Acids, Bases, and Salts

The governing concept is the Arrhenius description of bases. A base produces hydroxide ions, OH⁻, in aqueous solution; these ions account for its alkaline properties. Hence option B is correct. Hydrogen ions are associated with acids, while chloride and carbonate ions may be present in particular salts but are not the defining ions produced by every base. The answer refers to the general school-level definition.

The governing concept is the pH scale, which indicates the hydrogen-ion concentration of an aqueous solution. At ordinary school-level conditions, pH 7 is neutral; values below 7 indicate acidic solutions, while values above 7 indicate basic solutions. Therefore option A, seven, is correct. Ten and fourteen are alkaline-side values, and zero is an acidic pH value rather than the boundary asked for.

Common salt, the crystalline substance commonly used in food, is chemically called sodium chloride and has the formula NaCl. It consists of sodium ions and chloride ions in an ionic lattice, generally obtained from sea water or rock salt. Therefore option B is correct. Sodium hydroxide is caustic soda, sodium carbonate is washing soda, and sodium hydrogen carbonate is baking soda, so those names refer to different compounds.

Sodium hydroxide has the formula NaOH and dissociates in water to produce sodium ions and hydroxide ions, OH⁻. A substance that supplies hydroxide ions in aqueous solution is classified as a base; NaOH is specifically a strong base because it dissociates extensively. Therefore option B is correct. It is not an acid, salt, or neutral oxide. Sodium chloride is the salt involved as the brine starting material, not the NaOH product.

The governing concept is the chemical nature of common aqueous substances. Lemon juice contains citric acid and therefore has an acidic nature, so option B is correct. Soap solution is generally basic because of alkaline salts, lime water contains calcium hydroxide and is basic, and baking-soda solution is mildly basic because sodium hydrogen carbonate reacts with water to produce some hydroxide ions. Their household uses do not change these classifications.

The governing concept is the behaviour of bases in aqueous solution. Soap solution is generally basic because soap contains salts of weak acids and strong bases; it can turn red litmus blue. Therefore option C is correct. Vinegar contains acetic acid, lemon juice contains citric acid, and tamarind juice contains organic acids, so these three substances are acidic rather than basic. The classification refers to their usual aqueous chemical nature.

The governing concept is the colour change of acid-base indicators. Methyl orange appears red in an acidic solution and changes through orange near its transition range to yellow in a sufficiently basic solution. Hence option B is correct. Yellow is associated with the basic-side colour, while blue and green are not the standard methyl-orange colour for an acidic solution. The indicator itself does not make the solution acidic; it only signals the existing pH range.

The pH scale expresses hydrogen-ion concentration: lower pH means greater acidity, pH 7 is approximately neutral at ordinary conditions, and values above 7 indicate basic character. Comparing 2, 6, 7 and 12, the highest value is 12, so it represents the most basic solution and option C is correct. pH 2 and 6 are acidic, while pH 7 is neutral rather than basic.

Acidity increases as pH decreases because a lower pH corresponds to a greater concentration of hydrogen ions. Among the values 1, 4, 8 and 13, pH 1 is the lowest, so it indicates the most acidic solution and option D is correct. pH 4 is also acidic but weaker than pH 1, pH 8 is basic, and pH 13 is strongly basic.

When milk turns sour, microorganisms convert lactose into lactic acid through fermentation. The increased acid concentration raises hydrogen-ion concentration, so the pH decreases. Therefore option A is correct. A rise in pH would indicate a movement toward basicity, not sourness. The pH does not become fixed at 14, and it cannot remain unchanged when an acidic product is being formed.

A neutral solution has approximately equal hydrogen-ion and hydroxide-ion concentrations and is represented by pH 7 under standard conditions. Pure water is generally treated as neutral, so option B is correct. Tomato juice contains acids and is acidic, while soap solution and sodium hydrogen carbonate solution are basic. The word “pure” matters because dissolved impurities can change the pH of ordinary water.

Nettle stings may introduce acidic substances that cause irritation. The governing concept is neutralisation: an acid is reduced by reacting with a suitable base. Therefore, a mild base is the most appropriate choice because it can counteract the acidic effect without adding more acid. A strong acid or acidic juice would worsen the condition, while neutral oil does not neutralise the acid.

