Acids, Bases, and Salts

In the chlor-alkali process, aqueous sodium chloride (brine) is electrolysed. Chloride ions are discharged at the anode to form chlorine gas, water is reduced at the cathode to form hydrogen gas, and sodium hydroxide remains in the solution. Thus the three main products are chlorine, hydrogen, and sodium hydroxide. The other combinations belong to unrelated reactions.

The chlor-alkali process electrolyses brine. At the anode, oxidation occurs: chloride ions lose electrons and form chlorine gas, represented by 2Cl⁻ → Cl₂ + 2e⁻. Hydrogen is formed at the cathode by reduction of water. Oxygen and carbon dioxide are not the characteristic anode products in this process, so chlorine, option B, is correct.

In chlor-alkali electrolysis, the cathode is the site of reduction. Water molecules gain electrons there and produce hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine is released at the anode from chloride ions. Nitrogen and sulphur dioxide do not arise from brine electrolysis, so hydrogen is the correct answer.

Tasting a laboratory chemical is unsafe because acids and bases may be corrosive, poisonous, or irritating even in small quantities. Their taste is therefore not an acceptable identification method. A suitable indicator, pH paper, or another supervised laboratory test should be used instead. Option B is false because acids and bases are not always sweet; options C and D are also scientifically incorrect.

The governing concept is the reaction of an active metal with a dilute acid. Zinc displaces hydrogen from the acid, for example: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is combustible and, when tested with a burning splint, burns rapidly with a characteristic pop sound. Carbon dioxide extinguishes a flame, oxygen relights it, and chlorine has different bleaching and toxic properties. Therefore, option A is correct.

The governing concept is the acid–carbonate reaction and the limewater test. An acid reacts with a carbonate to form a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. When CO₂ is passed through limewater, it forms insoluble calcium carbonate, CaCO₃, which makes the solution milky. Hydrogen, nitrogen, and ammonia do not give this characteristic test. Therefore, option A is correct.

The governing concept is neutralisation. Most metal oxides are basic oxides, so they react with acids to produce a salt and water. For example, CuO + 2HCl → CuCl₂ + H₂O. This is not a metal-displacement reaction, so metal and hydrogen are not the general products. Carbon dioxide is usually associated with an acid reacting with a carbonate, not an ordinary metal oxide. Hence option A is correct.

The governing concept is the definition of an amphoteric oxide. If an oxide reacts with an acid as well as a base, it displays dual acid–base behaviour and is called amphoteric. Zinc oxide reacts with acids to form zinc salts and water, and with strong bases to form zincate species. Therefore, it cannot be classified as only acidic, only basic, or neutral on the evidence given. Option A correctly names the property proved by the observation.

The governing concept is neutralisation. Excess hydrochloric acid in the stomach can irritate the digestive system and contribute to indigestion. A mild base, such as the active ingredient in a suitable antacid, reacts with some of the excess acid and reduces its acidity, often producing salt and water. It does not create more acid, form metal, or convert water into gas. Therefore, the neutralisation explanation in option A is correct.

The governing concept is neutralisation. Bacteria can produce acids from food residues, and these acids attack tooth enamel. A basic toothpaste reacts with or neutralises some of this excess acid, helping the mouth move toward a less acidic pH and protecting enamel. Thus option A is correct. Toothpaste does not turn teeth into acid, remove all oral water, or fill teeth with carbon dioxide.

The governing concept is water of crystallisation. Anhydrous copper sulphate, CuSO4, is white because this water has been removed. When water is added, hydrated copper sulphate, CuSO4·5H2O, forms and the characteristic blue colour returns. Therefore option A correctly links the observed colour change with the presence of water; the other options do not describe this reversible hydration.

Washing soda is sodium carbonate decahydrate, Na2CO3·10H2O, and its aqueous solution is alkaline. Alkaline substances help loosen acidic or greasy dirt, while carbonate ions can assist cleaning by reacting with some contaminants. Thus option A is correct. It is not a strong acid, it is definitely a salt, and fragrance is not its cleaning function.

Baking powder supplies the acid–base reaction that releases carbon dioxide when the batter is heated. The gas becomes trapped in small bubbles, and their expansion gives the cake a lighter, raised structure. If too little baking powder is used, less carbon dioxide is generated and fewer bubbles expand. Therefore option A is correct; the other outcomes are unrelated.

