Acids, Bases, and Salts

The governing concept is the indicator test for bases. A substance that turns red litmus blue produces a basic aqueous medium, so the metal oxide behaves as a basic oxide under the stated conditions. Many metal oxides form hydroxides with water, for example sodium oxide forms sodium hydroxide. An acidic oxide would turn blue litmus red; colour alone cannot determine chemical nature, and the substance is not a gas.

A pH below 7 identifies the solution as acidic. Acids react with metal carbonates according to the pattern acid + carbonate → salt + water + carbon dioxide. Carbon dioxide is confirmed by passing it through limewater; insoluble calcium carbonate forms and makes the limewater milky. A glowing splint is associated with oxygen, while the litmus and white-fume choices do not identify carbon dioxide.

A basic aqueous solution indicates that the salt is a basic salt, typically formed from a strong base and a weak acid (like \(\text{H}_2\text{CO}_3\)). Carbonates \(\text{CO}_3^{2-}\) react with acids to form \(\text{CO}_2\). For \(\text{Na}_2\text{CO}_3\):
\[\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow\]
This releases \(\text{CO}_2\) and \(\text{CO}_3^{2-}\) makes the solution basic, so \(X\) is most likely \(\text{Na}_2\text{CO}_3\).

Electrical conductivity in an aqueous acid solution is caused by the movement of ions. When concentration and other conditions are the same, greater conductivity indicates a greater concentration of mobile ions, usually because the acid ionises more extensively. Therefore, the first acid forms more ions in solution, so option A is correct. Option B is opposite to the evidence, while C and D contradict the observed conductivity.

The acidic effect of an aqueous acid depends mainly on the concentration of hydrogen ions, represented as H₃O⁺ in water. Hydrochloric acid is a strong acid and ionises almost completely, whereas ethanoic acid is weak and ionises only partially at the same concentration. Thus hydrochloric acid produces more hydrogen ions and has the stronger effect, making option A correct. The other statements are chemically false.

Hydrogen chloride shows its acidic behaviour only when water allows it to ionise. In water, HCl produces hydronium ions, H₃O⁺, which cause blue litmus to turn red. Dry HCl gas and dry litmus do not provide the aqueous medium needed for this ion formation, so no colour change occurs. Therefore option A explains both observations; the other choices are incorrect.

Ammonia itself is a covalent gas, but in the presence of water it dissolves and reacts to produce hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The hydroxide ions make the solution basic and turn red litmus blue. Dry litmus lacks water, so this ion formation and colour change cannot occur. Hence option A is correct; the remaining choices misidentify the substance or indicator.

Carbonates react with acids to release carbon dioxide, for example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Sodium carbonate is also a salt of a strong base and a weak acid, so its aqueous solution is alkaline because carbonate ions hydrolyse water and produce OH⁻ ions. Sodium chloride is neutral, copper sulphate is not the expected carbonate, and ammonium chloride is acidic. Thus option A is correct.

An acid reacts with a hydrogen carbonate to form a salt, water, and carbon dioxide: acid + hydrogen carbonate → salt + H₂O + CO₂. Passing the gas through limewater produces insoluble calcium carbonate, making the limewater milky. A pop sound is the test for hydrogen, while litmus and phenolphthalein indicate acidity or basicity rather than confirming this gas. Therefore option A is correct.

Diluting a concentrated acid is highly exothermic: hydration of the acid particles releases heat. Adding acid slowly to a larger volume of water allows the released heat to spread through the water and be dissipated more safely, reducing the chance of sudden boiling and dangerous splashing. Water should not be poured into concentrated acid, because a small amount of water can heat rapidly and splatter acid. Thus option A is correct.

The governing concept is that dilution of a concentrated acid is highly exothermic, meaning it releases considerable heat. If a small amount of water is poured suddenly into acid, the water may heat and boil locally, causing the acid to splash violently. Therefore option A is correct. The solution does not freeze, acid cannot change into sugar, and option D is wrong because heat is released. For safety, acid should be added slowly to water with stirring.

The governing concept is the deliquescent and reactive nature of sodium hydroxide. Solid NaOH absorbs moisture from air, so it becomes wet and its composition is altered. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O. Thus option A is correct. Sodium hydroxide does not form gold with oxygen, does not make salt with nitrogen, and clearly does change in moist air.

The governing concept is the limited stability of bleaching powder, whose disinfecting and bleaching action depends on available chlorine. Moisture and prolonged exposure to air promote decomposition and reduce the amount of effective chlorine. A dry, tightly closed container slows this deterioration, so option A is correct. The purpose is not to convert bleaching powder into gypsum or sodium chloride, and taste has no relation to its storage or chemical activity.

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.