Acids, Bases, and Salts

An acidic lake contains an excess of hydrogen ions, so its pH is low. Lime is a basic material; it reacts with or consumes acidic species and reduces the hydrogen-ion effect. As acidity decreases, the pH rises toward the neutral range. This acid-base interaction is called neutralisation. Evaporation only removes solvent, sublimation changes a solid directly to gas, and crystallisation forms crystals; none explains the pH increase. Therefore option A is correct.

The key distinction is between an acid and an acidic oxide. Carbon dioxide is a non-metal oxide with acidic character. It reacts with water to form carbonic acid and reacts with bases such as calcium hydroxide: CO2 + Ca(OH)2 -> CaCO3 + H2O. Thus it can neutralise a base even though it is not a typical acid like hydrochloric acid. It is not a metal or a strong base, and gases are not universally inert. Option A is correct.

Methyl orange is an acid-base indicator: it appears red in a sufficiently acidic medium and yellow in a neutral or alkaline medium. Adding a base consumes some of the acid, so the hydrogen-ion concentration decreases and the pH rises. The colour change therefore shows movement from acidic conditions toward neutral or basic conditions. It does not indicate metal formation, carbon dioxide formation, or increasing acidity. Hence option A is correct.

The governing concept is that ions must become mobile for the acid-carbonate reaction to proceed efficiently. In dry baking powder, the solid particles have limited contact and the ions are not freely solvated, so vigorous reaction and carbon dioxide release are suppressed. When water is added, the substances dissolve and react; heating can then accelerate gas formation. The ingredients are not identical, and carbon dioxide does not form continuously without reaction. Therefore option A is correct.

When sodium hydroxide is added to an acid, hydroxide ions combine with hydrogen ions to form water: H+ + OH- -> H2O. This acid-base process is neutralisation, and the formation of water is generally exothermic, so some energy is released as heat and the temperature rises. The sodium hydroxide does not simply become a gas, the acid does not become a metal, and salt formation does not freeze water. Thus option A is correct.

The pH scale is logarithmic: pH = -log10[H+]. A rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. Here pH rises from 3 to 5, a change of two units, so the concentration decreases by 10^2 = 100 times. The answer is not two times merely because the pH changed by two units, nor five or one thousand times. Therefore option A is correct.

Copper oxide is a metal oxide and shows basic character. It reacts with hydrochloric acid in a neutralisation-type reaction: CuO + 2HCl -> CuCl2 + H2O. The copper salt formed dissolves and gives a blue-green solution, providing evidence that the oxide reacts with an acid to form salt and water. It does not increase acidity, behave as a gas, or form sugar. Therefore option A is correct.

Sodium hydroxide is a strong base, whereas carbon dioxide is an acidic oxide. When sodium hydroxide absorbs carbon dioxide from air, an acid–base reaction occurs: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Thus sodium carbonate can form. Calcium sulphate, ammonium chloride, and copper oxide require different reactants and are not products of this exposure.

Phenolphthalein turns pink in a basic solution, and turmeric changes from yellow to reddish-brown in the presence of a base. These observations indicate that the concentration of hydroxide ions is relatively high, so OH⁻ ions are likely in excess. Hydrogen ions would indicate acidity, while chloride and nitrate ions are spectator ions and do not by themselves produce these indicator changes.

Equal volume does not mean equal chemical amount. Neutralisation depends on the number of reacting H⁺ and OH⁻ ions, which is determined by concentration and acid or base strength. For example, equal volumes of 0.1 M HCl and 0.1 M NaOH can approach pH 7, but unequal concentrations or a weak reactant can leave an excess of one ion. Therefore A is correct.

The electrolysis of concentrated aqueous sodium chloride, or brine, is called the chlor-alkali process. It produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in solution. Sodium hydroxide is a strong base used in soap manufacture and in the paper industry. The other listed substances are not the characteristic base product of brine electrolysis.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it decomposes according to 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide reacts with limewater, calcium hydroxide, to form insoluble calcium carbonate, making the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Therefore option A is confirmed.

A base is a substance that reacts with an acid to produce salt and water; it need not dissolve in water. An alkali is specifically a base that dissolves in water and gives hydroxide ions in solution. Thus an insoluble substance showing neutralisation with an acid is a base but not an alkali. It is not automatically an acid, a salt, or an indicator.

Initially, red litmus turning blue shows that the solution is basic, because hydroxide ions change the indicator colour. When acid is added in sufficient amount, its hydrogen ions neutralise the hydroxide ions: H⁺ + OH⁻ → H₂O. If the final solution changes neither red nor blue litmus, it is approximately neutral. A strongly acidic or basic solution would still change one indicator.

Acid rain forms when acidic oxides, mainly sulfur dioxide and nitrogen oxides, dissolve in atmospheric moisture and produce acids such as sulfurous, sulfuric, or nitric acid. These acids increase the hydrogen-ion concentration of rainwater, and pH decreases as hydrogen-ion concentration increases. Therefore acid rain has a lower pH than normal rain. Common salt or absence of dissolved gas does not explain the acidity.

Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O, so it contains water of crystallisation. Controlled heating removes part of this water and produces plaster of Paris, CaSO₄·½H₂O. When water is added, plaster of Paris rehydrates to gypsum and hardens: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Hence option A correctly describes the reversible relationship.

The pH scale describes hydrogen-ion concentration: pH = −log[H⁺]. An acidic solution has a hydrogen-ion concentration greater than that of neutral water and therefore has pH below 7 at ordinary conditions. If the acid is very dilute, its hydrogen-ion concentration is only slightly greater than neutral, so its pH may be just below 7, such as 6 or 6.5. It is not necessarily 0.

The pH scale identifies how acidic or basic an aqueous solution is: values near 14 indicate a very strong base. A slippery feel is also commonly associated with alkaline solutions because they react with skin oils, but touching an unknown chemical is unsafe. Therefore option A is correct: it may be strongly basic and caustic. Pure water is near pH 7, a mild acid has pH below 7, and common salt solution is approximately neutral.

Bleaching powder is a chlorine-containing compound, commonly represented as calcium oxychloride. In moist water it releases available chlorine, which forms disinfecting chlorine species that damage or kill many disease-causing microorganisms. Hence option A gives the scientific reason for its use in water treatment. It does not convert water into sugar, produce calcium metal, or merely change the colour; disinfection is the essential function.

Carbonate ions release carbon dioxide when they react with hydrogen ions supplied by an acid: carbonate plus acid produces a salt, water, and CO2. Merely adding sodium carbonate to a solution that is not acidic need not produce visible gas. Thus option A correctly identifies the need for an acidic reaction medium. The gas is carbon dioxide, not hydrogen; sodium carbonate is soluble and is an ionic salt rather than a metal.