Acids, Bases, and Salts

The question tests examples of naturally occurring acids. Lemon juice contains citric acid, which contributes to the sour taste of citrus fruits. Curd contains lactic acid, formed during the conversion of milk sugar by beneficial bacteria. Therefore option A correctly matches each food with its characteristic acid. Options B reverses the examples, while hydrochloric acid and sulphuric acid are not the normal acids responsible for these foods’ sourness.

Dilution of a concentrated acid with water is an exothermic process, so heat is released. If a small quantity of water is poured into concentrated acid, the water may heat suddenly, boil, and produce acidic splashes. These splashes can cause severe burns. Therefore, the safe laboratory rule is to add concentrated acid slowly to a larger volume of water while stirring. Option A correctly describes the danger; the other choices contradict the heat-producing process.

A carbonate reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. Carbon dioxide passed through limewater forms insoluble calcium carbonate, making the limewater milky. Thus option A is the confirmatory observation. A pop sound is the test for hydrogen, while litmus change and a yellow precipitate are not the standard identification tests for carbon dioxide.

The acidic character of an acid appears when it is dissolved in water because the acid ionises or dissociates to produce hydrogen ions, more accurately represented in water as hydronium ions, H3O+. These ions cause indicator changes and other acidic properties. A dry solid acid cannot produce these aqueous ions, and oil or vacuum does not provide the necessary medium. Therefore, option A is correct.

Hydrogen chloride shows acidic behavior only after dissolving in water. In moist conditions, HCl reacts with water to form hydronium ions and chloride ions: HCl + H2O → H3O+ + Cl−. Hydronium ions cause blue litmus to turn red. Dry HCl gas and dry litmus provide no water for this ionisation, so no color change occurs. Hence option A is correct; HCl is not a base and can form an acid in water.

The reaction between an acid and a base in suitable amounts is called neutralisation. The hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt. For example, HCl + NaOH → NaCl + H2O. Option A is therefore correct. This is not decomposition or a simple combination without salt; formation of salt and water is the defining result.

The discomfort from a bee sting is commonly treated in this school-level context as the effect of an acidic substance. Baking soda, sodium hydrogen carbonate, is a mild base. Applied appropriately, it can partially neutralise the acidic material and reduce irritation. Vinegar, lemon juice, and hydrochloric acid are acidic, so they would not neutralise the sting. Therefore option A is the best answer under the stated neutralisation principle; actual stings should still receive suitable first aid.

Stomach irritation can result when excess hydrochloric acid is present. Antacids contain mild basic substances, such as magnesium hydroxide or other suitable compounds, which react with and neutralise some of this excess acid. The reaction reduces acidity rather than strengthening it; it does not involve producing poisonous carbon monoxide or converting water into acid. Therefore option A correctly expresses the neutralisation principle behind relief from acidity.

Sodium hydroxide dissolves in water to give a strongly basic solution containing hydroxide ions. Bases change the indicator litmus from red to blue, so the red litmus paper becomes blue when it contacts sodium hydroxide solution. The change is an indicator response, not a gas-producing reaction; bubbles are unnecessary. A base does not normally make red litmus colorless or green. Thus option A is the only correct observation.

Turmeric contains a natural indicator substance called curcumin. This indicator remains yellow in an acidic or neutral environment but changes to reddish-brown in a basic environment. Soap solution is generally basic because its ingredients produce an alkaline solution in water. Therefore, applying soap to a turmeric stain causes the observed reddish-brown color. Option A gives the reason; the other choices do not describe the indicator reaction.

Limewater contains dissolved calcium hydroxide and is basic. Soap solution is also generally basic because soap hydrolysis produces an alkaline solution. Thus both substances in option A have basic nature. Lemon juice, vinegar, curd, and tamarind juice contain acids, while soda water contains dissolved carbon dioxide and is weakly acidic; orange juice is also acidic. Therefore none of options B, C, or D contains two bases.

The pH scale indicates the acidic or basic nature of an aqueous solution. A lower pH means a greater concentration of hydrogen ions, so the solution is more acidic; a higher pH indicates a more basic solution. Therefore, option A is correct. Option B reverses the pH relationship, while pure water is approximately neutral and contains ions from the slight ionisation of water.

At about room temperature, pure water is considered neutral because it produces hydrogen ions and hydroxide ions in equal concentrations through self-ionisation. This balance gives a pH close to 7. Thus option A is correct. A strong acid would have a pH well below 7, and a strong base would have a pH well above 7; pure water is not simply a salt solution.

Very acidic soil can interfere with nutrient availability and may harm plant roots. Slaked lime, calcium hydroxide, is a basic substance that reacts with and neutralises excess acidity in the soil. Hence option A is correct. Vinegar, lemon juice, and acid rainwater are acidic, so adding them would increase rather than correct the soil’s acidity.

Bacteria in the mouth can use sugars to produce acids. As the pH of the mouth falls below about 5.5, the mineral component of tooth enamel begins to dissolve more readily, increasing the risk of decay. Therefore, option A is correct. A pH of 7 is neutral, while 8.5 and 14 represent alkaline conditions and do not describe the critical acidic level for enamel damage.

Bacteria can ferment food sugars in the mouth and produce acids, which lower pH and contribute to enamel demineralisation. Mildly basic substances in toothpaste help neutralise some of this acid and support oral protection. Thus option A is correct. Toothpaste is not intended to transform teeth, remove all saliva, or produce carbon dioxide; those statements do not describe its relevant chemical action.

Sodium hydroxide is hygroscopic and can absorb moisture, but it also absorbs carbon dioxide from air. The absorbed carbon dioxide reacts with sodium hydroxide, for example: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Therefore option A is correct. Nitrogen, oxygen, and noble gases do not explain the gradual formation of sodium carbonate under ordinary storage conditions.

Cold, dilute chlorine reacts with sodium hydroxide to produce sodium hypochlorite: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O. Hypochlorite is a bleaching and disinfecting substance, so it is used to remove colour from materials such as textiles and paper. Option A is correct. It does not sweeten food, convert metals into gold, or acidify drinking water.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Thus sodium carbonate, water, and carbon dioxide are formed, making option A correct. The reaction does not produce sodium metal, oxygen, or only hydrogen, and the substance does not remain unchanged.

Baking powder generally contains sodium hydrogen carbonate and a suitable edible acid. When the mixture becomes moist and is heated, an acid-base reaction produces carbon dioxide; the gas bubbles expand and become trapped in the batter, making the cake rise. Option A is correct. The process does not convert oxygen into gold, make the dough completely acidic, or freeze water.