Acids, Bases, and Salts

Excess hydrochloric acid in the stomach can produce a burning sensation and discomfort. An antacid contains a mild base, such as a suitable hydroxide, carbonate, or related basic compound. It reacts with some excess acid and neutralises it, reducing acidity without adding more acid. Therefore option A correctly states the main chemical action. The other choices describe gas control, water removal, or acid increase, none of which explains antacid neutralisation.

Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)2, which behaves as a weak or mildly basic substance in this context. Stomach acidity is associated with excess hydrochloric acid. Magnesium hydroxide reacts with and neutralises part of that acid, thereby reducing irritation. Hence option A identifies the relevant property. It is not a strong acid, and the statements about being merely a metal or not being a salt do not explain its antacid action.

A pH below 5.5 indicates that the mouth is sufficiently acidic to attack the mineral structure of tooth enamel. Acids can dissolve or weaken enamel, the hard protective outer covering of a tooth. Once enamel is damaged, decay can progress more easily. Therefore option A is correct. The distractors incorrectly assign the effect to a base, salt, or water and do not describe acid erosion of enamel.

Bacteria acting on food particles in the mouth can produce acids. These acids lower mouth pH and may gradually weaken tooth enamel. Mildly basic substances in toothpaste can neutralise or reduce some of this acidity, helping restore a less acidic environment during cleaning. Thus option A states the purpose correctly. Toothpaste is not intended to acidify teeth, convert them into salt, or stop saliva, so B, C, and D are unsuitable.

An ant sting commonly introduces formic acid into the skin, which contributes to irritation and a burning sensation. Baking soda, sodium hydrogen carbonate, is mildly basic. When applied appropriately as a dilute solution, it can react with and partially neutralise the acid, helping reduce irritation. Therefore option A is correct. The other choices either reverse the acid-base effect or mention unrelated changes involving skin acidity or carbon dioxide.

Acidic soil contains excess acidity, which can make conditions unsuitable for the healthy growth of some crops. Slaked lime is calcium hydroxide, Ca(OH)2, a basic substance. It reacts with acidic components in the soil and helps neutralise them, raising the soil pH toward a more suitable range. Therefore option A is correct. The other options describe changes that are unrelated to the agricultural use of lime.

Basic soil has an excess of alkaline character and therefore a pH higher than the desired range for some plants. During decomposition, organic matter can produce humic and other weak acidic substances. These substances can help lower excessive alkalinity and improve the soil’s chemical condition. Hence option A is the best answer. The remaining choices either increase basicity or describe unrelated changes involving metal formation or water removal.

In the chlor-alkali process, an aqueous solution of sodium chloride, called brine, is electrolysed. The products include sodium hydroxide in solution, chlorine gas at the anode, and hydrogen gas at the cathode. Thus option A names the correct industrial process. Rusting is corrosion, photosynthesis is a biological food-making process, and combustion is reaction with oxygen; none of these produces sodium hydroxide from brine.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode, water gains electrons and produces hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Therefore the anode–cathode order is chlorine and hydrogen, making option A correct; option B reverses the electrodes, while C and D name gases not produced in this process.

In the chlor-alkali process, an aqueous solution of sodium chloride is electrolysed. Chlorine is released at the anode, while water is reduced at the cathode to form hydrogen and hydroxide ions. The sodium ions remaining in solution combine with hydroxide ions, producing sodium hydroxide: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Thus option A is correct. The other hydroxides are not formed from brine because calcium, magnesium, and ammonium ions are absent.

Bleaching powder, commonly represented as calcium oxychloride, is useful in water treatment because it releases chlorine or hypochlorous substances in the presence of moisture. These powerful oxidising species damage and kill many disease-causing microorganisms, reducing contamination. Therefore option A correctly states its disinfecting action. It does not sweeten water, convert water into a metal, or transform it into oil; options B, C, and D have no chemical basis.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, calcium hydroxide. The reaction is commonly represented as Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O, although the product may contain related calcium compounds depending on conditions. Thus chlorine is the required gas and option A is correct. Hydrogen, oxygen, and nitrogen do not provide the characteristic chlorinating and disinfecting product, so options B, C, and D are unsuitable.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide forms bubbles in the dough or batter, and these expanding bubbles make the food rise. Hence option A is correct. The process is not caused by stopping hydrogen, absorbing oxygen, or solidifying water, so B, C, and D are incorrect.

