Acids, Bases, and Salts

The governing concept is the formation and hydration of a salt. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O, obtained by carefully heating gypsum. When water is added, it changes back into gypsum, CaSO₄·2H₂O, and hardens into a solid mass. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is a different calcium salt.

The governing concept is the preparation and use of bleaching powder, also called calcium oxychloride in school-level chemistry. Chlorine gas is passed over dry slaked lime, Ca(OH)₂, to form bleaching powder. Thus option B is correct. Hydrogen and nitrogen do not produce bleaching powder, while carbon dioxide is not the reacting gas in this preparation.

Bleaching powder is prepared by passing chlorine over dry slaked lime, whose chemical formula is Ca(OH)₂. The relevant reaction is commonly represented as Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O. Therefore option A is correct. Common salt is used in the chlor-alkali process, gypsum forms plaster of Paris on heating, and washing soda is sodium carbonate; none is the direct solid used here.

The governing concept is disinfection by available chlorine. When bleaching powder is added to water, it releases chlorine-containing species that produce available chlorine, including hypochlorous acid under suitable conditions. These oxidising substances damage or kill many disease-causing microorganisms. Therefore option A is correct; sweetness, metal formation, and freezing are unrelated to the disinfecting action.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O, so it contains water of crystallisation. On controlled heating, part of this water is removed and calcium sulphate hemihydrate, CaSO₄·½H₂O, is produced. This substance is called plaster of Paris, making option A correct. Washing soda, baking soda, and common salt are different salts and are not formed by heating gypsum.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When water is added, it rehydrates and changes back into gypsum, calcium sulphate dihydrate, CaSO₄·2H₂O. The re-formed crystals interlock and make the material set into a hard mass, so option A is correct. It does not form sodium chloride, turn into a gas, or produce sugar.

Copper sulphate crystals are hydrated copper sulphate, CuSO₄·5H₂O, and contain a definite number of water molecules within their crystal structure. At this school level, the characteristic blue appearance is associated with the hydrated crystalline form and its water of crystallisation. Therefore option A is the expected answer. Sodium ions, chlorine gas, and hydrogen gas are not constituents responsible for these crystals’ blue colour.

Blue copper sulphate crystals are CuSO₄·5H₂O, which contain water of crystallisation. Heating drives this water out of the crystal lattice, producing anhydrous copper sulphate, CuSO₄, which is white or pale grey. Consequently the blue colour disappears, so option A is correct. No sodium is added, and chlorine or sugar is not formed in this dehydration change.

Water of crystallisation means the definite number of water molecules chemically associated with one formula unit of certain salts and incorporated into their crystal structure. For example, CuSO₄·5H₂O contains five water molecules per formula unit. This is not simply river water or any water mixed with an acid, and it is not a gas formed from a base. Therefore option A is correct.

The hardness of water is mainly caused by dissolved calcium and magnesium ions, often present as bicarbonates, chlorides, or sulphates. Washing soda, Na₂CO₃·10H₂O, supplies carbonate ions that precipitate these ions as insoluble calcium carbonate and magnesium carbonate. Removing them softens the water, so option A is correct. Sodium and chloride ions generally do not cause ordinary hardness.

Baking soda is sodium hydrogen carbonate, NaHCO₃. Carbonates and hydrogen carbonates react with acids to form a salt, water, and carbon dioxide; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Effervescence is caused by the released carbon dioxide gas, so option B is correct. Hydrogen, chlorine, and oxygen are not products of this ordinary acid–bicarbonate reaction.

Baking powder contains sodium hydrogen carbonate along with a mild edible acid. When the mixture becomes moist and is heated, they react and release carbon dioxide gas. The gas expands and gets trapped as bubbles in the batter or dough, increasing its volume and making the food light and spongy. Hydrogen, chlorine, and nitrogen are not the gases responsible for this rising effect, so option A is correct.

On heating, baking soda, or sodium hydrogen carbonate, undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate remains as the principal solid product, while carbon dioxide escapes as a gas and water is formed as vapour. Sodium chloride, calcium carbonate, and ammonium chloride are not products of this decomposition, so option A is correct.

