Acids, Bases, and Salts

The governing concept is neutralisation and pH correction. Acidic soil has an excess of acidic substances and therefore a relatively low pH. Slaked lime, calcium hydroxide, Ca(OH)₂, is a base; it reacts with acidic components and reduces their acidity. This raises the soil pH toward a more suitable range for plants. Thus option A is correct, while the other choices contradict basic chemistry or describe impossible changes.

The governing concept is adjustment of an unsuitable soil pH. Highly basic soil has a high pH and may not support every crop well. Decomposing organic matter, such as compost or manure, produces organic acids and other substances gradually. These can reduce excessive alkalinity and improve soil conditions over time. Therefore option A is the best answer. The process does not make soil metallic, produce chlorine, or generally raise an already high pH.

The governing concept is the reaction of acids with carbonates. Marble consists mainly of calcium carbonate, CaCO₃. Acids in acid rain react with this carbonate, producing a salt, water, and carbon dioxide; in general, acid + carbonate → salt + H₂O + CO₂. Repeated reaction removes or roughens the surface and causes deterioration. Therefore option A is correct; marble is not sugar and acid rain is not basic.

The governing concept is the ecological effect of pH change. Acid rain introduces acidic substances into lakes, lowering the water’s pH. Aquatic organisms function within limited chemical conditions; excessive acidity can damage tissues, disturb enzyme activity, affect reproduction, and alter the availability of dissolved minerals. Thus very low pH can disrupt life processes, making option A correct. It does not turn water into food or make it basic.

The governing concept is the reaction of acidic food with a reactive metal surface. Curd contains lactic acid and is therefore acidic. If it is stored for a long time in a copper vessel, the acid may react with copper and form copper compounds or salts that can contaminate the food. This is why acidic foods are not normally stored in copper or brass vessels. Hence option A is correct; curd is not acid-free and copper is not universally safe.

The governing concept is the colour response of acid–base indicators. Phenolphthalein becomes pink in a basic medium, while methyl orange is yellow in a neutral or basic medium and red in an acidic medium. Since both observed colours are consistent with a basic solution, option A is correct. An acidic solution would make methyl orange red, and insolubility is not a measure of pH.

The governing concept is neutralisation, in which an acid reacts with a base to form salt and water. Red methyl orange shows that the solution is acidic; colourless phenolphthalein is also consistent with an acidic or neutral medium. Therefore, adding a suitable base will consume excess hydrogen ions and move the solution toward neutrality. More acid would worsen the condition, while sugar and sodium chloride do not neutralise the acid.

The governing concept is the identification of bases by indicators. Turmeric changes to reddish-brown in a basic medium, and a base changes red litmus to blue. In an aqueous basic solution, hydroxide ions are present in excess relative to hydrogen ions. Thus option A is correct. Hydrogen ions would be associated with acidity, while chloride and nitrate ions are spectator ions and do not determine basicity by themselves.

The governing concept is the pH scale and neutralisation. Universal indicator is red in a strongly acidic range and green near pH 7. Adding a base consumes hydrogen ions from the acid, so acidity decreases and the pH moves toward neutrality. The change to green therefore indicates that neutralisation was nearly complete. The solution did not become more acidic or strongly basic, and a chemical reaction did occur.

The governing concept is acid–base neutralisation shown through pH colour changes. Blue indicates a basic solution, whereas green is characteristic of a nearly neutral solution around pH 7. Adding acid consumes some hydroxide ions and lowers the basicity, so the mixture approaches neutrality. Hence option A is correct. It did not become more basic, strongly acidic, or unchanged as the distractors claim.

The governing concept is neutralisation and the definition of pH. An acidic solution contains an excess of hydrogen ions. Added hydroxide ions react with them according to H⁺ + OH⁻ → H₂O, decreasing the hydrogen-ion concentration. Since pH rises as hydrogen-ion concentration falls, option A correctly explains the increase. The other choices either describe the opposite trend or unrelated processes.

The governing distinction is between a base and an alkali. A base reacts with an acid to form salt and water, so the described insoluble substance can be a base. An alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Because the substance is stated to be insoluble, it cannot be classified as an alkali. Therefore option A is correct; the other choices contradict the given reaction or add unsupported properties.

The governing concept is the neutralisation behaviour of metal oxides. A substance that reacts with an acid to produce salt and water removes the acid’s hydrogen ions and behaves as a base. Most metal oxides therefore show basic nature, so option A is correct in this school-level context. An acidic oxide would generally react with a base, while neutral, gaseous, or sweet properties do not follow from the stated reaction.

The governing reaction is neutralisation between a metal oxide and an acid. Copper oxide reacts with dilute hydrochloric acid to form copper chloride and water: CuO + 2HCl → CuCl₂ + H₂O. Dissolved copper(II) ions give the solution its characteristic blue-green colour. Therefore a copper salt forms; sugar, hydrogen gas, or water alone cannot explain the colour.

Hardness is mainly caused by dissolved calcium and magnesium ions. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions that react with them to form insoluble precipitates such as calcium carbonate and magnesium carbonate. These precipitates can be separated, so the concentration of hardness-causing ions decreases. It does not acidify water, produce hydrogen gas, or destroy sodium ions.

The governing concept is the sequence of human nutrition. First, ingestion brings food into the alimentary canal. Digestion breaks complex substances into simpler soluble molecules; absorption transfers them into the blood, and assimilation uses them in cells. Finally, egestion removes undigested material from the body. Hence option A gives the correct order. The other choices mix unrelated processes or place nutrition stages in the wrong sequence.