Acids, Bases, and Salts

An acid reacts with a carbonate to produce a salt, water and carbon dioxide. For example, sodium carbonate with hydrochloric acid gives sodium chloride, water and CO₂. Passing this gas through limewater produces insoluble calcium carbonate, making the limewater milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Thus option A is correct. The other tests indicate acidic, basic or hydrogen-related behaviour, not carbon dioxide.

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 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 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.

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.

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.

Blue copper sulfate crystals are copper sulfate pentahydrate, CuSO4·5H2O. Their blue colour is associated with water of crystallisation. On heating, this water is removed and white anhydrous copper sulfate, CuSO4, is formed. The change is therefore a dehydration process, not a conversion of copper into gold or sulphate into a gas. Option A correctly identifies the governing concept and cause of the colour change.

Heating blue CuSO4·5H2O removes its water of crystallisation and produces white anhydrous CuSO4. When water is added again, the copper sulfate hydrates and water molecules become associated with its crystal structure, restoring the blue hydrated form. Thus option A is correct. Copper does not become a gas, sulphate is not destroyed, and no salt-to-sugar conversion occurs.

This experiment demonstrates the characteristic test for carbon dioxide. A metal carbonate reacts with an acid to produce a salt, water and CO₂. When the gas is bubbled through lime water, it reacts with calcium hydroxide and forms insoluble calcium carbonate, making the solution appear milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. The colour changes in B and C are unsupported, while D ignores gas formation. Hence A is correct.

The governing environmental concept is pH and its effect on aquatic ecosystems. Direct discharge of acidic waste increases the hydrogen-ion concentration of water, so its pH decreases. If the change is large, fish, aquatic plants and microorganisms may be harmed because their biological processes require a suitable pH range. The other options describe impossible or unrelated changes. Treatment and neutralisation of waste before discharge can reduce this danger, so A is correct.

The governing concept is the reaction of acids with carbonates. Marble consists mainly of calcium carbonate, CaCO₃. Acids formed from pollutants in rainwater react with this mineral, gradually dissolving or roughening the surface and producing salts, water and carbon dioxide. This chemical weathering erodes inscriptions and decorative details. Acid rain does not convert marble into iron or sugar, and cooling-related expansion is not the principal cause. Therefore option A is correct.

The governing classification is based on solubility in water. A base is a substance that can neutralise an acid, but only a base that dissolves in water is specifically called an alkali; examples include sodium hydroxide and potassium hydroxide. Thus every alkali belongs to the wider group of bases, whereas insoluble bases such as copper(II) oxide are not alkalis. Options B, C and D contradict the definition. Hence A is correct.

The governing concept is the association of common organic acids with their natural sources. Ant venom contains methanoic acid, also called formic acid, so option A is the correct match. Lemon is chiefly associated with citric acid rather than lactic acid; curd contains lactic acid rather than citric acid; and tartaric acid is commonly associated with tamarind or grapes, not tomato. Therefore only option A correctly pairs the acid and source.

The governing concept is the relationship between the properties of salts and their practical uses. Bleaching powder releases chlorine or active chlorine and is used to disinfect drinking water by killing many disease-causing microorganisms. Sodium chloride is common salt, not a material for bone casts; plaster of Paris is used for casts, not for raising food batter; and washing soda is used in cleaning and softening water, not for producing acid rain. Hence A is correct.

Sour foods commonly contain acids such as acetic, citric, or lactic acid. These acids can react gradually with copper or with components of brass, producing metal compounds that may contaminate the food and can be harmful if consumed. Therefore, glass, stainless steel, or food-safe containers are preferred. Options B and C describe impossible changes, while D is opposite to the chemical reactivity involved.