Acids, Bases, and Salts

Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. When water is added, it undergoes hydration and reforms gypsum, calcium sulphate dihydrate, CaSO4·2H2O: CaSO4·1/2H2O + 3/2H2O → CaSO4·2H2O. The interlocking gypsum crystals produce a hard mass, which explains its use in casts and moulds. The other products are unrelated.

Gypsum is CaSO4·2H2O. Controlled heating removes only part of its water of crystallisation and produces plaster of Paris, CaSO4·1/2H2O. If heating is excessive, the remaining water can also be driven off, producing anhydrous calcium sulphate, CaSO4, commonly called dead-burnt plaster. This material does not rehydrate and set properly, so option A explains the loss of the desired product.

The chlor-alkali process is the electrolysis of concentrated sodium chloride solution, commonly called brine. At the anode, chloride ions are oxidised to chlorine gas; at the cathode, water is reduced to hydrogen gas and hydroxide ions. The sodium ions combine with hydroxide ions to form sodium hydroxide solution. Therefore option A gives all three correct products. The other options contain substances not formed in this process.

During electrolysis of brine, chloride ions move towards the positively charged anode. At the anode they lose electrons according to the oxidation process 2Cl⁻ → Cl₂ + 2e⁻. The chlorine atoms combine to form chlorine gas. Hence option A, the anode, is correct. Hydrogen is produced at the cathode, so option B confuses the two electrode reactions, while C and D are inconsistent with electrolysis.

In the chlor-alkali process, water is reduced at the negatively charged cathode. The cathodic reaction can be represented as 2H₂O + 2e⁻ → H₂ + 2OH⁻, so hydrogen gas is released at the cathode. Therefore option B is correct. Chlorine is formed at the anode by oxidation of chloride ions; the remaining choices do not describe an electrode reaction and cannot produce hydrogen.

Very acidic soil can harm roots and reduce the availability of some nutrients. Slaked lime, Ca(OH)₂, is a basic substance that reacts with excess acidic components and raises the soil pH toward a more suitable range. This is an agricultural application of neutralisation, so option A is correct. Adding more acid would worsen the condition, while salt and sugar do not provide the required neutralising action.

Excessively basic soil can be improved by adding organic manure. As organic matter decomposes, it produces humus and mild acidic substances that can gradually reduce the soil's alkalinity. Thus option A is the most appropriate choice in the school-level context. Lime, sodium hydroxide, and washing soda are basic or alkaline materials, so adding them would generally increase rather than correct the basic nature of the soil.

Acidic strength in water is related to the concentration of hydronium ions produced by ionisation. Hydrochloric acid is a strong acid and ionises almost completely, whereas acetic acid is a weak acid and ionises only partially at the same concentration. Consequently, HCl produces more H₃O⁺ ions and shows a stronger acidic effect. Therefore option A is correct; colour, carbon content, and physical state are irrelevant distractors.

A solution conducts electricity when it contains mobile charged particles. Hydrogen chloride produces hydronium and chloride ions when dissolved in water, and its aqueous solution has a high concentration of ions because HCl is a strong acid. Therefore option C is the best answer. Dry HCl gas and dry sugar lack freely moving ions, while pure water has only a very small concentration of ions and conducts much less effectively.

Turning blue litmus red is a characteristic test for an acidic solution. In aqueous school-level chemistry, acids produce hydrogen ions, more precisely hydronium ions formed when H⁺ associates with water. These ions account for acidic properties, and their movement also helps the solution conduct electricity. Therefore option A is correct. Hydroxide ions indicate basicity, while sodium and carbonate ions do not produce the stated acidic behaviour.

A solution that turns red litmus blue is basic, and a slippery or soapy feel is another common indication of basicity. In aqueous chemistry, bases furnish hydroxide ions, OH⁻, either directly or through reactions with water. These ions are responsible for the characteristic basic properties. Thus option A is correct. Hydrogen ions are associated with acids, while chloride and sulphate ions are spectator ions in many common solutions and do not by themselves explain the observation.

A soda-acid fire extinguisher uses sodium hydrogen carbonate, NaHCO₃, together with an acid. Their reaction produces carbon dioxide, for example NaHCO₃ + HCl → NaCl + H₂O + CO₂. The released CO₂ does not support combustion and forms a layer that helps cut off oxygen from the fire. Therefore option A is correct. Sodium hydroxide and calcium oxide are bases, while sodium chloride does not release CO₂ with acid.

Baking powder contains sodium hydrogen carbonate and a mild edible acid. When the mixture is moistened and heated, the acid reacts with the hydrogen carbonate and produces carbon dioxide gas. The gas expands and forms bubbles trapped in the batter, making the cake rise and become porous. Oxygen does not change into water, sodium metal is not formed, and nitrogen is not responsible for the expansion.

Baking soda is mainly sodium hydrogen carbonate, a single chemical compound. Baking powder is a mixture containing baking soda and a mild dry acid, often with starch to keep it stable. When liquid and heat are supplied, the acid reacts with the hydrogen carbonate and releases carbon dioxide. Therefore A gives the real composition difference; the other statements confuse it with chlorine, common salt, or claim incorrectly that the two are identical.

Hard water contains dissolved calcium and magnesium ions, which react with soap to form scum and prevent good lather. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions. These combine with calcium and magnesium ions to form insoluble carbonates that can precipitate and be removed. Thus A is correct; washing soda does not acidify water, convert it into gas, or generate chlorine.

Marble is composed mainly of calcium carbonate, CaCO₃. Acids present in polluted rainwater react with this carbonate, producing a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated reaction gradually dissolves or erodes the surface, so A is correct. Water alone is not the key cause, and acid rain neither makes metal nor is completely blocked.

Both reactions release carbon dioxide because the carbonate or hydrogen carbonate ion reacts with hydrogen ions from the acid. The general equations are metal carbonate + acid → salt + water + CO₂ and metal hydrogen carbonate + acid → salt + water + CO₂. Therefore A is the common product. Hydrogen is associated with some acid-metal reactions, while oxygen and ammonia are not products of these carbonate reactions.

An acid reacting with a metal carbonate produces all three named products: salt, water, and carbon dioxide. For example, 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. An acid and a metal generally produce salt and hydrogen; an acid-base reaction produces salt and water; and an acid-metal oxide also generally produces salt and water. Hence A is the only complete match.

For an aqueous solution, pH = −log[H⁺]. Adding water dilutes the solution, lowering the hydrogen-ion concentration and increasing its pH from 2 to 4. Because pH 4 is still below 7, the solution remains acidic, but it is less acidic than before. It has not necessarily become basic, and dilution does not remove all ions. Thus A is the most accurate statement.

Litmus is an acid-base indicator. In an acidic medium, blue litmus changes to red because the indicator responds to the presence of acidic hydrogen ions. Therefore the solution is acidic, so A is correct. A basic solution would turn red litmus blue, while a neutral solution generally produces no litmus colour change. “Salty” describes the possible presence of a salt but does not by itself identify this indicator result.