Acids, Bases, and Salts

Plaster of Paris is calcium sulfate hemihydrate, CaSO4·1/2H2O. It reacts with water and changes back into gypsum, CaSO4·2H2O, forming a hard solid mass. This setting reaction is useful when making casts, but unwanted moisture can make the powder set during storage and spoil it before use. Hence option A is correct; it does not become acid, sugar, or burn in moisture.

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.

Hydrochloric acid reacts with sodium carbonate in an acid–carbonate reaction. The balanced equation is 2HCl + Na2CO3 → 2NaCl + H2O + CO2. Thus sodium chloride is the salt, and water plus carbon dioxide are the other products. Carbon dioxide produces effervescence and turns limewater milky, providing a useful test. Therefore option A is correct; hydrogen, oxygen, chlorine, and ammonia are not formed in this reaction.

The governing concept is the acid–carbonate reaction. When an acid reacts with sodium hydrogen carbonate, the products are a salt, water and carbon dioxide: acid + NaHCO₃ → salt + H₂O + CO₂. The escaping carbon dioxide forms visible bubbles, producing effervescence. Hydrogen is not released, salt vapour is not formed, and freezing water cannot explain the observation. Therefore option A is correct.

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 concept is the aqueous definition of an acid. In water, an acid increases the concentration of hydrogen ions, more accurately forming hydronium ions, H₃O⁺. This explains acidic behaviour and indicator changes. Acids are not all safe; some are corrosive. Acids turn blue litmus red, not red litmus blue. Option D is also incorrect because acids are substances, not necessarily metals. Thus option A gives the scientifically correct general statement.

The governing concept is the basic behaviour of substances in aqueous solution. A soluble base dissociates or reacts with water to increase hydroxide-ion concentration, giving OH⁻ ions in the solution. This is associated with turning red litmus blue. Option B reverses the litmus result, while C is too absolute because some bases are soluble and D describes an acid-like taste, not a property of bases. Therefore A is the most correct statement.

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 colour change of an acid–base indicator. Methyl orange is red in an acidic solution and changes through an orange transition range toward yellow in an alkaline solution. Because the colour depends on the solution’s pH, it helps distinguish acidic and alkaline conditions during testing or titration. Option B describes its alkaline colour incompletely, while C and D give incorrect colours and interpretations. Therefore 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.

Blue litmus turning red is a standard indication that hydrogen ions are present and the solution is acidic. Phenolphthalein is colourless in acidic and neutral media, but it becomes pink in a sufficiently basic solution. Thus the two observations agree with an acidic solution. Basic and strongly basic options conflict with the litmus result, and “metallic” is not a solution nature identified by these indicators.

Sodium hydroxide is a base, so its initial pH is above 7. Adding hydrochloric acid supplies hydrogen ions that neutralise hydroxide ions: H⁺ + OH⁻ → H₂O. The pH therefore falls toward 7 near the equivalence point. If more acid is added after neutralisation, excess hydrogen ions make the solution acidic and the pH drops below 7. Hence A is correct.

Neutralisation depends on the amounts of reactive hydrogen ions and hydroxide ions, not simply on the measured volumes. Equal volumes may contain unequal concentrations, and weak or strong acids and bases may provide different effective ion amounts. For example, the same volume of 1 M acid and 0.1 M base will not neutralise completely. Therefore option A is correct; the other statements deny or misdescribe neutralisation.

Litmus is an acid-base indicator. A basic solution changes red litmus paper to blue because the indicator responds to the alkaline medium. Blue litmus turning red instead indicates an acid. Limewater becoming milky is associated with carbon dioxide reacting with calcium hydroxide, while a pop sound from a burning splint tests for hydrogen gas. Thus only option A is evidence of a possible base.

Lemon juice contains citric acid, while baking soda is sodium hydrogen carbonate, NaHCO₃. An acid reacting with a hydrogen carbonate produces a salt, water, and carbon dioxide: acid + hydrogen carbonate → salt + H₂O + CO₂. The carbon dioxide escapes as visible bubbles or fizz. No metal is formed, and the observation itself proves that a reaction occurs, so option A is correct.