Acids, Bases, and Salts

The governing concept is environmental acidification. Acid rain contains acids formed when atmospheric pollutants react with water and oxygen. When this rain enters lakes or streams, it can lower their pH. Aquatic organisms may then experience disturbed enzyme activity, reproduction, respiration, and mineral balance; sensitive species can decline or die. Hence option A is correct, while B, C, and D do not describe the chemical or biological effect.

The governing concept is acid-base identification followed by neutralisation. A solution that turns red litmus blue is basic, while one that turns blue litmus red is acidic. When suitable amounts of an acid and a base react, hydrogen ions combine with hydroxide ions to form water, and the remaining ions form a salt: H+ + OH− → H2O. Thus option A is correct; the other products are unsupported.

Neutralisation is governed by the net ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l). An acidic solution contains hydrogen or hydronium ions, and sodium hydroxide dissociates to supply OH⁻ ions. These ions combine to form water; Na⁺ and the acid’s negative ion remain as dissolved salt ions. Therefore A is correct. The other choices describe no valid ionic process in this neutralisation.

A base is a substance that reacts with an acid to form salt and water, usually in a neutralisation reaction. Solubility is the key distinction here: an alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Since the stated substance is insoluble, it can be called a base but not an alkali. Therefore option A is correct; C is the reacting counterpart and D only detects chemical nature.

Bleaching powder, commonly represented as CaOCl₂ in school-level chemistry, is prepared by passing chlorine gas over dry slaked lime, Ca(OH)₂. The word “dry” matters because excess moisture can interfere with obtaining the intended product. Sodium chloride is a raw material in chlorine manufacture but is not the direct absorbing substance here. Gypsum and washing soda have different uses and do not give bleaching powder in this preparation, so A is correct.

Dilution increases the volume without adding a basic substance. Consequently, the hydrogen-ion concentration decreases, so the pH rises toward 7, but the remaining H⁺ ions can still keep the solution acidic. Water does not automatically convert an acid into a base, nor does it destroy all ions. The exact pH change depends on concentration and acid strength, but the concept described in A explains why strong dilution does not necessarily produce basicity.

Acidic behaviour in aqueous solution depends on the formation of hydrogen or hydronium ions. Water allows an acid to ionise or dissociate, and these ions change blue litmus to red. In the dry state, ions are not freely available to move and interact with the indicator, so the colour change may not occur. Thus A gives the appropriate reason. The other choices contradict the definition and use of acid–base indicators.

The gas is carbon dioxide. First, carbon dioxide reacts with lime water, calcium hydroxide, to form insoluble calcium carbonate, which produces the milky appearance. When carbon dioxide is passed in excess, the precipitate changes into soluble calcium hydrogen carbonate. Acids reacting with carbonates release carbon dioxide, so option A is correct; the other reactions do not normally produce this gas.

Electrical conductivity in an aqueous acid solution depends mainly on the concentration and mobility of ions. Since both acids have the same stated concentration, the one showing greater conductivity is producing more mobile ions, indicating a greater degree of ionisation. Thus option A is the best conclusion. Conductivity alone should not be used to claim that an acid has become a base, and option C contradicts the observation.

A red litmus paper turns blue when a basic medium is present. The gas shows this effect only when moisture is available, because water enables the gas to dissolve and produce ions responsible for basicity. Ammonia is the familiar example: it dissolves in water and forms an alkaline solution. Hence option A is correct; the dry paper lacks the aqueous medium needed for the colour change.

In the chlor-alkali process, concentrated aqueous sodium chloride is electrolysed. Water is reduced at the cathode, producing hydrogen gas and hydroxide ions: 2H2O + 2e− → H2 + 2OH−. Hydrogen burns rapidly with a characteristic ‘pop’ when a burning splint is brought near it. Therefore option A is correct; chlorine is produced at the anode, not the cathode.

Gypsum is calcium sulfate dihydrate, CaSO4·2H2O. Controlled heating removes one and a half molecules of water to form plaster of Paris, calcium sulfate hemihydrate, CaSO4·1/2H2O. Excessive heating can remove the remaining water and produce anhydrous calcium sulfate, which does not set properly with water. Thus option A is correct.

Methyl orange is red in a strongly acidic medium, while phenolphthalein remains colourless in acidic and neutral solutions. Together these observations indicate that the unknown solution is acidic. Neutralisation requires an alkali to supply hydroxide ions that react with hydrogen ions to form water. Sodium hydroxide is therefore suitable, making option A correct. Vinegar, lemon juice, and dilute acid would add more acid rather than neutralise it.

