Acids, Bases, and Salts

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 concept is that neutralisation depends on the reacting amounts of H⁺ and OH⁻, not simply on the measured volumes. Equal volumes can contain different concentrations, and acids or bases can differ in strength and degree of ionisation. If the acid supplies more effective H⁺ than the base can remove through OH⁻, excess acid remains and the mixture stays acidic. Therefore A is the best reason.

The governing concept is the stoichiometry of neutralisation. Equal volumes do not necessarily contain equal numbers of reactive particles because concentration and strength may differ. If the base provides more effective OH⁻ ions than the acid can neutralise with H⁺ ions, hydroxide remains after mixing and the final solution is basic. Thus option A is correct. Equal volume alone fixes neither pH nor neutrality, and neutralisation does produce water.

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 concept is the acid–base reaction. Acids react with bases to produce salt and water, and many non-metal oxides show the same behaviour when they react with bases. For example, carbon dioxide reacts with sodium hydroxide to form sodium carbonate and water. Thus the oxide has acidic nature; it is not basic, metallic, or completely neutral.

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 idea is that colour alone is not a reliable chemical test. Many acids and bases, such as dilute hydrochloric acid and sodium hydroxide solution, are colourless, while the colour of a substance may come from another dissolved chemical. An indicator, universal indicator, or pH paper should be used because its colour change is linked to acidity or basicity. Therefore option A is correct.

The governing principle is laboratory safety. Unknown acids and bases may be corrosive, poisonous, or harmful even in small amounts, so tasting them can cause burns, poisoning, or internal injury. Their identity and pH must instead be tested with suitable indicators, pH paper, or approved laboratory procedures. Therefore option A is correct; the other choices make false claims about sweetness, colour, or the accuracy of taste.

Blue litmus turning red is the standard indication of an acidic solution. Deep red on pH paper generally corresponds to a very low pH, indicating strong acidity rather than neutrality or basicity. Taken together, the observations support the conclusion that the solution is strongly acidic. The result does not prove that it contains only one substance or only salt, so option A is the safest and best answer.

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.

A base changes red litmus paper to blue, so option A is correct. Litmus is a natural indicator that shows different colours in acidic and basic substances. Red litmus turns blue in a basic medium; it does not turn green or yellow. Exam tip: bases turn red litmus blue, whereas acids turn blue litmus red.

A pH greater than 7 generally indicates a basic (alkaline) solution. A pH of 7 is neutral, whereas a pH below 7 indicates an acidic solution. “Salty” only suggests the possible presence of a salt; it does not necessarily mean that the pH is greater than 7. Exam tip: Use 7 as the reference point on the pH scale—below it is acidic and above it is basic.

A pH below 7 indicates a relatively higher concentration of hydrogen ions
(H^+
), so the solution is acidic. A pH of 7 is neutral, whereas a pH above 7 is basic. “Saline” may indicate dissolved salts, but it does not by itself determine whether a solution is acidic or basic. Exam tip: remember 7 as the midpoint of the pH scale—below it is acidic and above it is basic.

On the pH scale, 7 is neutral. A solution with pH below 7 has acidic properties, so it is an acidic solution. Basic solutions have pH above 7, whereas a neutral solution has pH 7. Exam tip: remember pH < 7 as acidic, pH = 7 as neutral, and pH > 7 as basic.

On the pH scale, a value of 7 indicates a neutral solution, so neutral is correct. Pure water has a pH of about 7; it is neither acidic nor basic. Basic solutions have pH values greater than 7, whereas acidic solutions have pH values below 7. Exam tip: remember that pH 7 is neutral, below 7 is acidic, and above 7 is basic.

Litmus is an acid–base indicator. In an acidic solution, blue litmus changes to red, so option A is correct. This colour change occurs because the indicator responds to the acidic conditions. A base generally changes red litmus to blue, while green and yellow are not the standard litmus result for this test. Thus the blue-to-red rule identifies an acid.