Acids, Bases, and Salts

The governing concept is controlled dehydration of gypsum. Gypsum is calcium sulfate dihydrate, CaSO4·2H2O. On moderate heating it loses part of its water and forms plaster of Paris, calcium sulfate hemihydrate, CaSO4·½H2O. Excessive heating can remove still more water and produce an unsuitable anhydrous form, so the material may not set properly. Therefore option A is correct; gypsum does not become sodium chloride, calcium does not escape as a gas, and chlorine is not added.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulfate crystals are CuSO4·5H2O. Heating removes this crystallisation water and produces white anhydrous CuSO4. Adding water restores the hydrated blue form. Hence the observation proves that the crystal water is associated with the colour, so option A is correct. Copper sulfate is neither sugar nor common salt, and water does not convert copper into gold.

The governing concepts are electrical neutrality and ionic conduction, which are different properties. A neutral sodium chloride solution has balanced positive and negative charges overall, so its net charge is zero. However, dissolved NaCl separates into mobile Na+ and Cl− ions. These ions move toward opposite electrodes and carry electric current, making option A correct. Neutrality does not mean absence of ions; therefore option D is a false generalisation.

The governing concept is that an aqueous solution conducts when it contains mobile charged particles. Sugar dissolves as neutral molecules and does not ionise appreciably, so there are no sufficient charge carriers. Sodium chloride dissociates in water into Na+ and Cl− ions, which move through the solution and carry current. Thus option A is correct. Sugar is not a strong acid, sodium chloride does produce ions, and the two solutions do not conduct equally.

The governing concept is electrolysis of concentrated brine, an aqueous solution of sodium chloride. At the anode, chloride ions lose electrons and form chlorine gas: 2Cl− → Cl2 + 2e−. At the cathode, water is reduced and produces hydrogen gas: 2H2O + 2e− → H2 + 2OH−. Therefore the order anode then cathode is chlorine and hydrogen, so option A is correct.

The governing concept is electrolysis of brine. Sodium chloride solution contains Na+, Cl−, water, and supporting ions. Chlorine gas is released at the anode, while hydrogen gas is formed at the cathode from water. The remaining Na+ and OH− ions stay in the aqueous phase and form sodium hydroxide in the solution. Therefore option A is correct. NaOH is not obtained as an anode gas or as solid sodium metal at the cathode.

The governing concept is the thermal decomposition of sodium hydrogen carbonate. On heating, baking soda decomposes as 2NaHCO3 → Na2CO3 + H2O + CO2. The released carbon dioxide is passed through limewater, where it reacts with calcium hydroxide to form insoluble calcium carbonate, making the limewater milky. Hence option A is correct. A pop sound tests hydrogen, while the other indicators do not specifically confirm carbon dioxide.

The governing concept is the thermal decomposition of sodium hydrogen carbonate. On heating, baking soda, NaHCO₃, produces sodium carbonate, Na₂CO₃, along with carbon dioxide and water: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate hydrolyses in water and produces hydroxide ions, so its solution is basic. Therefore, option A is correct; it is not acidic, oily, or non-conducting.

The governing concept is an acid–carbonate reaction. Baking powder contains sodium hydrogen carbonate and a mild edible acid. When the mixture becomes moist and is heated, the acid reacts with NaHCO₃ and releases carbon dioxide, for example: acid + NaHCO₃ → salt + H₂O + CO₂. The gas bubbles expand in the batter and make it rise. Thus option A is correct; the other choices do not describe this reaction.

The governing concept is incomplete neutralisation in a baking mixture. During heating, baking soda can decompose and leave sodium carbonate. If no edible acid is present, this basic substance is not sufficiently neutralised. Residual sodium carbonate may give the food an unpleasant bitter or alkaline taste. Therefore option A is correct. Hydrogen is not the relevant product, sodium chloride is not formed here, and some change can occur.

The governing concept is the effect of pH on tooth enamel. Bacteria use sugars and produce acids, lowering the pH in dental plaque. Tooth enamel is largely calcium phosphate, which becomes more vulnerable to dissolution in sufficiently acidic conditions. When the mouth pH falls below about 5.5, mineral loss can begin and cavities may develop. Hence option A is correct; low pH does not strengthen enamel or convert sugar into a base.

The governing concept is acid–base neutralisation. Bacteria in the mouth can convert food sugars into acids, which lower pH and attack enamel. A mildly basic toothpaste reacts with or neutralises some of these acids, reducing their harmful effect and helping the mouth return toward a safer pH. Therefore option A is correct. It should not dissolve enamel, force pH to zero, or increase acidity.

The governing concept is neutralisation and pH correction. Acidic soil has an excess of acidic substances and therefore a relatively low pH. Slaked lime, calcium hydroxide, Ca(OH)₂, is a base; it reacts with acidic components and reduces their acidity. This raises the soil pH toward a more suitable range for plants. Thus option A is correct, while the other choices contradict basic chemistry or describe impossible changes.

The governing concept is adjustment of an unsuitable soil pH. Highly basic soil has a high pH and may not support every crop well. Decomposing organic matter, such as compost or manure, produces organic acids and other substances gradually. These can reduce excessive alkalinity and improve soil conditions over time. Therefore option A is the best answer. The process does not make soil metallic, produce chlorine, or generally raise an already high pH.

The governing concept is the reaction of acids with carbonates. Marble consists mainly of calcium carbonate, CaCO₃. Acids in acid rain react with this carbonate, producing a salt, water, and carbon dioxide; in general, acid + carbonate → salt + H₂O + CO₂. Repeated reaction removes or roughens the surface and causes deterioration. Therefore option A is correct; marble is not sugar and acid rain is not basic.

The governing concept is the ecological effect of pH change. Acid rain introduces acidic substances into lakes, lowering the water’s pH. Aquatic organisms function within limited chemical conditions; excessive acidity can damage tissues, disturb enzyme activity, affect reproduction, and alter the availability of dissolved minerals. Thus very low pH can disrupt life processes, making option A correct. It does not turn water into food or make it basic.

The governing concept is the reaction of acidic food with a reactive metal surface. Curd contains lactic acid and is therefore acidic. If it is stored for a long time in a copper vessel, the acid may react with copper and form copper compounds or salts that can contaminate the food. This is why acidic foods are not normally stored in copper or brass vessels. Hence option A is correct; curd is not acid-free and copper is not universally safe.

The governing concept is the colour response of acid–base indicators. Phenolphthalein becomes pink in a basic medium, while methyl orange is yellow in a neutral or basic medium and red in an acidic medium. Since both observed colours are consistent with a basic solution, option A is correct. An acidic solution would make methyl orange red, and insolubility is not a measure of pH.

The governing concept is neutralisation, in which an acid reacts with a base to form salt and water. Red methyl orange shows that the solution is acidic; colourless phenolphthalein is also consistent with an acidic or neutral medium. Therefore, adding a suitable base will consume excess hydrogen ions and move the solution toward neutrality. More acid would worsen the condition, while sugar and sodium chloride do not neutralise the acid.

The governing concept is the identification of bases by indicators. Turmeric changes to reddish-brown in a basic medium, and a base changes red litmus to blue. In an aqueous basic solution, hydroxide ions are present in excess relative to hydrogen ions. Thus option A is correct. Hydrogen ions would be associated with acidity, while chloride and nitrate ions are spectator ions and do not determine basicity by themselves.