Baking powder contains baking soda, sodium hydrogen carbonate, together with a mild edible acid such as tartaric acid. When moistened and heated, the acid reacts with NaHCO₃ and produces carbon dioxide. The gas bubbles expand through the mixture and make cakes or bread light and spongy. Therefore option A is correct. The acid is not added to form a metal, darken food, or prevent gas formation; B, C, and D contradict the purpose of baking powder.

Baking soda is sodium hydrogen carbonate, NaHCO₃, whereas washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. The different names reflect different compositions, formulas, and uses: baking soda is used in baking and as a mild antacid, while washing soda is used for cleaning and softening hard water. Thus option A is correct. The other pairs name unrelated salts or acids and do not identify these two substances.

Hard water contains dissolved calcium and magnesium ions. Washing soda, sodium carbonate decahydrate, dissolves to provide carbonate ions. These react with Ca²⁺ and Mg²⁺ to form insoluble carbonates, such as CaCO₃, which can be removed: Ca²⁺ + CO₃²⁻ → CaCO₃(s). Consequently, option A is correct. Washing soda does not soften water by making it acidic, gaseous, or black, so B, C, and D are implausible.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Careful heating removes part of its water of crystallisation and produces Plaster of Paris, calcium sulphate hemihydrate: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Limited heating is important; excessive heating can remove more water and reduce the desired setting property. Therefore A is correct, while calcium does not burn and gypsum does not become sulphur or common salt.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. In water, it supplies carbonate ions that react with dissolved calcium and magnesium ions responsible for permanent hardness, forming insoluble carbonates that can be separated. For example, Ca²⁺ + CO₃²⁻ → CaCO₃(s). Hence option A is correct. Baking soda does not provide enough carbonate for this standard treatment, while Plaster of Paris and bleaching powder have different compositions and uses.

The governing concept is water of crystallisation: some salts contain a fixed number of water molecules within their crystal structure. Blue copper sulphate is CuSO₄·5H₂O. On heating, these water molecules are driven off, producing anhydrous CuSO₄, which is white. Thus option A is correct. The copper does not become a gas, sulphate does not turn into water, and oxygen is not added.

The governing concept is hydration and water of crystallisation. Heating removes water from blue CuSO₄·5H₂O and forms white anhydrous CuSO₄. When water is added again, the salt incorporates the water molecules and the hydrated crystal form is restored, giving the characteristic blue colour. Therefore option A is correct; the other options describe unrelated substances or products.

The governing concept is that water of crystallisation can influence the structure and appearance of a salt. When this water is removed by heating, the hydrated salt may become anhydrous and its colour can change; for example, blue hydrated copper sulphate becomes white. Therefore option A is correct. Removal does not always convert a salt into a gas or metal, and a visible change is possible.

The governing concept is that combustion requires a continuous supply of oxygen. In a soda-acid extinguisher, a reaction produces carbon dioxide, which is non-combustible and denser than air. It spreads around the flame and displaces or cuts off atmospheric oxygen, so combustion cannot continue. Option A is correct. CO₂ does not supply oxygen, form hydrogen, or extinguish fire by making it acidic.

The governing concept is the pH scale: a lower pH indicates a higher concentration of hydrogen ions and therefore greater acidity. Acid rain forms when pollutants such as sulphur and nitrogen oxides produce acids in atmospheric moisture, making its pH lower than ordinary rain. Hence option A is correct. A pH of 14 is strongly basic, and acid rain is not always basic or free from dissolved substances.

The governing concept is the reaction of acids with carbonates. Marble is mainly calcium carbonate, CaCO₃. Acids present in acid rain react with it, gradually dissolving or altering the stone and sometimes producing salts, water, and carbon dioxide. Therefore option A is correct. Acid rain cannot turn marble into gold; it is not neutral, and marble is a solid rock rather than a gas.

The governing concept is the chlor-alkali process. Common salt, sodium chloride, is dissolved in water to make brine. Electrolysis of brine produces sodium hydroxide in solution, chlorine gas at the anode, and hydrogen gas at the cathode: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Thus option A is correct; salt has many industrial uses and does not form only acids.