Brine is a concentrated aqueous solution of sodium chloride. During its electrolysis, chloride ions are discharged at the anode to produce chlorine, while water is reduced at the cathode to produce hydrogen and hydroxide ions. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. This complete process is therefore called chlor-alkali electrolysis, making option A correct.

The chlor-alkali process electrolyses aqueous sodium chloride, or brine. Chloride ions form chlorine gas at the anode, and water forms hydrogen gas at the cathode. Hydroxide ions remain in solution and combine with sodium ions to produce sodium hydroxide. Thus the three important products are chlorine, hydrogen, and sodium hydroxide; the other combinations do not describe brine electrolysis, so A is correct.

In chlor-alkali electrolysis, brine supplies chloride ions and water. At the anode, oxidation occurs: chloride ions lose electrons and form chlorine gas, represented by 2Cl⁻ → Cl₂ + 2e⁻. Hydrogen is produced at the cathode, while carbon dioxide is unrelated to this process. Under the standard school-level treatment of concentrated brine, chlorine is therefore the correct answer, option B.

At the cathode, reduction takes place. In the aqueous brine solution, water molecules gain electrons and produce hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine forms at the anode, whereas nitrogen and sulphur dioxide are not products of this electrolysis. Hence hydrogen is the cathode gas and option A is correct.

Laboratory chemicals may be corrosive, poisonous, or irritating even when only a small quantity is present. Therefore tasting an unknown acid or base can injure the mouth, throat, or digestive system and is never an acceptable identification method. Indicators, universal indicator, or pH paper provide safer evidence of acidic or basic character. Since acids and bases can be harmful, option A is correct.

The governing concept is neutralisation. In a typical acid-base reaction, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining positive and negative ions combine to form a salt. For example, HCl + NaOH → NaCl + H₂O. Thus water is only one product; salt is also formed. The other options contradict the reaction, so B is correct.

The governing concept is thermal decomposition of sodium hydrogen carbonate, commonly called baking soda. On heating, two formula units decompose as: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Thus sodium carbonate, carbon dioxide, and water are formed. Sodium chloride requires a chloride source and does not appear in this reaction, so it is not a product. Therefore option D is correct; the other three are explicitly present in the equation.

The governing concept is neutralisation. Bacteria acting on food residues in the mouth produce acids, and these acids can attack tooth enamel and begin decay. Basic toothpaste supplies alkaline substances that react with and reduce the acidity. Therefore option A is correct. Making teeth acidic or merely changing colour does not prevent acid damage, and saliva cannot be converted into a gas by toothpaste.

The governing concept is acid–base neutralisation. The irritating material associated with an ant sting is acidic, while baking soda, sodium hydrogen carbonate, is a mild base. Contact between them reduces the acidic effect and can lessen burning or irritation. Hence option A is correct. Increasing acidity, producing oxygen, or removing water is not the relevant chemical process responsible for relief.

The governing indicator rule is that a basic solution changes red litmus to blue. Sodium carbonate dissolves in water and undergoes hydrolysis, producing a solution with alkaline character. The observation therefore identifies its aqueous solution as basic, so option B is correct. An acidic solution would turn blue litmus red, a neutral solution would show no such change, and “metallic” is not a solution nature indicated by litmus.

The governing reaction pattern is acid plus carbonate producing salt, water, and carbon dioxide. Sodium carbonate reacts with hydrochloric acid as Na2CO3 + 2HCl → 2NaCl + H2O + CO2↑, so option B is correct. Zinc with acid usually releases hydrogen, sodium hydroxide with acid gives neutralisation, and magnesium oxide with acid gives salt and water without carbon dioxide.

Red litmus turning blue demonstrates that the solution is basic, while phenolphthalein becoming pink confirms an alkaline medium. A pH greater than 7 is therefore required. Of the choices, pH 11 clearly represents a basic solution; pH 2 and 5 are acidic, and pH 7 is neutral, so they cannot explain both indicator changes.