Acids, Bases, and Salts

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.

This is an application of neutralisation. Since the bee sting is described as acidic, a mild base can partly neutralise the acid and reduce its irritating effect. Sodium hydrogen carbonate, commonly called baking soda, is mildly basic, so option C is correct. Vinegar, lemon juice and tamarind juice are acidic and would not be suitable for neutralising an acidic sting. In real treatment, the wound should also be handled according to appropriate medical guidance.

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.

Plant roots absorb mineral nutrients most effectively when soil conditions, including pH, are suitable. The governing concept is nutrient availability: extremely acidic or extremely basic soil can change the solubility of minerals and may damage roots, making absorption difficult. Thus option A is correct. Neutral soil is not automatically harmful, and the other choices are oversimplified rather than scientifically reliable conditions.

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.

Magnesium hydroxide is a sparingly soluble base. The governing concept is neutralisation: its hydroxide ions react with excess hydrochloric acid in the stomach, reducing acidity and discomfort. For example, the overall reaction can be represented as Mg(OH)2 + 2HCl → MgCl2 + 2H2O. Therefore option B is correct; the other options do not describe an antacid reaction.

An aqueous solution of sodium chloride is called brine. In the chlor-alkali process, brine undergoes electrolysis. Chlorine is released at the anode, hydrogen at the cathode, and sodium hydroxide remains in the solution, subject to industrial separation and operating conditions. Therefore option B is correct. The other choices are not the name of the industrial electrolysis process that begins with brine.

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.

Hydrogen is one of the products formed during electrolysis of brine in the chlor-alkali process. A major industrial use of hydrogen is hydrogenation, in which hydrogen adds to unsaturated bonds in vegetable oils, often in the presence of a catalyst, converting them into more saturated fats. Therefore option A is correct. The other choices are not standard uses of this hydrogen product.

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 prepared by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)₂. In simplified form, chlorine reacts with calcium hydroxide to produce bleaching powder along with other products. Therefore, option B is correct. Hydrogen and oxygen do not provide the required chlorinating action, while carbon dioxide reacts differently and is not used for this preparation. The lime must be dry so that the product forms properly.

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.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it decomposes according to the equation 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The gaseous product asked for is therefore carbon dioxide, so option D is correct. The released carbon dioxide forms bubbles in dough and helps cakes or other baked foods become light and porous. Hydrogen, chlorine, and oxygen are not products of this decomposition.

Baking soda is chiefly sodium hydrogen carbonate, NaHCO₃, whereas baking powder is a mixture containing baking soda and a mild edible acid, commonly such as tartaric acid, along with a suitable drying agent. When moistened and heated, the acid reacts with the bicarbonate and produces carbon dioxide. Therefore, option B is correct. Baking soda alone does not contain the added edible acid, and baking powder is neither common salt nor chemically identical to baking soda.

When baking soda is used in excess, heating can leave sodium carbonate or another alkaline residue if it is not fully neutralised by an acid. Alkaline substances generally produce an unpleasant bitter or soapy taste, so option A is correct. Baking powder includes an edible acid that reacts with the bicarbonate and reduces this alkaline residue. Formation of common salt, freezing of water, and acid rain are unrelated to the taste change in cake.