Lime, commonly represented by calcium oxide or calcium hydroxide in this context, has a basic character. The governing concept is neutralisation: the basic material reacts with acidic components of soil and raises the soil pH toward a more suitable range. Therefore, option B is correct. Adding lime does not remove all water or transform soil into metal, and it cannot increase acidity.

Bacteria in dental plaque can break down food sugars and produce acids. The governing concept is acid-base neutralisation: these acids can attack and demineralise tooth enamel, whereas a mildly basic toothpaste helps neutralise some of the acid and supports oral protection. Hence option A is correct. Toothpaste is not intended to dissolve enamel, force the pH to 1, or make food more acidic.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. The products are chlorine gas, hydrogen gas, and sodium hydroxide solution; the sodium hydroxide is the alkali that remains in the solution. Thus option C is correct. Calcium hydroxide, magnesium hydroxide, and ammonium hydroxide are different bases and are not the product formed in this process.

Chlorine is a reactive halogen with strong disinfecting action. The governing application is water treatment: controlled amounts of chlorine or suitable chlorine compounds kill many disease-causing microorganisms, so option C is correct. Chlorine is not used as an ordinary fuel, does not make cakes rise, and does not form tooth enamel. Safe handling and correct dosage are essential in real treatment systems.

Sodium hydroxide is a strong industrial alkali with several important applications. It is used in soap manufacture through saponification and in the paper industry during pulping and chemical processing. Hence option A is correct. The words “only” in options B and C make them false, while option D is poorly formed and does not identify a valid principal industrial use. The broad, established answer is soap and paper production.

Bleaching powder is used for decolourising because it has bleaching properties. In the presence of moisture, it can release substances with oxidising action, which chemically changes many coloured substances into colourless products. Thus, option C correctly identifies the relevant property. Forming oil or sugar has no connection with colour removal, and bleaching powder is not described as always acidic; its bleaching action is the governing concept.

The prefix “deca-” means ten, and “hydrate” refers to water associated with a compound. Thus, sodium carbonate decahydrate, Na₂CO₃·10H₂O, contains ten water molecules of crystallisation for each formula unit of sodium carbonate. Therefore, option A is correct. The term does not indicate ten metals or ten acids, and it cannot mean that the crystal has no water. The water is present in a definite chemical ratio.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of certain salt crystals. For example, washing soda is Na₂CO₃·10H₂O and blue copper sulphate is CuSO₄·5H₂O. Therefore, option B is correct. It is not an uncertain amount present in every salt, nor is it restricted to boiling water. The term also does not mean that the water itself is always acidic.

Methyl orange is an acid–base indicator whose colour depends on the acidity of the solution. In an acidic medium it appears red, while in a basic medium it appears yellow. Since the question specifies a basic solution, the correct observation is yellow, so option C is correct. Red belongs to the acidic range; blue is associated with other indicators such as litmus, and black is not the normal methyl-orange colour.

Blue litmus turning red is a characteristic test for an acidic solution. A second independent observation supports the same conclusion: acids react with metal carbonates to form a salt, water, and carbon dioxide gas, for example acid + carbonate → salt + H₂O + CO₂. Thus the solution is acidic, making option D correct. A basic or neutral solution would not normally produce this combined indicator pattern.

The first observation is decisive: blue litmus turns red in an acidic medium. The second observation is consistent with it because phenolphthalein is colourless in acidic solutions, whereas it becomes pink in a basic range. Together, the tests support the conclusion that the unknown solution is acidic, so option B is correct. Colourlessness with phenolphthalein alone could also occur in a neutral solution, but the litmus result rules neutrality out in this case.

Red litmus turning blue is evidence of a basic medium. Phenolphthalein provides a second confirming observation because it becomes pink in a basic solution and remains colourless in acidic or neutral conditions. Since both tests point to the same chemical nature, the solution is basic and option C is correct. A neutral solution would not normally change red litmus or produce the pink phenolphthalein colour, while the last option is contradictory and unsupported.

According to the Arrhenius concept, a base increases the concentration of hydroxide ions in aqueous solution. Sodium hydroxide dissociates almost completely in water: NaOH(aq) → Na⁺(aq) + OH⁻(aq), so it is a strong base and clearly gives OH⁻ ions. Hydrochloric acid, lemon juice, and vinegar are acidic substances that increase hydrogen-ion concentration instead. Therefore, option C is correct.