Sodium hydroxide is deliquescent, so it absorbs water vapour from air and becomes wet. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O. Water absorption changes its physical condition, while carbonate formation lowers chemical purity. Therefore option A gives both reasons; the other choices describe no relevant atmospheric process.

A gas that turns lime water milky is carbon dioxide, so the unknown substance should contain a carbonate or hydrogencarbonate ion. Sodium carbonate reacts with an acid to release CO2 and forms a basic aqueous solution because carbonate ions hydrolyse water to produce hydroxide ions. Sodium chloride is nearly neutral, ammonium chloride is acidic, and copper sulfate is not the expected carbonate source. Hence option A is correct.

Neutrality and electrical conductivity describe different properties. A neutral sodium chloride solution has nearly equal concentrations of hydrogen and hydroxide ions, so its pH is close to 7. However, dissolved sodium chloride dissociates into mobile Na+ and Cl− ions. These charged particles move towards opposite electrodes and carry electric current. Therefore option A is correct; being neutral does not mean that a solution lacks ions. The other options are chemically incorrect.

Marble consists mainly of calcium carbonate, CaCO3. Acid rain contains acids such as sulfuric acid and nitric acid, which react with carbonate minerals. The reaction consumes calcium carbonate and produces soluble salts, water, and carbon dioxide; repeated reaction gradually erodes the polished surface. Thus option A is correct. Sodium chloride and copper sulphate are salts, while sodium hydroxide is a base and is not the principal constituent responsible for marble damage.

Baking soda is sodium hydrogen carbonate, NaHCO3. On heating, it undergoes thermal decomposition: 2NaHCO3 → Na2CO3 + CO2 + H2O. The solid residue is therefore sodium carbonate, Na2CO3, and its aqueous solution is alkaline because carbonate ions undergo hydrolysis and produce hydroxide ions. Option A is correct. Sodium chloride is neutral, calcium sulphate is a different salt, and ammonium chloride gives an acidic solution.

pH is related to hydrogen-ion concentration: pH = −log[H+]. An acidic solution contains excess H+ ions. When OH− ions are added, they neutralise these ions according to H+ + OH− → H2O. The hydrogen-ion concentration consequently decreases, so the solution becomes less acidic and its pH increases. Option A correctly states this neutralisation. The other options do not describe the relevant ionic reaction or pH change.

A basic oxide reacts with an acid in a neutralisation reaction to form salt and water. For example, CuO + 2HCl → CuCl2 + H2O. Since copper oxide consumes the acid and produces these two products, it shows basic character and is classified as a basic oxide. Option A is correct. An acidic oxide generally reacts with a base, while copper oxide is neither a gas nor an ordinary salt in this reaction.

Neutralisation depends on the amounts of reacting H+ and OH−, not merely on the volumes mixed. Equal volumes can contain different numbers of reactive particles if the concentrations differ; strong and weak substances can also contribute ions differently. If the acid supplies more effective H+ than the base can neutralise with OH−, an excess remains and the final solution is acidic. Thus option A is correct. Equal volume does not guarantee neutral pH.

An aqueous solution of sodium chloride is called brine. Electrolysis of brine is the chlor-alkali process. At the cathode, water produces hydrogen gas and hydroxide ions; sodium ions remain in solution and combine with hydroxide ions to form sodium hydroxide. Chlorine gas is released at the anode. Therefore option A is correct. Calcium carbonate, ammonium chloride, and copper oxide are not products formed as the important base in this process.

The pH scale is logarithmic, and pH = −log[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 4 to pH 7 is three units, so the concentration changes by 10³ = 1000 times in the decreasing direction. Hence option C is correct; the other numerical options do not apply the logarithmic scale correctly.

Bleaching powder works mainly because it contains available chlorine, which gives it bleaching and disinfecting properties. Moisture and exposure to air can cause gradual decomposition and reduce the amount of effective chlorine. A dry, tightly closed container limits this deterioration and preserves its activity. Thus option A is correct; the other choices describe unrelated or chemically incorrect changes.

Many acids react with metals above hydrogen in the reactivity series and release hydrogen gas: metal + acid → salt + H₂. Magnesium is sufficiently reactive, so brisk bubbles appear, whereas silver is below hydrogen and normally does not displace it from a dilute non-oxidising acid. The observation therefore demonstrates different metal reactivities, making option A correct.