high concentration of hydrogen ions, H⁺, and its pH is low. Added hydroxide ions react with hydrogen ions in the neutralisation reaction H⁺ + OH⁻ → H₂O. This removes free H⁺ ions from the solution, so the hydrogen-ion concentration decreases and pH rises. The solution may eventually approach neutrality, depending on the amounts added. Options B, C, and D do not describe this reaction. Therefore A is correct.

The governing concept is the reaction of carbon dioxide with lime water, which is aqueous calcium hydroxide. Initially, carbon dioxide forms insoluble calcium carbonate, Ca(OH)2 + CO2 → CaCO3 + H2O, producing milkiness. When carbon dioxide is passed in excess, the precipitate reacts further to form soluble calcium hydrogen carbonate, CaCO3 + CO2 + H2O → Ca(HCO3)2. The precipitate dissolves, so option A is correct; the other conditions do not cause this change.

The governing concept is bleaching by oxidation. Bleaching powder releases available chlorine, which forms oxidising substances in the presence of moisture. These substances react with many coloured compounds and destroy or alter their colour, making the fabric appear white. Therefore option A is correct. Bleaching powder does not convert cloth into a metal, increase its mass as its useful action, or transform water into sugar; those choices do not describe bleaching chemistry.

The governing concept is that acids react differently with metals and carbonates. Magnesium reacts with an acid to produce hydrogen, for example Mg + 2HCl → MgCl2 + H2. Sodium carbonate reacts with the acid to form a salt, water, and carbon dioxide: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. Thus the second gas is carbon dioxide, so option A is correct; hydrogen belongs to the magnesium reaction, while oxygen and ammonia are not products here.

The governing concept is ionisation in aqueous solution. Electric current in a solution is carried by mobile ions, not by neutral molecules alone. Water helps an acid ionise or dissociate to produce hydronium ions and corresponding anions; these charged particles move and conduct electricity. Therefore option A is correct. The observation contradicts the claim that acids never form ions, and colour or solid state does not explain the increased conductivity.

The governing concept is the pH scale, which indicates hydrogen-ion concentration: pH = −log[H+]. Lower pH values correspond to greater hydrogen-ion concentration and stronger acidity. Therefore pH 0 represents a highly acidic solution, not the absence of acid, so option A is correct. Neutrality is approximately pH 7, while pH values above 7 generally indicate basic solutions; pH is used for both acids and bases.

The governing concept is the behaviour of salts in water and their reactions with acids. Sodium carbonate dissolves to give a basic solution because carbonate ions hydrolyse water and produce hydroxide ions. With an acid, it releases carbon dioxide: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. Thus option A satisfies both conditions. Sodium hydroxide is basic but does not release CO2 with acid, while sodium chloride and copper sulphate do not meet both requirements.

The governing concept is the reactivity of metals with dilute acids. A metal above hydrogen in the reactivity series can generally displace hydrogen from an acid, producing hydrogen gas; for example, magnesium reacts with hydrochloric acid. Metals below hydrogen, such as copper, usually do not release hydrogen in this way. Thus option A is correct. Options B, C, and D are false because metals differ in reactivity, colour is irrelevant, and some metals do react with acids.

The governing concept is the identification of bases through their properties and ions in aqueous solution. A base changes red litmus to blue and often feels slippery or soapy, indicating the presence of hydroxide ions, OH⁻, in water. Therefore option A is correct. Hydrogen ions are associated with acidic solutions and would turn blue litmus red. Chloride ions alone are not responsible for basicity, and carbonate is not always the principal ion in every base.

The governing concept is the chlor-alkali process. Common salt, sodium chloride (NaCl), is obtained abundantly from seawater and dissolved in water to form brine. Electrolysis of brine produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in solution: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Therefore option A is correct; the other salts do not give this chlor-alkali combination.