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 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.

For an aqueous solution, pH 7 represents neutrality under ordinary school-level conditions. At this point, the acidic effect associated with hydrogen ions and the basic effect associated with hydroxide ions are considered balanced. Hence option C is correct. Option A would describe an acidic solution with pH below 7, and option B would describe a basic solution with pH above 7. A pH value does not mean that water is absent, so option D is incorrect.

A universal indicator is a mixture of indicators that displays different colours over a range of pH values. Green is conventionally associated with a pH near 7, the neutral point of the scale. Thus, in the usual school interpretation, green indicates a neutral solution, making option B correct. Acidic solutions generally produce yellow, orange, or red shades, whereas strongly basic solutions tend toward blue or violet shades.

The colour of a universal indicator changes according to the pH of a solution. Red is found at the low-pH end of the scale, where the concentration of hydrogen ions is relatively high and the solution is strongly acidic. Therefore option A is correct. A neutral solution near pH 7 appears green, a mildly basic solution may appear blue-green, and a strongly basic solution generally gives a violet or purple shade.

Universal-indicator colours are linked to the pH scale. Violet or purple occurs near the high-pH end, where hydroxide-ion concentration is relatively high and the solution shows strong basicity. Consequently, option C is correct. Acidic solutions have pH values below 7 and usually give yellow-to-red shades, while a neutral solution near pH 7 is green. A slightly acidic solution cannot normally produce the violet colour associated with strong alkalinity.

An active metal displaces hydrogen from a dilute acid. The general reaction is: metal + acid → salt + hydrogen gas. For example, zinc reacts with dilute hydrochloric acid as Zn + 2HCl → ZnCl₂ + H₂. Thus option A is correct. Carbon dioxide is typically released when an acid reacts with a carbonate, while oxygen and chlorine are not the general products of an acid-active-metal reaction.

Hydrogen is a combustible gas, so it can burn rapidly when mixed with air. In the standard laboratory test, a burning splint is brought near the mouth of the container; hydrogen ignites with a characteristic ‘pop’ sound. Hence option A is correct. A whistling, rattling, or ordinary blast sound is not the accepted identification test. The test should be performed carefully because hydrogen-air mixtures may ignite suddenly.

Acids react with carbonates to produce three products: a salt, water, and carbon dioxide. The general equation is acid + carbonate → salt + water + CO₂. For example, hydrochloric acid reacts with calcium carbonate to form calcium chloride, water, and carbon dioxide. Therefore option B is correct. Hydrogen is associated with acid-metal reactions, whereas nitrogen and oxygen are not the characteristic gases released by acid-carbonate reactions.

Lime water is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. The calcium carbonate formed is insoluble and appears as a white suspension, making the lime water milky. Therefore option A is correct. The red, blue, and black changes are not the characteristic result of this carbon-dioxide test.