Acids, Bases, and Salts

Methyl orange is a synthetic acid-base indicator with a transition range in which it appears red in an acidic medium and yellow in a basic or sufficiently alkaline medium. Therefore option B matches both observations. Phenolphthalein is colourless in acid and pink in base; blue litmus changes to red in acid but does not become yellow in base; turmeric usually changes from yellow to reddish-brown in a base.

The governing laboratory-safety principle is never to taste an unknown chemical. Acids and bases may be corrosive or poisonous, and even a small amount can injure the mouth or internal organs. Identification should instead use safe tests such as litmus, a universal indicator, or pH paper, following proper instructions. Thus option A is correct; taste cannot provide an exact pH and indicators are specifically designed for this purpose.

Blue litmus turning red establishes that the unknown solution is acidic. Acids react with zinc, an active metal, to form a salt and hydrogen gas: acid + Zn → salt + H2. The same acid reacts with sodium carbonate to form a salt, water, and carbon dioxide: acid + carbonate → salt + H2O + CO2. Therefore option A correctly identifies the two gases.

Use the definition pH = −log[H+]. If the hydrogen-ion concentration decreases by a factor of 100, that is a decrease of 10², so the pH increases by 2 units. Starting from pH 3, the new value is 3 + 2 = 5. Therefore option B is correct. A decrease in hydrogen-ion concentration cannot lower the pH to 1, while the other listed values imply a much larger change.

Blue litmus turning red means the salt solution is acidic. In the usual school-level classification, salts of strong acids and weak bases undergo hydrolysis that increases the hydrogen-ion concentration, producing an acidic solution. Hence option B is the most likely combination. A strong acid–strong base salt is generally neutral, while a weak acid–strong base salt is generally basic; weak–weak salts require comparison of strengths.

The governing concept is the reaction of carbon dioxide with lime water. Carbon dioxide first reacts with calcium hydroxide to form insoluble calcium carbonate, which makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. When excess CO₂ is passed, the carbonate reacts further with water and carbon dioxide to form soluble calcium hydrogen carbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Therefore, option C is correct; calcium carbonate is only the initial precipitate.

The governing concept is acid strength and hydrogen-ion concentration. Hydrochloric acid is a strong acid, so at the same formal concentration it ionises almost completely in water and produces a relatively high concentration of H⁺ or H₃O⁺ ions. Ethanoic acid is weak and ionises only partially, so it produces fewer hydrogen ions. Since pH = −log[H⁺], a greater hydrogen-ion concentration gives a lower pH. Thus option B is correct; the other statements contradict the properties of these acids.

The governing concept is water of crystallisation and its loss from a hydrated salt. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. On exposure to dry air, its crystals may lose part or all of this combined water and become less crystalline and powdery. This spontaneous loss of water of crystallisation is called efflorescence, so option B is correct. Deliquescence is the opposite type of behaviour, in which a substance absorbs moisture and may dissolve.

The governing concept is electrolysis of concentrated brine in the chlor-alkali process. At the anode, chloride ions are oxidised and chlorine gas is released: 2Cl⁻ → Cl₂ + 2e⁻. Chlorine is a strong disinfectant because it forms substances such as hypochlorous acid in water, which destroy many disease-causing microorganisms. Hence chlorine is used to disinfect drinking-water supplies and swimming pools, making option B correct. Carbon dioxide, not chlorine, is associated with raising cakes.

The governing concept is controlled dehydration of a hydrated salt. Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. On careful heating, it loses only one and a half molecules of water per formula unit and forms calcium sulfate hemihydrate, CaSO₄·½H₂O, called plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Therefore option B is correct. Excessive heating can remove more water and affect the useful setting property.

The governing concept is the colour range of an acid–base indicator. Phenolphthalein is colourless in acidic and approximately neutral conditions, but it changes to pink in a sufficiently basic medium, generally around pH 8.2–10.0. As base is added to the original acidic solution, hydrogen ions are neutralised and the pH rises. Once the indicator’s transition range is reached, pink appears. Therefore option A is correct; increasing acidity would keep it colourless.

The governing concept is the hygroscopic and reactive nature of sodium hydroxide. Solid NaOH absorbs water vapour from air, so it becomes moist and its effective concentration or purity changes. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Both processes reduce the quality of the stored chemical. Therefore option A is correct; oxygen, nitrogen, light, sound, glass, and rubber do not explain the stated loss under ordinary storage conditions.

The governing concept is the identification of a basic carbonate salt by two tests. Turning red litmus blue shows that the aqueous solution is basic. Releasing carbon dioxide on adding an acid is characteristic of a carbonate or hydrogen carbonate: CO₃²⁻ + 2H⁺ → CO₂ + H₂O. Sodium carbonate dissolves in water, gives an alkaline solution through hydrolysis, and produces carbon dioxide with acids. Thus option B fits both observations; sodium chloride is neutral, while ammonium chloride and copper sulfate do not give this combination.

The governing concept is the relationship between pH and acid attack on tooth enamel. A pH below 5.5 indicates an acidic environment. Tooth enamel is mainly a calcium phosphate mineral, and acids can react with it, gradually dissolving or demineralising the surface. Therefore frequent or prolonged exposure to such a solution may damage enamel and contribute to tooth decay. Option A is correct. The solution is not neutral or necessarily basic, and low pH does not strengthen teeth.

The governing concept is the difference between pH and electrical conductivity. A pH near 7 means that the solution is approximately neutral, not that it contains no ions. Sodium chloride is a soluble ionic compound and dissociates in water: NaCl(aq) → Na⁺(aq) + Cl⁻(aq). These charged ions move toward opposite electrodes and carry electric current. Therefore option A is correct. Neutrality does not prevent conductivity; the claims that there are no ions or that NaCl is a hidden acid are incorrect.

Marble is mainly calcium carbonate, CaCO₃. The governing concept is the reaction of an acid with a carbonate: acid + carbonate produces a salt, water, and carbon dioxide. Acid rain contains acids that slowly attack the carbonate surface, causing effervescence or dissolution and erosion of monuments. Therefore option A is correct; the other choices do not describe the relevant reaction or even a valid chemical process.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The solid product is sodium carbonate, and its aqueous solution is basic because carbonate ions react with water to produce hydroxide ions. Hence option B is correct. Sodium chloride and calcium sulphate are not the products of this decomposition, while calcium carbonate has the wrong composition.

An acidic solution contains an excess of hydrogen ions, H⁺. The governing neutralisation concept is H⁺ + OH⁻ → H₂O. When hydroxide ions are added, they first consume hydrogen ions to form water, so the hydrogen-ion concentration decreases and the pH begins to rise. The solution does not instantly become solid, and neutralisation does not convert salt into metal. Therefore option A is correct.

The pH scale is logarithmic, and pOH = 14 − pH at 25°C. At pH 13, pOH is 1; at pH 11, pOH is 3. Thus the hydroxide-ion concentration changes from about 10⁻¹ M to 10⁻³ M, a decrease by 10², or 100 times. The solution remains basic at pH 11, but it is much less basic. Therefore option A is correct.

A metal above hydrogen in the reactivity series can displace hydrogen from a dilute, non-oxidising acid: metal + acid → salt + H₂. Magnesium is above hydrogen and is more reactive than copper, so it produces hydrogen bubbles. Copper is below hydrogen and generally does not liberate hydrogen from dilute hydrochloric or dilute sulphuric acid. Thus option A is correct; the other statements are chemically false.