Acids, Bases, and Salts

Red cabbage juice contains pigments called anthocyanins, whose colour changes in acidic and basic media. Because this visible change can indicate the nature of a solution, the juice is used as a natural acid-base indicator. Thus option A is correct; it is not a metal, a salt itself, or a fuel.

Onion is commonly described as an olfactory indicator because its smell helps indicate the medium. In a basic substance, the characteristic onion smell becomes weak or may disappear, so option A is correct. A smell cannot become blue, turn into a solid, or necessarily increase greatly merely because the medium is basic.

Vanilla essence is used as an olfactory indicator in school-level acid-base tests. Its characteristic smell becomes faint or disappears in a basic solution, making option A correct. The other choices confuse smell with colour or with a precipitate: a smell does not become red or blue, and this observation does not mean that a solid precipitate forms.

Litmus is an acid-base indicator. An acidic substance changes blue litmus paper to red, whereas a base changes red litmus to blue. Since the question describes the blue-to-red change, the substance has acidic nature and option A is correct. Neutral substances show no characteristic litmus change, and metallic is not an acid-base classification here.

Red litmus is an indicator used to recognise a basic medium. A base changes red litmus paper to blue, while an acid produces the opposite blue-to-red change. Therefore the substance described has basic nature, so option A is correct. Neutral solutions normally do not cause this characteristic change, and gaseous describes a state rather than acid-base nature.

Hydrochloric acid, HCl, releases hydrogen ions in aqueous solution and therefore shows acidic behaviour. Acids turn blue litmus paper red, so option B is correct. Sodium hydroxide and lime water are bases and turn red litmus blue. Sodium chloride solution is generally neutral in this school-level comparison and does not produce the stated blue-to-red change.

The pH scale describes hydrogen-ion concentration: values below 7 indicate an acidic solution, 7 is neutral, and values above 7 indicate a basic solution. Since pH 2 is well below 7, the solution is acidic, making option A correct. Basic and neutral do not match this value; salty describes a composition, not a pH classification.

On the pH scale, 7 represents neutrality. A value greater than 7 indicates a basic or alkaline solution, while values below 7 indicate acidity. Because pH 12 is greater than 7, the solution is basic and option A is correct. It is not acidic or neutral, and gaseous refers to physical state rather than chemical nature.

Acidity increases as pH decreases because a lower pH represents a greater hydrogen-ion concentration. Comparing the two values, pH 3 is lower than pH 6; therefore the pH 3 solution is more acidic. Option A is correct. The solutions are not equally acidic, and both are acidic rather than neither being acidic.

The governing concept is the pH scale. For aqueous solutions, a higher pH indicates a lower hydrogen-ion concentration and greater basicity. Since 11 is higher than 8, the solution with pH 11 is more basic, so option A is correct. The solutions are not equally basic, and neither can be rejected merely because both values lie on the basic side of the scale.

The governing concept is the Arrhenius description of acids. When an acid dissolves in water, it increases the concentration of hydrogen ions, commonly represented as H⁺; therefore option A is correct. In water, these ions are associated with water molecules as hydronium, H₃O⁺. Hydroxide ions are characteristic of bases, while sodium and calcium ions are not produced by every acid.

According to the Arrhenius concept, a base produces hydroxide ions, OH⁻, when dissolved in water. These ions are responsible for the alkaline nature of the solution and can cause indicators such as red litmus to turn blue. Hydrogen ions are associated with acids, whereas chloride and nitrate ions may occur in salts but do not define a base. Therefore, option A is correct.

Hydrogen chloride shows acidic behaviour when it dissolves in water and ionises to form hydrogen ions, commonly represented as H₃O⁺ in aqueous solution. Dry HCl gas has no water in which this ionisation can occur, and dry litmus also has no moisture for the colour-changing reaction. Therefore, dry HCl does not change dry litmus. The gas is not basic or metallic, so option A is correct.

Dilution of a concentrated acid with water is exothermic, meaning that considerable heat may be released. The safe laboratory procedure is to add the acid slowly to a larger volume of water while stirring, allowing the heat to spread through the water. Adding water directly to concentrated acid can make the small amount of water boil suddenly and splash corrosive acid. Thus, option A is correct.

Sodium hydroxide, NaOH, dissociates in water to form Na⁺ and OH⁻ ions. The hydroxide ions make the aqueous solution basic, and a basic solution changes red litmus paper to blue. Hydrochloric acid and carbon dioxide produce acidic solutions, while sodium chloride solution is approximately neutral. Therefore, sodium hydroxide is the only suitable answer, making option B correct.

Litmus is an acid-base indicator. Blue litmus turns red when it comes into contact with an acidic solution, so the observed colour change shows that the solution has acidic character. A basic solution would turn red litmus blue, and a neutral solution would normally produce no change. The observation cannot prove that the solution is a saturated salt solution. Hence, option A is correct.

An acid reacting with a metal carbonate follows the general pattern: acid + metal carbonate → salt + water + carbon dioxide. For example, hydrochloric acid reacts with calcium carbonate as 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. The released carbon dioxide turns limewater milky. Hydrogen is not the characteristic gas in this reaction, so option A is correct.

Sodium hydroxide dissociates in water to produce hydroxide ions, OH⁻. The presence of these ions gives the solution basic properties, including the ability to turn red litmus paper blue. Hydrochloric acid is acidic and would show the opposite litmus behaviour. Sodium chloride solution and distilled water are approximately neutral, so they normally do not produce this colour change. Therefore, option A is correct.

The standard test for carbon dioxide uses lime water, which is an aqueous solution of calcium hydroxide. When carbon dioxide is bubbled through it, calcium carbonate forms and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Vinegar is acidic, while oil and salt water do not give this characteristic test. Therefore, option A is correct.

Blue litmus turning red is the characteristic indicator response of an acidic solution. Acids provide hydrogen ions in water, which cause this colour change. A basic solution generally turns red litmus blue, while a neutral solution produces no litmus change. The observation does not prove that the liquid contains only salt, because salts may be acidic, basic, or neutral. Thus option A is correct.

Neutralisation is the reaction between an acid and a base. 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. Therefore, salt and water are the general products, making option A correct; the other choices do not describe this reaction pattern.

Excess hydrochloric acid in the stomach can cause irritation and a burning sensation. An antacid contains a mild base, such as magnesium hydroxide, which reacts with and neutralises some of the excess acid. This raises the pH toward a less acidic value and provides relief. A strong acid would worsen acidity, while common salt and a metal are not suitable antacids. Hence option A is correct.

Milk of magnesia is a suspension containing magnesium hydroxide, Mg(OH)₂, a weak base. In the stomach it reacts with excess hydrochloric acid and reduces its acidic effect through neutralisation: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. It does not remove excess water, oxygen, or salt. Therefore, option A is correct.

Tooth enamel is mainly composed of calcium compounds and can be attacked when the mouth becomes sufficiently acidic. A mouth pH below about 5.5 is generally regarded as harmful because acid dissolution of enamel becomes significant. A pH of 7 is neutral, whereas 10 and 14 are alkaline values. Therefore, the accepted threshold in this school-level context is 5.5, so option A is correct.

Bacteria can produce acids from sugars left in the mouth after eating. These acids lower the local pH and may gradually damage tooth enamel. Mildly basic toothpaste helps neutralise or reduce the effect of these acids without being strongly caustic. It is not intended to make teeth acidic, colour the tongue, or convert saliva into gas. Thus option A correctly states its purpose.