Acids, Bases, and Salts

Bacteria in the mouth can convert food sugars into acids. When the pH around the teeth falls below about 5.5, the mineral hydroxyapatite in enamel can begin to dissolve more readily, increasing the risk of dental caries. Hence option B is correct. A pH of 7 is neutral, while 10 and 14 are alkaline and do not represent the critical acidic threshold.

After eating, oral bacteria may produce acids from sugars, lowering the pH at the tooth surface and promoting enamel demineralisation. A mildly basic toothpaste helps neutralise some of these acids, reducing acidity and supporting tooth protection. Therefore option A is correct. Dissolving enamel, making pH zero, or increasing acid would worsen damage rather than prevent it.

Acidic soil has a pH below the desirable range for many crops. Slaked lime, calcium hydroxide, is a basic substance that reacts with excess soil acidity and raises the pH toward a more suitable level. Thus option A is correct. Vinegar, lemon juice, and dilute acid are acidic materials, so adding them would generally increase rather than correct soil acidity.

The governing concept is neutralisation and soil pH management. Highly basic soil has a pH above 7, so adding more alkaline substances such as lime or sodium hydroxide would make the problem worse. Organic matter, such as compost or manure, decomposes gradually and can produce mildly acidic substances that help reduce excessive alkalinity and improve soil structure. Therefore, option B is correct; soap is not a suitable soil amendment.

The governing concept is the preparation and hydration of salts. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is prepared by carefully heating gypsum, CaSO₄·2H₂O, to remove part of its water of crystallisation. When water is added, plaster of Paris changes back into gypsum and sets into a hard mass. Washing soda, common salt, and calcium carbonate are different compounds, so option A is correct.

The governing concept is the preparation and use of bleaching powder. Chlorine gas is passed over dry slaked lime, Ca(OH)₂, to form bleaching powder, commonly represented at school level as calcium oxychloride, CaOCl₂. Chlorine is therefore the required gas. Hydrogen, carbon dioxide, and nitrogen do not provide the active bleaching constituent in this preparation. Hence option B is correct.

The governing concept is chemical disinfection by available chlorine. In water, bleaching powder releases chlorine-containing species that produce active chlorine, including hypochlorous acid under suitable conditions. These species oxidise and destroy many disease-causing microorganisms, making the water safer. The process does not sweeten water, convert it into a metal, or freeze it. Therefore, option A correctly explains why bleaching powder is used for disinfection.

The governing concept is electrolysis of brine, an aqueous solution of sodium chloride. At the anode, chloride ions are oxidised to chlorine gas; at the cathode, water is reduced to hydrogen gas and hydroxide ions. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. Thus the three main products are chlorine, hydrogen, and sodium hydroxide, making option A correct. The other combinations are unrelated products.

The governing concept is the electrode reaction during electrolysis of brine. Brine contains chloride ions, and 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 oxygen and carbon dioxide are not the main anode products in this process. Therefore, option B is correct.

The governing concept is reduction at the cathode during brine electrolysis. At the cathode, water gains electrons and produces hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine is formed at the anode by oxidation of chloride ions. Nitrogen and sulphur dioxide are not products of the chlor-alkali process. Hence, the gas formed at the cathode is hydrogen, so option A is correct.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulphate crystals are represented as CuSO4·5H2O, meaning that a fixed number of water molecules are incorporated into the crystal structure. This hydrated form has its characteristic blue colour. On heating, the water is removed and anhydrous copper sulphate becomes white. Hence option A is correct; sodium ions, chlorine gas and carbon dioxide are not responsible for this colour.

The governing concept is dehydration of a hydrated salt. Blue copper sulphate crystals contain water of crystallisation, commonly written as CuSO4·5H2O. When the crystals are heated, this bound water is driven off, producing anhydrous copper sulphate, which is white. Therefore option A gives the correct cause of the colour change. Heating does not form sodium, add carbon dioxide, or convert the salt into sugar.

The governing concept is controlled dehydration of gypsum. Gypsum is calcium sulphate dihydrate, CaSO4·2H2O. On careful heating, it loses part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO4·1/2H2O, commonly called plaster of Paris. Thus option A is correct. Washing soda, baking soda and common salt are different salts and are not products of gypsum heating.

