Acids, Bases, and Salts

Indicators reveal the nature of a solution through characteristic colour changes. A basic solution turns red litmus blue because it provides an alkaline medium. It also changes yellow turmeric to reddish brown, which is another common test for a base. An acid would turn blue litmus red, while a neutral solution normally causes neither stated change. Therefore the two observations consistently support option B, basic.

Phenolphthalein is colourless in acidic and generally neutral conditions, but it becomes pink in a basic medium. Therefore, when base is added to an acidic solution in sufficient amount to make the final solution basic, the indicator changes from colourless to pink. Adding acid to a base would remove the pink colour as the medium becomes acidic. The salt choices do not necessarily create a basic solution, so option A is correct.

The pH scale expresses hydrogen-ion concentration on a logarithmic scale. Values below 7 indicate acidity, and a higher pH means a lower hydrogen-ion concentration and therefore weaker acidity. A rise from pH 3 to pH 6 represents a 1000-fold decrease in hydrogen-ion concentration, although the solution remains acidic because 6 is still below 7. Thus option A is correct, while option C is false.

The pH scale is inversely related to hydrogen-ion concentration: a lower pH means a greater concentration of H⁺ ions and stronger acidity. Both pH 2 and pH 5 are below 7, so both solutions are acidic, but pH 2 is lower by three units. That corresponds to 10³, or 1000 times, greater hydrogen-ion concentration under comparable conditions. Therefore option A is correct.

Acid strength is determined by the extent to which an acid ionizes in water and produces hydrogen ions, not by colour or by whether it contains oxygen. Hydrochloric acid is a strong acid and ionizes almost completely, whereas ethanoic acid is a weak acid and ionizes only partially at the same concentration. Consequently, HCl produces a higher concentration of H⁺ ions and is considered stronger. Hence option A is correct.

Hydroxide ions are responsible for the basic character of an aqueous solution. Sodium hydroxide is a soluble strong base, so it dissociates almost completely in water: NaOH → Na⁺ + OH⁻. Ethanoic acid produces H⁺ ions and is weak, sodium chloride is approximately neutral in water, and carbon dioxide water is acidic because carbonic acid forms. Therefore sodium hydroxide is expected to provide the greatest OH⁻ concentration, making option A correct.

Tooth enamel is mainly mineral and can dissolve when acids lower the pH in the mouth. Oral bacteria may produce acids from sugars, and enamel becomes increasingly vulnerable below the critical pH of about 5.5. Keeping the mouth above approximately 5.5 reduces the risk of demineralisation and tooth decay. The lower values in options B, C and D represent more acidic conditions, so option A is the correct threshold.

The governing concept is neutralisation. After food remains in the mouth, bacteria may break down sugars and produce acids. These acids can attack tooth enamel and contribute to tooth decay. Basic toothpaste contains mildly alkaline substances that react with and reduce the effect of the acids. Therefore option A is correct. The other options are incorrect because toothpaste does not convert teeth into acid, stop saliva completely, or protect teeth by depositing ordinary salt.

The governing concept is acid-base neutralisation. Acidic soil has an excess of acidic substances, so a suitable treatment should be mildly basic rather than another acid. Slaked lime, calcium hydroxide, is basic and can react with excess acidity, helping raise the soil pH toward a more suitable range. Thus option A is correct. Vinegar, lemon juice, and dilute hydrochloric acid are acidic, so adding them would generally increase rather than reduce soil acidity.

The governing idea is that soil treatment should counter the unwanted pH. Excessively basic soil can be improved by adding organic manure, because decomposition of organic matter produces acidic substances gradually and can lower the soil's basicity. Therefore option A is the best answer. Slaked lime, caustic soda, and sodium carbonate are basic or alkaline substances; adding them would not normally reduce basicity and could make the condition worse.

This is a neutralisation reaction between a base and an acid. The balanced equation is NaOH + HCl → NaCl + H₂O. The sodium ion comes from sodium hydroxide, while the chloride ion comes from hydrochloric acid; together they form sodium chloride, and hydrogen ions combine with hydroxide ions to form water. Hence option A is correct. The other salts require carbonate, calcium, or sulphate ions, none of which are present in the reactants.

The governing concept is the colour change of an acid-base indicator. Sodium carbonate is a salt of a strong base and a weak acid, so its aqueous solution is alkaline because carbonate ions react partly with water to produce hydroxide ions. Phenolphthalein is pink in a basic medium. Therefore option A correctly explains the observation. It is not caused by strong acidity, universal pink colour, or insolubility; sodium carbonate dissolves in water.

The governing concept is the nature of salts formed from strong acids and strong bases. Common salt is sodium chloride, produced by the reaction of hydrochloric acid, a strong acid, with sodium hydroxide, a strong base. NaCl does not appreciably hydrolyse in water, so its aqueous solution is nearly neutral and has pH close to 7 at ordinary conditions. Therefore option A is correct; the other choices contradict its composition or solubility.

The governing concept is thermal decomposition and the confirmatory test for carbon dioxide. On heating, sodium hydrogen carbonate decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide reacts with lime water, calcium hydroxide, to form insoluble calcium carbonate, producing a milky appearance. Thus option A is correct. Hydrogen, oxygen, and ammonia do not arise from this decomposition and do not give this characteristic lime-water test.

The governing concept is an acid-carbonate reaction. Baking soda, sodium hydrogen carbonate, reacts with the mild acid in baking powder when moisture is added and often releases carbon dioxide: acid + NaHCO₃ → salt + water + CO₂. The gas forms bubbles that expand during heating, making dough or batter rise and become porous. Therefore option A is correct. The reaction does not produce hydrogen or sodium metal, and the purpose is not to keep food basic.

The governing concept is the composition and acid-base reaction of baking agents. Baking soda is mainly sodium hydrogen carbonate, a single chemical compound that needs an external acid to release carbon dioxide efficiently. Baking powder is a mixture containing baking soda, one or more dry acidic substances, and commonly a starch-like filler. Moisture and heat allow the components to react and produce leavening gas. Hence option A is correct; the other statements confuse composition or falsely claim identity.

The governing concept is removal of hardness-producing ions by precipitation. Hard water contains dissolved calcium and magnesium ions, which react with soap to form scum and prevent lather. Washing soda, sodium carbonate, supplies carbonate ions that can form sparingly soluble calcium carbonate and magnesium carbonate, removing these ions from the water. Therefore option A is correct. It does not convert water into gas, generate oxygen, or soften water by making it acidic.

Bleaching powder contains reactive chlorine compounds that provide its bleaching and disinfecting action. On prolonged exposure to humid air and carbon dioxide, these active constituents may undergo unwanted reactions and gradually lose available chlorine. Therefore, the powder becomes less effective with time. Option A is correct; the other options describe impossible or scientifically unrelated changes. It should be stored in a tightly closed, dry container.

Drinking water can contain disease-causing bacteria and other microorganisms. Bleaching powder releases active chlorine, which disinfects water by damaging or killing many such microbes when used in an appropriate controlled amount. Thus option A correctly explains its use in water treatment. It does not sweeten water, convert it into milk, or remove dissolved sodium chloride; those claims are unrelated to disinfection.

Gypsum is calcium sulfate dihydrate, CaSO4·2H2O, so it contains water of crystallisation. Controlled heating removes about three-fourths of this water and produces Plaster of Paris, CaSO4·1/2H2O. If heating is excessive, the remaining water can also be removed and anhydrous calcium sulfate may form, which does not set properly with water. Therefore option A is correct.