Acids, Bases, and Salts

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 chemical laboratory safety. Acids and bases can cause burns, damage materials, and react dangerously if they are mixed or mistaken for another substance. Separate, compatible containers with tight closures and clear labels identify the contents and reduce accidental contact or mixing. Therefore option A is correct. Mixing them may cause heat or splashing, a drinking bottle invites accidental ingestion, and removing labels removes essential safety information.

The governing concept is neutralisation and soil pH management. Acidic soil has a pH below the preferred range for some plants. Slaked lime, Ca(OH)2, is a basic substance; when applied in a suitable measured quantity, it neutralises some soil acidity and raises the pH. Hence option A is correct. Vinegar, lemon juice, and dilute acids are acidic, so adding them would generally increase rather than reduce soil acidity.

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 gas evolution and expansion in a food mixture. Sodium hydrogen carbonate reacts with an acid to release carbon dioxide, for example NaHCO3 + H+ → Na+ + H2O + CO2. During heating, the carbon dioxide forms and expands as many small bubbles trapped in the batter or dough. These bubbles increase the volume and create a lighter, softer texture, so option A is correct.

The governing concept is neutrality in an aqueous solution. Hydrogen ions are associated with acidity, while hydroxide ions are associated with basicity. When their effects are nearly balanced, neither acidic nor basic character dominates, so the solution is approximately neutral. At ordinary conditions, a neutral aqueous solution has pH about 7; therefore option A is correct. pH 0 and pH 2 indicate acidity, while pH 14 indicates strong basicity rather than neutrality.

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.

The governing concept is neutralisation and the pH scale. A base supplies hydroxide ions, which react with hydrogen ions from the acid: H⁺ + OH⁻ → H₂O. If only a small amount of base is added, it consumes only part of the available H⁺ ions. Their concentration therefore decreases and pH rises, but some acidity remains, so complete neutralisation has not occurred. Hence option A is correct.

The governing concept is that acid or base behaviour of many gases appears only in the presence of water. Moisture allows the gas to dissolve and form ions; those ions then interact with the litmus indicator and change its colour. Dry litmus lacks the water needed for this ion formation, so it remains unchanged. Therefore option A is correct. The other options incorrectly call the indicators defective or deny that the gas is a substance.

The governing concept is the different composition and uses of two sodium salts. Baking soda, sodium hydrogen carbonate (NaHCO₃), releases carbon dioxide on heating or reacting with an acid; the gas makes dough and cakes rise. Washing soda, sodium carbonate decahydrate (Na₂CO₃·10H₂O), removes calcium and magnesium ions and is used to soften hard water. Thus option A is correct.

The governing concept is chemical safety, which depends on a substance’s nature, strength, concentration, and exposure rather than simply whether it is an acid or a base. Concentrated sulphuric acid can cause severe burns, while strong sodium hydroxide can also damage skin and tissues. Some dilute acids and bases are safe in controlled uses, but that does not make every one harmless. Therefore option A is correct; the absolute claims in B, C, and D are false.

The governing concept is the use of characteristic tests for acids and bases. Magnesium is above hydrogen in the reactivity series and reacts with an acid to release hydrogen gas, for example: Mg + 2HCl → MgCl₂ + H₂. Separately, a solution that changes red litmus to blue is basic. Therefore the first solution is acidic and the second is basic, so option A is correct; the other choices reverse or ignore these observations.

The governing concept is the colour scale of a universal indicator. Green generally corresponds to a nearly neutral solution around pH 7, while yellow indicates a lower pH and therefore mild acidity. Adding acid increases the hydrogen-ion concentration and shifts the pH below neutral, producing the yellow colour. Hence option B is correct. The solution did not become more basic, remain unchanged, or turn solid, so A, C, and D do not fit the observation.

The governing concept is the relationship between pH and hydroxide concentration in a basic solution. A pH of 11 indicates alkalinity, so hydroxide ions are present in relatively high effective concentration. Bringing the pH closer to 7 means neutralising the base, usually by adding acid; hydrogen ions react with hydroxide ions according to H⁺ + OH⁻ → H₂O. Thus the effective amount of OH⁻ decreases, making option C correct. Chloride and nitrate ions do not determine this change.

The governing concept is that acids show their characteristic properties through ions in aqueous solution. Dry hydrogen chloride gas does not produce hydrogen ions without water, so dry litmus shows no acidic colour change. In moisture, HCl dissolves and ionises as HCl + H₂O → H₃O⁺ + Cl⁻. The hydronium ions turn blue litmus red, making option A correct; the other statements contradict acid behaviour.