Acids, Bases, and Salts

The governing concept is the nature of salts formed from strong acids and strong bases. Sodium chloride is produced from hydrochloric acid and sodium hydroxide, so its aqueous solution is generally neutral, with pH close to 7. Dilution lowers the concentrations of both sodium and chloride ions but does not create a significant excess of hydrogen or hydroxide ions. Hydrochloric acid and sodium hydroxide would remain acidic or basic, while sodium carbonate is basic.

The governing concept is the chemical action of available chlorine. Bleaching powder, commonly represented as calcium oxychloride, releases chlorine or hypochlorous species that oxidise coloured substances and many microorganisms. Thus it is used for bleaching cloth and paper and for disinfecting drinking water and surfaces. Gypsum is mainly used in plaster products, washing soda is a cleaning and softening agent, and sodium chloride is common salt rather than a bleaching disinfectant.

The governing concept is salt hydrolysis and the relative strengths of the reacting acid and base. A strong acid with a strong base generally gives a neutral salt, whereas a strong acid with a weak base tends to give an acidic salt, and a weak acid with a strong base tends to give a basic salt. Therefore the acid and base strengths determine the salt’s behaviour in water; container shape, colour, and zero temperature do not.

The governing concept is the pH scale and the colour range of a universal indicator. An excess of hydroxide ions over hydrogen ions means the solution is basic and has a pH above 7. Universal indicator usually changes through green near neutral to blue and then violet as basicity increases. Red, orange, and yellow represent acidic ranges, so they do not fit the stated ion balance. The exact blue or violet shade depends on the pH.

The governing concept is laboratory safety when handling a soluble, corrosive base. A strong base can damage skin and eyes, and dissolving some bases in water releases heat; therefore direct contact and careless mixing are unsafe. The substance should be handled with gloves, eye protection, suitable tools, clear labelling, and controlled procedures. Option B is correct. Eating it or storing it without a label creates additional hazards, while bare-hand use ignores its caustic nature.

Red litmus turning blue indicates that the salt solution is basic. A salt formed from a weak acid and a strong base commonly gives a basic solution because the anion of the weak acid hydrolyses water and produces hydroxide ions; sodium carbonate is a familiar example. A strong-acid/weak-base salt is generally acidic, while a strong-acid/strong-base salt is approximately neutral. Hence A is correct.

Carbonates react with acids to release carbon dioxide, for example: sodium carbonate plus hydrochloric acid produces sodium chloride, water, and CO₂. The blue colour produced from red litmus also shows that the aqueous solution is basic. Sodium carbonate satisfies both tests. Sodium chloride is nearly neutral, ammonium chloride is acidic, and copper sulphate does not give carbon dioxide with an acid. Therefore A is correct.

Diluting many concentrated acids is exothermic: hydration of ions releases heat. If water is poured rapidly into acid, the small amount of water can heat suddenly, boil locally, and eject acid droplets. Adding acid slowly to a larger quantity of water allows heat to spread more safely. The hazard is therefore rapid heat release and splashing, not gas formation, solidification, or colour change. Option A is correct.

Strength describes the extent to which a base ionises in water; a strong base, such as sodium hydroxide, ionises almost completely. Concentration describes how much dissolved base is present in a given volume of solution. Thus a strong base may be dilute, and a weak base may be concentrated. The two properties are independent, so option A is the accurate distinction.

Water of crystallisation is a definite number of water molecules incorporated into the crystal structure of some salts. Heating can remove this water, producing a colour change and sometimes visible moisture; adding water may restore the hydrated form and its original colour. Hydrated copper sulphate is a standard example. Common salt does not explain this reversible colour change, and oxygen or chlorine gas are not crystal water. Hence A is correct.

Gypsum has the formula CaSO₄·2H₂O, so it contains two water molecules per calcium sulphate unit. Controlled heating removes one and a half of these water molecules and forms plaster of Paris, CaSO₄·½H₂O. Therefore half a water molecule remains per calcium sulphate unit. The other values do not match the balanced formula or the known dehydration process, so A is correct.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion undergoes hydrolysis and produces hydronium ions, making the solution acidic; chloride ions do not significantly offset this effect. Consequently blue litmus turns red. A strong-acid/strong-base salt would be nearly neutral, and the other choices contradict its ionic nature. Thus A is correct.

The governing concept is the reactivity of metals with acids. A sufficiently reactive metal displaces hydrogen from a dilute acid, producing hydrogen gas, whereas a less reactive metal may show little or no reaction. If gas is evolved rapidly, that metal is reacting faster under the same conditions. Colour, container shape, and the acid’s name do not explain the difference. Therefore option A is correct.

Hard water contains dissolved calcium and magnesium ions, which interfere with soap and produce scum. Washing soda, sodium carbonate (Na2CO3), supplies carbonate ions. These ions precipitate calcium carbonate and magnesium carbonate, removing the hardness-causing ions from the water. No hydrogen gas is required, sodium ions are not destroyed, and the water does not become an acid. Thus option A is correct.

The governing idea is that mouth bacteria can metabolise food residues, especially sugars, and release acids. These acids lower the oral pH; when the pH remains low, tooth enamel can lose minerals and decay becomes more likely. Cleaning or rinsing removes residues and reduces acid production, helping the pH move away from the harmful acidic range. It does not dissolve enamel or increase acidity, so option A is correct.

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.

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.

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.