Acids, Bases, and Salts

The governing concept is pH and the effect of acidity on living organisms. Acid rain introduces acidic substances into lakes, ponds and rivers, so the water’s pH may fall below its normal range. Many aquatic organisms, eggs and plants are sensitive to such changes, which can disturb biological processes. Therefore option A is correct; sweetness, permanent alkalinity and colour alone do not explain the harm.

The governing concept is dilution and the logarithmic pH scale. Adding water decreases the concentration of hydrogen ions in an acidic solution. Since pH = −log[H⁺], a lower hydrogen-ion concentration gives a higher pH. The value moves toward the neutral value 7, although it does not necessarily become exactly 7. Thus option A is correct; the other choices contradict dilution.

The governing concept is dilution of a base. When water is added, the concentration of hydroxide ions per unit volume decreases. Consequently, the solution becomes less strongly basic and its pH falls toward the neutral value 7. It need not become exactly 7 after a limited amount of water is added. Therefore option A is correct; dilution does not raise pH above 14, make it zero, or leave it invariably unchanged.

The governing concept is electrical conduction in solutions. An electric current requires charged particles that can move. In an aqueous solution, dissolved electrolytes may separate into mobile positive and negative ions; these ions carry charge in opposite directions when a potential difference is applied. Therefore option A is correct. Colour and smell are sensory properties, while sugar generally does not produce ions in water, so they do not explain conduction.

The governing concept is the difference between electrical neutrality and electrical conduction. Sodium chloride dissolves and dissociates in water into mobile Na⁺ and Cl⁻ ions. Equal positive and negative charges make the solution approximately neutral overall, but the ions can still move and carry current. Hence option A is correct. Being neutral does not mean being ion-free, so options B, C and D are wrong.

The governing concept is laboratory safety and the controlled identification of chemicals. Acids and bases can be corrosive, poisonous or irritating even in small amounts, so tasting an unknown substance may cause burns or poisoning. Safe tests include suitable indicators, pH paper and teacher-approved procedures. Thus option A is correct. Taste is not always wrong, but it is unsafe; the other statements are scientifically overgeneralized or irrelevant.

The governing concept is the acid–base behavior of indicators. Blue litmus turns red in an acidic medium, showing that the solution has an excess of hydrogen ions, represented in water as hydronium ions. Therefore option A is the best answer. Hydroxide ions indicate basicity, while sodium and chloride ions alone do not establish that the solution is acidic; colourlessness is also irrelevant to the test.

The governing concept is the characteristic response of acid–base indicators. A basic solution changes red litmus to blue, and turmeric develops a reddish-brown colour in the presence of a base. Since both independent observations point to alkalinity, option B is correct. An acid would produce the opposite litmus change, neutrality would not give these characteristic responses, and “gaseous” describes a state rather than the solution’s chemical nature.

The governing concept is the pH scale, on which 7 is neutral, values below 7 are acidic and values above 7 are basic at ordinary school-level interpretation. Because 5 is less than 7, the solution is acidic; option A is correct. It may contain a salt, so “salt-free” cannot be inferred from pH. It is neither neutral nor basic, making options B, C and D incorrect.

The pH scale indicates the acidic or basic nature of an aqueous solution: pH below 7 is acidic, pH 7 is neutral, and pH above 7 is basic. Since the given pH is 9, it is greater than 7, so the solution is basic and option B is correct. Option A describes a pH below 7, option C applies near pH 7, and volatility is a physical property unrelated to pH.

Neutrality in an aqueous solution means that acidic and basic effects are balanced; at about 25°C, the concentrations of hydrogen ions and hydroxide ions are approximately equal. On the usual school-level pH scale, this condition corresponds to pH 7. Therefore option B, seven, is correct. A pH of 3 is acidic, while 11 and 14 indicate basic solutions, so those distractors do not describe neutrality.

A universal indicator is a mixture of indicators that displays different colours over a range of pH values. A neutral solution has a pH of approximately 7, and the indicator colour associated with this middle point is green. Thus option B is correct. Red generally indicates strong acidity, while blue and violet indicate increasingly basic conditions; therefore those colours are not expected for a neutral solution.

Universal indicator changes colour according to the pH of a solution. Red occurs at the low-pH end of its colour range, showing a high concentration of hydrogen ions and therefore a strongly acidic nature. Hence option A is correct. Green is associated with near-neutral conditions, while blue or violet indicates basic conditions. The word salty describes taste or composition and is not a pH classification.

The universal-indicator colour scale progresses from red at low pH through green near pH 7 to blue and violet at high pH. Violet therefore represents a very high pH and a strongly basic solution. Option C is correct. Acidic and slightly acidic solutions have pH below 7 and appear toward the red or orange side, while a neutral solution is represented near green, so options A, B, and D are unsuitable.

An active metal reacts with an acid to form a salt and release hydrogen gas, for example: zinc + hydrochloric acid → zinc chloride + hydrogen. Hydrogen is tested by bringing a burning splint near the collected gas; it burns rapidly with a characteristic ‘pop’ sound. Therefore option B is correct. Limewater milkiness tests carbon dioxide, while turmeric and phenolphthalein indicate alkaline conditions rather than hydrogen.

When a carbonate reacts with an acid, the products are a salt, water, and carbon dioxide; in ionic form, carbonate ions react with hydrogen ions to produce CO₂ and water. Passing this gas through limewater forms insoluble calcium carbonate, which makes the solution milky. Thus option B is correct. Hydrogen gives a pop test, whereas oxygen and nitrogen do not produce the characteristic limewater reaction.

Limewater is a dilute solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The calcium carbonate formed is insoluble and appears as a white suspension, producing the milky appearance. Therefore option A is correct. The other substances listed do not form the characteristic precipitate responsible for this observation.

Initially, carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, causing milkiness. When excess CO₂ is passed, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble in water, so the solid precipitate disappears and the milkiness is removed. Hence option A is correct; the other choices do not explain this reaction.

The reaction between an acid and a base is called neutralisation. Hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions combine to form a salt. For example, HCl + NaOH → NaCl + H₂O. Therefore option A, salt and water, is correct. The other pairs are not the general products of an acid-base neutralisation reaction.

Neutralisation occurs when an acid reacts with a base. The acid supplies hydrogen ions, H⁺, while the base supplies hydroxide ions, OH⁻. These ions combine in the net ionic reaction H⁺ + OH⁻ → H₂O, producing water. Sodium and chloride ions may remain as spectator ions, while the other pairs do not directly form water in this process. Therefore, option A is correct.