Acids, Bases, and Salts

On heating, sodium hydrogen carbonate undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The gas released is therefore carbon dioxide. It can be identified because it turns limewater milky, although this test is not required to select the answer. Oxygen, hydrogen, and chlorine are not products of this decomposition, so option B is correct.

Sodium hydroxide is a base. In water, it dissociates into \(Na^+\) and \(OH^-\) ions, so option B is correct. Hydrochloric acid forms \(H_3O^+\) ions in water, whereas sodium chloride mainly gives \(Na^+\) and \(Cl^-\) ions. Exam tip: a substance that gives \(OH^-\) ions in aqueous solution is a base.

The governing concept is salt hydrolysis and the strengths of the parent acid and base. Sodium chloride is produced from the strong acid HCl and the strong base NaOH. Neither ion hydrolyses water appreciably, so the solution is approximately neutral, with pH near 7. Sodium carbonate and sodium acetate are basic salts, while ammonium chloride is acidic because it comes from a weak base and a strong acid. Therefore, option A is correct.

The governing concept is neutralisation: an acidic substance is treated with a suitable base. Slaked lime, Ca(OH)₂, supplies hydroxide ions that react with excess hydrogen ions in acidic soil, reducing its acidity. In simplified form, H⁺ + OH⁻ → H₂O. Dilute hydrochloric acid and carbon dioxide would add to acidity, while sodium chloride is essentially neutral and is not the appropriate corrective material. Thus option A is correct.

The governing concept is the colour scale of a universal indicator. Around pH 7, the indicator appears green, showing a neutral solution. Acidic solutions have pH values below 7 and generally produce colours ranging from yellow to red, whereas basic solutions have pH above 7 and tend toward blue or violet. The colour does not merely prove that a salt is present. Therefore, the student’s statement should be corrected as in option A.

In the chlor-alkali process, brine (aqueous sodium chloride solution) is electrolysed. At the cathode, water is reduced to form hydrogen gas: \(2H_2O + 2e^- \rightarrow H_2 + 2OH^-\). Chlorine is produced at the anode, so option B is incorrect. Exam tip: reduction always takes place at the cathode during electrolysis.

The governing concept is acid-base neutralisation in an antacid. Magnesium hydroxide, Mg(OH)₂, is a mild base and reacts with excess stomach hydrochloric acid: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. This consumes acid and provides relief when used appropriately. Sodium chloride does not neutralise the acid, while ethanoic acid and lemon juice are acidic and would not be suitable antacids. Hence option A is correct.

The governing concept is the pH scale and the natural acidity of rainwater. Unpolluted rain is mildly acidic, with pH about 5.6, mainly because atmospheric carbon dioxide dissolves in water to form weak carbonic acid. Rain with pH below approximately 5.6 is commonly classified as acid rain, often due to stronger acids formed from sulfur and nitrogen oxides. Values 7, 10, and 14 do not represent this threshold, so option A is correct.

The governing concept is the relation between a salt’s solution and the strengths of the acid and base from which it is formed. Sodium chloride comes from strong HCl and strong NaOH. Its Na⁺ and Cl⁻ ions do not significantly react with water, so the solution remains nearly neutral, with pH close to 7. Sodium carbonate and sodium acetate hydrolyse to give basic solutions, while ammonium chloride gives an acidic solution. Therefore, option A is correct.

The governing concept is neutralisation or reduction of excess alkalinity in soil. During decomposition, organic matter can produce acidic substances and organic acids. These react with some basic components of alkaline soil and help lower its excessive basicity, improving soil conditions. Organic matter does not turn soil into metal, stop gas movement, or generally make an already basic soil more alkaline. Therefore, the scientifically appropriate answer is option A.

The governing concept is that hydrogen chloride shows acidic behaviour only after dissolving in water. Moisture on the litmus paper allows HCl gas to dissolve and ionise, producing hydrochloric acid and hydrogen ions: HCl(aq) → H⁺ + Cl⁻. These ions turn blue litmus red. Dry HCl gas cannot ionise effectively without water, so the word “moist” is essential. Hence option A is correct.

The governing concept is the colour change of an acid-base indicator. Methyl orange is red in an acidic medium and yellow in a neutral or basic medium. Therefore, when it is added to a basic solution, the observed colour is yellow. Pink and black are not the characteristic methyl-orange colours, and red indicates an acidic rather than a basic condition. Thus option B is the only correct answer.

The governing concept is safe handling of an exothermic dilution. Concentrated sulphuric acid releases a large amount of heat when mixed with water. The safe method is to add the acid slowly to a larger volume of water while stirring continuously, allowing heat to disperse and reducing the chance of local boiling and splashing. Adding water directly to concentrated acid can make the small water layer boil suddenly. Therefore, option A is correct.

Phenolphthalein is an acid–base indicator whose colour depends on the nature of the solution. In an acidic or neutral medium it remains colourless, but in a sufficiently basic medium it changes to pink. Therefore, the correct answer is option A, pink. Colourless, yellow, and blue do not represent the characteristic alkaline colour of phenolphthalein.

Litmus is an acid–base indicator that gives a characteristic colour change. When blue litmus paper turns red, the tested substance has an acidic nature, because acids change blue litmus to red. Thus option A is correct. A base generally turns red litmus blue, while a neutral solution normally causes no such acid-indicating change; metallic nature is not identified by this test.

Red litmus is used to test whether a substance is basic. A base changes red litmus paper to blue, so the observed colour change identifies an alkaline or basic nature. Therefore option A is correct. Acids show the opposite response by changing blue litmus red, whereas a neutral solution generally does not produce this characteristic change. The test does not indicate whether a substance is gaseous.

Turmeric contains a natural colour substance that acts as an indicator in alkaline conditions. Soap solution is generally basic, so it changes the yellow turmeric colour to reddish brown. Hence option A is correct. Blue is not the characteristic turmeric response to soap, while white and black are not the expected indicator colours. The observation demonstrates the use of turmeric as a natural acid–base indicator.

An olfactory indicator is a substance that helps distinguish acidic and basic media through a change or loss of smell rather than through a colour change. Onion extract is commonly taught as an example of this type of indicator. Therefore option A is correct. It is not a metal, salt, or heat indicator because those descriptions do not state the relevant smell-based property.

Vanilla extract is classified as an olfactory indicator because its smell can be used to distinguish acidic and basic conditions. The defining principle is detection through odour, not through a colour change or an electrical measurement. Thus option A is correct. Metallic, electrical, and hardness indicators describe unrelated properties and do not explain vanilla’s use in acid–base testing.

The pH scale is related logarithmically to hydrogen-ion concentration: pH = −log[H⁺]. Therefore, when pH decreases, the hydrogen-ion concentration increases and the solution becomes more acidic. Option A is correct. A lower pH does not make acidity zero or neutral; option B reverses the relationship, and option D describes pH near seven rather than a lower pH.