Acids, Bases, and Salts

The relevant concept is bleaching by oxidation. In the presence of moisture, bleaching powder releases available chlorine, which produces oxidising substances such as hypochlorous acid. These oxidise many coloured compounds and make them colourless. Therefore option A is correct. The other choices describe no chemical property responsible for bleaching and are unrelated to colour removal.

Aqueous sodium chloride solution is called brine. When brine undergoes electrolysis, chlorine is produced at the anode, hydrogen at the cathode, and sodium hydroxide remains in the solution. Because the process produces chlorine and an alkali, it is named the chlor-alkali process. Hence option A is correct; the other choices describe different physical or chemical processes.

In the chlor-alkali process, concentrated brine, NaCl(aq), is electrolysed. The overall reaction is 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chlorine and hydrogen are released as gases, while sodium hydroxide is obtained in the solution. Therefore option A is correct. The other hydroxides are not formed from sodium chloride electrolysis in this process.

The governing concept is electrode-wise discharge during electrolysis of brine. Chloride ions move toward the positive anode and are oxidised: 2Cl⁻ → Cl₂ + 2e⁻. Thus chlorine gas is formed at the anode, so option B is correct. Hydrogen is formed at the cathode, while carbon dioxide and ammonia are not products of the chlor-alkali process.

During electrolysis of brine, water is reduced at the negative cathode. The reduction reaction is 2H₂O + 2e⁻ → H₂ + 2OH⁻, so hydrogen gas is released at the cathode. Therefore option B is correct. Chlorine forms at the anode, while oxygen and carbon dioxide are not the expected cathode products in the chlor-alkali process.

The governing definition is that an alkali is a base that dissolves in water and produces hydroxide ions in the solution. For example, sodium hydroxide dissolves readily in water. Therefore the correct answer is option B. An acid donates hydrogen ions, a salt is an ionic product of an acid-base reaction, and an indicator changes colour; none of these terms specifically means a water-soluble base.

The key distinction is solubility. Every alkali is a base, but only a base that dissolves in water is called an alkali. Bases such as copper(II) hydroxide are poorly soluble, so they are not classified as alkalis. Hence option A is correct. Bases can form salts with acids and contain or produce ions, so options B, C, and D are scientifically incorrect.

The governing concept is bacterial fermentation and pH. Food sugars left on teeth are metabolised by oral bacteria, which produce organic acids. The increased hydrogen-ion concentration lowers mouth pH, making the tooth enamel vulnerable to demineralisation and decay. Thus option A is correct. Toothpaste is generally basic, saliva does not always form a base, and teeth do not produce hydrogen gas.

The relevant concept is acid-base neutralisation. Acids produced by bacteria can attack tooth enamel, while the mildly basic substances in toothpaste react with and neutralise some of those acids. This reduces acidity and helps protect enamel, so option A is correct. Increasing acid would worsen decay, dissolving enamel would damage teeth, and pH zero represents extreme acidity rather than protection.

The governing concept is neutralisation. Acidic soil has a pH below the suitable range, while slaked lime, calcium hydroxide, is a basic substance. Its hydroxide ions react with acidic components in the soil and reduce excess acidity, helping bring the pH closer to a level suitable for plant growth. Therefore option A is correct. Option B reverses the effect, while C and D describe unrelated changes that do not occur.

The governing idea is correction of soil pH. Highly basic soil has an excessively high pH, and suitable organic matter can decompose gradually to produce mildly acidic substances. These substances may reduce excess alkalinity and improve the soil’s chemical balance over time. Thus option A is correct. The change is gradual, not instant; organic matter does not turn soil into metal or automatically remove every dissolved salt, so B, C, and D are unsuitable.

The governing concept is the reaction of acids with metals. Curd contains lactic acid and is therefore acidic. On prolonged contact, the acid may react with copper and form copper compounds or salts that can contaminate the food and make it unsafe. Hence option A is correct. Option C is false because curd does contain acid, while B and D describe changes that have no chemical basis in this situation.

The governing concept is the reaction between acids and carbonates. Marble is mainly calcium carbonate, CaCO3. Acids in rain react with this carbonate, producing salts, water, and carbon dioxide; the surface can gradually dissolve, lose polish, and become damaged. Therefore option A gives the relevant reason. Marble is not pure sodium or sugar, and acid rain is acidic rather than basic, so B, C, and D are incorrect.

The governing concept is the effect of acids on pH and aquatic life. Acid rain carries acidic substances into lakes, where they can lower the water’s pH. Many aquatic organisms require a limited pH range for enzyme activity, growth, reproduction, and shell formation. Consequently, excessive acidity can disturb the ecosystem, making option A correct. It does not make water sweet or always basic, and its effect is not merely a colour change.

The governing concept is electrical conduction in electrolytic solutions. An electric current requires charged particles that can move. In an aqueous solution, dissolved acids, bases, or salts may separate into mobile positive and negative ions, which carry charge between the electrodes. Therefore option A is correct. Colour and smell are observable properties, not charge carriers, and sugar generally dissolves without producing ions, so B, C, and D do not explain conduction.

The governing concept is laboratory chemical safety. Acids and bases used in experiments may be corrosive, poisonous, or harmful even in small amounts, and their appearance cannot reliably reveal their danger. Tasting an unknown substance can injure the mouth, throat, or internal organs. Thus option A is correct. Safe identification should use an indicator such as litmus or pH paper under supervision; B, C, and D are scientifically false.

The governing concept is the colour change of an acid-base indicator. Litmus is a natural indicator: blue litmus changes to red when it is placed in an acidic solution, because the indicator responds to the acidic conditions. Hence option A is correct. A basic solution would turn red litmus blue, while a neutral solution normally causes no such characteristic change. “Metallic” is not a solution nature identified by litmus.

The governing concept is the litmus test for bases. Red litmus turns blue when it contacts a basic or alkaline solution, so the observed colour change identifies the solution as basic. Therefore option B is correct. An acidic solution turns blue litmus red, and a neutral solution generally produces no characteristic litmus change. Being salt-free is unrelated to this indicator result and cannot be concluded from the observation, so A, C, and D are incorrect.

The governing concept is the pH scale. At approximately 25°C, a solution with pH 7 is considered neutral, values below 7 indicate increasing hydrogen-ion concentration and acidic nature, and values above 7 indicate basic nature. Thus a pH less than 7 identifies an acidic solution, making option A correct. Basic and neutral describe the other pH ranges, while insolubility concerns dissolving and is unrelated to pH.

The pH scale describes the acidic or basic nature of an aqueous solution. A pH of 7 is neutral; values below 7 indicate acidity, while values above 7 indicate increasing basicity because hydroxide-ion concentration is relatively greater. Therefore, a solution with pH greater than 7 is basic or alkaline, so option B is correct. Option A applies below 7, option C applies at 7, and volatility is unrelated to pH.