Acids, Bases, and Salts

The governing concept is that acidic behaviour requires ions in an aqueous medium. Hydrogen chloride molecules do not release mobile hydrogen ions merely because they are present as dry gas; in water they ionise, represented as HCl + H2O → H3O+ + Cl−. Dry litmus therefore shows no acidic colour change. Option A is correct. Litmus can respond to acids as well as bases, HCl is not basic, and the important missing condition is water rather than total chemical inactivity of litmus.

The governing concept is salt hydrolysis and the relative strengths of the parent acid and base. A salt made from a strong acid and a weak base contains a cation that hydrolyses water and produces hydronium ions; for example, ammonium chloride gives an acidic solution because NH4+ reacts with water. Therefore option A is correct. A strong-acid/strong-base salt is generally neutral, while a weak-acid/strong-base salt is generally basic. Colour cannot determine acidity.

The governing concept is electrical conduction by mobile ions and the distinction between strong and weak acids. At equal concentration, hydrochloric acid ionises almost completely in water, producing many H3O+ and Cl− ions. Ethanoic acid is weak and only partially ionises, so it supplies fewer charge carriers. Consequently HCl has higher conductivity, making option A correct. Equal labels as acids do not imply equal ionisation, and option B contradicts the stated equal concentration.

The governing reaction pattern is acid plus carbonate or hydrogencarbonate. In both cases, the acid supplies H+ to form carbonic acid, which decomposes into water and carbon dioxide: CO3^2− + 2H+ → H2O + CO2, while HCO3− + H+ → H2O + CO2. The remaining ions form a salt. Thus option A lists all main products; hydrogen, oxygen and nitrogen are not produced in these reactions.

The governing concept is neutralisation. A bee sting is commonly treated in school chemistry as introducing an acidic substance, so a mild, weakly basic material can partially neutralise it and reduce irritation. Baking soda, sodium hydrogen carbonate, is mildly basic and is therefore the suitable choice in option A. Lemon juice and vinegar are acidic, so they would not neutralise an acidic sting, while hydrochloric acid would add corrosive acidity rather than provide relief.

Phenolphthalein is a base-sensitive indicator: it is pink in a sufficiently basic solution and colourless when the solution is neutral or acidic. Sodium hydroxide supplies hydroxide ions, but added acid reacts with them in a neutralisation reaction to form water. As the hydroxide concentration and pH fall, the pink colour disappears. Therefore A is correct; acid does not create the colour, and neither solid sodium hydroxide nor gaseous decomposition is involved.

Dilution increases the volume while the amount of dissolved acid remains the same. Consequently, the concentration of hydrogen ions, written as [H+], decreases. Since pH is related to concentration by pH = −log[H+], a decrease in [H+] produces an increase in pH. The solution becomes less acidic, but it does not necessarily become basic. Thus A is correct; B reverses the effect and C and D are false.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)2. The commonly represented reaction is 2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O. Thus dry slaked lime is the required reactant. Sodium chloride is used to prepare brine, calcium carbonate is a limestone compound, and sodium hydroxide is not the standard material in this preparation. Therefore A is correct.

The chlor-alkali process electrolyses concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced more readily than sodium ions: 2H2O + 2e− → H2 + 2OH−. Therefore hydrogen gas is released at the cathode. Chlorine gas forms at the anode, while sodium hydroxide remains in solution; sodium metal is not deposited from this aqueous solution, and oxygen is not the main cathode product. Hence A is correct.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO4·5H2O. The attached water molecules are called water of crystallisation and give the crystals their blue appearance. On heating, this water is driven off, producing anhydrous copper sulphate, CuSO4, which is white. The change is not caused by release of oxygen, copper metal, or sulphur dioxide. Adding water can restore the blue hydrated form, so A is correct.

Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. It reacts with water to form gypsum, CaSO4·2H2O: CaSO4·1/2H2O + 1 1/2H2O → CaSO4·2H2O. This hydration makes the material set and harden, so unwanted moisture can spoil its storage and handling properties. It does not form an acid, release chlorine, or produce sodium carbonate. Therefore A is correct.

Indicator colour changes reveal the solution’s acid-base nature. Methyl orange is red in an acidic medium and changes toward yellow as the medium becomes less acidic or basic. Phenolphthalein is colourless in acidic and neutral conditions, becoming pink only in a sufficiently basic solution. Both observations are therefore consistent with an acidic solution. A is correct; a basic solution would usually make phenolphthalein pink, while “metallic” is not an aqueous acid-base classification.