Indicators provide the governing evidence for classification. A basic solution changes red litmus to blue, and phenolphthalein is pink in a basic medium. Since both independent observations indicate the same nature, the solution is basic and option B is correct. An acidic solution would turn blue litmus red and keep phenolphthalein colourless; a neutral solution would not produce these changes. Saltiness alone does not determine pH.

The question tests examples of naturally occurring acids. Lemon juice contains citric acid, which contributes to the sour taste of citrus fruits. Curd contains lactic acid, formed during the conversion of milk sugar by beneficial bacteria. Therefore option A correctly matches each food with its characteristic acid. Options B reverses the examples, while hydrochloric acid and sulphuric acid are not the normal acids responsible for these foods’ sourness.

The reaction between an acid and a base in suitable amounts is called neutralisation. The hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt. For example, HCl + NaOH → NaCl + H2O. Option A is therefore correct. This is not decomposition or a simple combination without salt; formation of salt and water is the defining result.

The discomfort from a bee sting is commonly treated in this school-level context as the effect of an acidic substance. Baking soda, sodium hydrogen carbonate, is a mild base. Applied appropriately, it can partially neutralise the acidic material and reduce irritation. Vinegar, lemon juice, and hydrochloric acid are acidic, so they would not neutralise the sting. Therefore option A is the best answer under the stated neutralisation principle; actual stings should still receive suitable first aid.

Stomach irritation can result when excess hydrochloric acid is present. Antacids contain mild basic substances, such as magnesium hydroxide or other suitable compounds, which react with and neutralise some of this excess acid. The reaction reduces acidity rather than strengthening it; it does not involve producing poisonous carbon monoxide or converting water into acid. Therefore option A correctly expresses the neutralisation principle behind relief from acidity.

Sodium hydroxide dissolves in water to give a strongly basic solution containing hydroxide ions. Bases change the indicator litmus from red to blue, so the red litmus paper becomes blue when it contacts sodium hydroxide solution. The change is an indicator response, not a gas-producing reaction; bubbles are unnecessary. A base does not normally make red litmus colorless or green. Thus option A is the only correct observation.

Turmeric contains a natural indicator substance called curcumin. This indicator remains yellow in an acidic or neutral environment but changes to reddish-brown in a basic environment. Soap solution is generally basic because its ingredients produce an alkaline solution in water. Therefore, applying soap to a turmeric stain causes the observed reddish-brown color. Option A gives the reason; the other choices do not describe the indicator reaction.

Limewater contains dissolved calcium hydroxide and is basic. Soap solution is also generally basic because soap hydrolysis produces an alkaline solution. Thus both substances in option A have basic nature. Lemon juice, vinegar, curd, and tamarind juice contain acids, while soda water contains dissolved carbon dioxide and is weakly acidic; orange juice is also acidic. Therefore none of options B, C, or D contains two bases.

The pH scale indicates the acidic or basic nature of an aqueous solution. A lower pH means a greater concentration of hydrogen ions, so the solution is more acidic; a higher pH indicates a more basic solution. Therefore, option A is correct. Option B reverses the pH relationship, while pure water is approximately neutral and contains ions from the slight ionisation of water.

At about room temperature, pure water is considered neutral because it produces hydrogen ions and hydroxide ions in equal concentrations through self-ionisation. This balance gives a pH close to 7. Thus option A is correct. A strong acid would have a pH well below 7, and a strong base would have a pH well above 7; pure water is not simply a salt solution.

Very acidic soil can interfere with nutrient availability and may harm plant roots. Slaked lime, calcium hydroxide, is a basic substance that reacts with and neutralises excess acidity in the soil. Hence option A is correct. Vinegar, lemon juice, and acid rainwater are acidic, so adding them would increase rather than correct the soil’s acidity.

Bacteria can ferment food sugars in the mouth and produce acids, which lower pH and contribute to enamel demineralisation. Mildly basic substances in toothpaste help neutralise some of this acid and support oral protection. Thus option A is correct. Toothpaste is not intended to transform teeth, remove all saliva, or produce carbon dioxide; those statements do not describe its relevant chemical action.