Acids, Bases, and Salts

Universal indicator gives a range of colours linked to pH. Blue generally indicates an alkaline solution, whereas green represents a pH close to neutral. Adding acid consumes some hydroxide ions and lowers the pH, moving the colour from blue toward green. Therefore option B is correct. The observation does not by itself prove that the solution became strongly acidic or that only a gas formed.

The relevant concept is the chemical nature of oxides, not their physical state. Carbon dioxide is a non-metal oxide; it dissolves partly in water to form carbonic acid and reacts with bases such as limewater. For example, CO2 + Ca(OH)2 produces CaCO3 precipitate and water. Hence option A is correct. It is not a metal oxide or a strong base, and the claim that it never dissolves is false.

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.

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.

Baking powder contains sodium hydrogen carbonate and a suitable edible acid. In the dry mixture, ions have limited mobility, so the reaction is slow. Water dissolves the components and allows ionic contact; on heating, carbon dioxide is released more rapidly: acid + hydrogen carbonate → salt + water + CO₂. The CO₂ bubbles expand and raise the dough. Therefore option A is correct.

Copper oxide is a metal oxide and shows basic character. The governing reaction is a neutralisation: basic metal oxide + acid → salt + water. For example, CuO + H₂SO₄ → CuSO₄ + H₂O; the dissolved copper salt gives the solution its characteristic colour. Formation of salt with an acid demonstrates basicity, so option B is correct. It is not an acidic oxide, neutral gas, or salt-free material.

Phenolphthalein is pink in a basic medium and colourless in an acidic or neutral medium. Methyl orange is yellow in a neutral or basic medium, so the two observations together indicate that the solution is basic. In a basic aqueous solution, hydroxide ions are present in excess relative to hydrogen ions. Chloride and sulphate ions do not determine this indicator combination. Therefore option B is correct.

Aqueous common salt, sodium chloride, undergoes electrolysis in the chlor-alkali process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. The sodium hydroxide formed is a strong base used in making soap, paper, and several chemicals. Calcium carbonate, ammonium chloride, and copper sulphate are different compounds and are not the important base obtained in this process. Therefore option A is correct.

The governing relation is pH = −log[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. Here pH rises from 2 to 4, an increase of two units, so [H⁺] decreases by 10² = 100 times. Thus option A is correct. Option B reverses the logarithmic relationship, while C treats pH as a linear scale and D ignores the change in pH.

Bleaching powder, commonly represented as CaOCl₂, owes its bleaching and disinfecting action to available chlorine. On prolonged exposure to atmospheric moisture and carbon dioxide, it gradually decomposes and loses available chlorine, reducing its effectiveness. A closed container limits contact with air and moisture, so option A is correct. The other options describe unrelated products or an impossible purpose; storage does not convert bleaching powder into salt, gypsum, or food.

Blue litmus turning red is a standard indication that the solution is acidic. However, hydrogen evolution depends on the reactivity of the metal as well as the acid. Metals above hydrogen in the reactivity series can usually displace hydrogen from dilute acids, whereas copper is below hydrogen and generally does not do so. Therefore option A correctly combines both observations; B and D contradict the litmus test, and C is chemically false.

A pH of 9 indicates that the soil is distinctly basic or alkaline. To move its pH toward a range more suitable for many crops, the general principle is to add a suitable mildly acidic amendment, often organic matter such as compost or well-managed manure, according to soil testing. Therefore option A is the best general answer. Slaked lime and sodium hydroxide would increase alkalinity, while soap is not an appropriate soil amendment.