The governing concept is rehydration and setting of plaster of Paris. Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. When water is added, it combines with the material and converts it back into gypsum, CaSO4·2H2O. The newly formed crystalline gypsum interlocks and makes the mixture hard. Therefore option A is correct; no sodium chloride, gas, or sugar is produced in this setting reaction.

The governing concept is precipitation of hardness-causing ions. Hard water contains dissolved calcium and magnesium ions, which interfere with soap and produce scum. Washing soda, sodium carbonate (Na2CO3), reacts with these ions to form insoluble carbonates such as calcium carbonate and magnesium carbonate. The precipitates can be removed, reducing hardness. Hence option A is correct; gas formation, acidification and chlorine removal do not explain softening.

The governing concept is an acid–carbonate reaction that releases a gas. Baking soda is sodium hydrogen carbonate, NaHCO3. With an acid, it produces a salt, water and carbon dioxide; for example, NaHCO3 + H+ forms CO2 and H2O along with the salt. Carbon dioxide bubbles become trapped in the mixture, expanding it and making the food rise and become porous. Therefore option A is correct.

The governing concept is the reaction of food acids with reactive metals. Curd contains lactic acid, and many pickles contain acids such as acetic acid. These acids can react with copper when food is stored in a copper vessel for a long time, forming copper salts that may contaminate the food and be harmful in excess. Thus option A is correct. The other choices contradict the presence of acids or make unsupported claims about copper.

The governing concept is the reaction between acids and carbonates. Marble is mainly calcium carbonate, CaCO₃. Acids in rain react with this carbonate, producing soluble calcium salts, water and carbon dioxide; the surface can therefore dissolve and lose detail gradually. Thus option A is correct. Iron, sugar and basic rain do not explain marble damage, so options B, C and D are incorrect.

The governing concept is pH and the effect of acidity on living organisms. Acid rain introduces acidic substances into lakes, ponds and rivers, so the water’s pH may fall below its normal range. Many aquatic organisms, eggs and plants are sensitive to such changes, which can disturb biological processes. Therefore option A is correct; sweetness, permanent alkalinity and colour alone do not explain the harm.

The governing concept is dilution and the logarithmic pH scale. Adding water decreases the concentration of hydrogen ions in an acidic solution. Since pH = −log[H⁺], a lower hydrogen-ion concentration gives a higher pH. The value moves toward the neutral value 7, although it does not necessarily become exactly 7. Thus option A is correct; the other choices contradict dilution.

The governing concept is dilution of a base. When water is added, the concentration of hydroxide ions per unit volume decreases. Consequently, the solution becomes less strongly basic and its pH falls toward the neutral value 7. It need not become exactly 7 after a limited amount of water is added. Therefore option A is correct; dilution does not raise pH above 14, make it zero, or leave it invariably unchanged.

The governing concept is electrical conduction in solutions. An electric current requires charged particles that can move. In an aqueous solution, dissolved electrolytes may separate into mobile positive and negative ions; these ions carry charge in opposite directions when a potential difference is applied. Therefore option A is correct. Colour and smell are sensory properties, while sugar generally does not produce ions in water, so they do not explain conduction.

The governing concept is the difference between electrical neutrality and electrical conduction. Sodium chloride dissolves and dissociates in water into mobile Na⁺ and Cl⁻ ions. Equal positive and negative charges make the solution approximately neutral overall, but the ions can still move and carry current. Hence option A is correct. Being neutral does not mean being ion-free, so options B, C and D are wrong.

The governing concept is laboratory safety and the controlled identification of chemicals. Acids and bases can be corrosive, poisonous or irritating even in small amounts, so tasting an unknown substance may cause burns or poisoning. Safe tests include suitable indicators, pH paper and teacher-approved procedures. Thus option A is correct. Taste is not always wrong, but it is unsafe; the other statements are scientifically overgeneralized or irrelevant.

The governing concept is the acid–base behavior of indicators. Blue litmus turns red in an acidic medium, showing that the solution has an excess of hydrogen ions, represented in water as hydronium ions. Therefore option A is the best answer. Hydroxide ions indicate basicity, while sodium and chloride ions alone do not establish that the solution is acidic; colourlessness is also irrelevant to the test.