A very low pH indicates that the soil is strongly acidic. Slaked lime, Ca(OH)2, is a basic substance, so it reacts with and neutralises excess acidic substances in the soil. This raises the pH toward a range more suitable for crop growth. The purpose is not to increase acidity, add ordinary salt, or remove water. Because the treatment is an acid-base neutralisation, option A gives the correct reason.

The stomach normally contains hydrochloric acid, which supports digestion, but an excess can irritate the stomach lining and cause discomfort. An antacid contains a mild base, such as a hydroxide, carbonate, or bicarbonate, which reacts with excess hydrochloric acid in a neutralisation reaction. The acidity is reduced rather than strengthened. It does not form chlorine or convert water into acid. Therefore option A correctly explains the relief.

Curd and many sour foods contain acids, such as lactic acid in curd. These acids can react with copper or with metals present in brass, causing corrosion and producing metal salts. Some such salts may be harmful if they dissolve into food and are consumed. The concern is therefore a chemical reaction and possible contamination, not sweetness, conversion into gold, or transformation into water. Hence option A is correct.

The red-litmus-to-blue change shows that the solution is basic. When an acid reacts with a base, the characteristic products are salt and water; heat is often released because the reaction is exothermic. This is therefore a neutralisation reaction, so option A is correct. Decomposition involves one compound breaking down, displacement involves replacement by another element, and precipitation specifically produces an insoluble solid.

Acid strength describes the extent to which an acid ionises in water. A strong acid ionises almost completely, whereas a weak acid ionises only partly. Concentration is different: it describes how much acid is present per unit volume of solution. Thus a dilute strong acid and a concentrated weak acid are both possible. Option A correctly separates these ideas; the other statements wrongly treat them as identical or absolute properties.

Baking soda is sodium hydrogen carbonate. It reacts with the edible acid in baking powder to form carbon dioxide, water, and a salt. The reaction is useful because carbon dioxide bubbles become trapped in the dough and make it rise, light, and soft. Option A is correct because the acid provides a controlled release of gas. It does not convert baking soda into metal, harden the dough, or produce chlorine.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating near 373 K, it loses one and a half molecules of water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Thus option A is correct; no calcium metal, chlorine, or sodium carbonate is produced.

A base is a substance that can neutralise an acid, but not every base dissolves appreciably in water. A base that dissolves in water produces hydroxide ions and gives an alkaline solution; such a soluble base is called an alkali. Therefore option A is the most correct. Option B confuses acids with bases, option C contradicts the existence of soluble bases, and option D ignores the colour change of indicators.

Sodium chloride is formed by neutralisation of hydrochloric acid, HCl, a strong acid, with sodium hydroxide, NaOH, a strong base: HCl + NaOH → NaCl + H₂O. The Na⁺ and Cl⁻ ions do not hydrolyse appreciably in water, so the solution remains nearly neutral with pH close to 7. Option A is correct. Salts from weak acids or weak bases generally produce acidic or basic solutions instead.

The pH scale describes the acidic or basic nature of an aqueous solution. A higher concentration of hydroxide ions, [OH⁻], makes the solution more basic. Because pOH decreases as [OH⁻] increases and pH + pOH = 14 at 25°C, the pH rises, generally above 7. Thus option A is correct. Option B describes increasing acidity, while C and D are not general consequences of adding hydroxide ions.

Electric current in a liquid requires mobile charged particles. When an acid dissolves in water, it ionises or dissociates to produce ions such as H₃O⁺ and a corresponding negative ion. These ions move between the electrodes and carry charge, so conductivity is evidence for their presence. Therefore option A is correct. Colour change, loss of mass, or freezing does not by itself prove that ions are present.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, commonly called brine. At the anode, chloride ions form chlorine gas; at the cathode, water produces hydrogen gas and hydroxide ions. The sodium ions combine with hydroxide ions to form sodium hydroxide in solution. Hence chlorine and sodium hydroxide are valid products, making option A correct. The other pairs belong to unrelated reactions or processes.

Limewater is a dilute solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is bubbled through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. Calcium carbonate is insoluble and forms a white suspension, making the limewater milky. This visible, characteristic change makes option A correct. The other options give products or colour changes that do not describe this identification test.