Acids, Bases, and Salts

The governing concept is neutralisation. Excess hydrochloric acid in the stomach can cause symptoms such as burning or indigestion. An antacid is a mild base, or a substance that reacts with and neutralises excess acid without being strongly corrosive. Therefore, option A is correct. A strong acid would increase acidity, while a metal or a precipitate is not a general name for an acidity-relieving medicine.

The governing concept is the basic nature of magnesium hydroxide, Mg(OH)₂. Milk of magnesia is a suspension of this sparingly soluble base in water. It can react with and neutralise excess hydrochloric acid in the stomach, which is why it is used as an antacid. Therefore, option A is correct. It is not a strong acid, a gas, or simply a metallic substance; those descriptions do not represent its chemical nature.

The governing concept is the effect of acidity on tooth enamel. When the pH in the mouth falls below about 5.5, the medium becomes sufficiently acidic to promote demineralisation of enamel, especially if acids remain in contact with the teeth. This increases the risk of tooth decay, so option A is correct. pH 7 is neutral, while pH 10 and pH 14 are alkaline and do not represent the critical acidic threshold.

The governing concept is neutralisation of acids by a base. Bacteria can produce acids from food residues in the mouth, and these acids may attack tooth enamel. Mildly basic toothpaste helps neutralise or reduce the effect of those acids during brushing, supporting oral hygiene. Therefore, option A is correct. It does not make teeth acidic, convert saliva into gas, or turn teeth into salt; those choices describe no relevant chemical purpose.

The governing concept is the indicator test for bases. Soap solution is generally alkaline because soap undergoes hydrolysis in water and produces an alkaline medium. Bases change red litmus paper to blue, so option B is correct. Red would indicate no relevant change, green is associated with some universal-indicator readings rather than ordinary litmus paper, and colourless is not the expected litmus response. The result assumes a normal aqueous soap solution.

The governing concept is neutralisation: an antacid should contain a mild base that reacts safely with excess hydrochloric acid in the stomach. Magnesium hydroxide is suitable because Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Sodium hydroxide is a strong, corrosive alkali, while acetic acid and hydrochloric acid would add acidity rather than reduce it. Therefore, option A is correct.

An olfactory indicator is identified through a change in smell rather than a change in colour. Onion is an olfactory indicator because its characteristic smell is affected by the nature of the medium and may be detected differently in acidic and basic conditions. Turmeric, litmus and phenolphthalein are colour indicators; they show changes through colour. Thus, onion, option B, is the correct answer.

The chemical name of bleaching powder is calcium oxychloride, commonly represented as \(\mathrm{CaOCl_2}\). It is used for bleaching textiles and paper pulp and for disinfecting water. Sodium carbonate is washing soda, not bleaching powder. Exam tip: Remember bleaching powder together with calcium oxychloride and \(\mathrm{CaOCl_2}\).

The pH scale shows that a pH below 7 represents an acidic substance; therefore, soil with pH 5 is acidic. Quicklime, CaO, is a basic oxide. In moist soil it forms calcium hydroxide, which neutralises excess acidity and raises the pH toward a more suitable range. HCl would increase acidity, while ammonium chloride is an acidic salt and sodium chloride is not the appropriate corrective material. Option A is correct.

The governing idea is the litmus test for acidic, basic and neutral solutions. An aqueous solution of sodium chloride, formed from a strong acid and a strong base, is generally neutral and therefore does not change either red or blue litmus. Hydrochloric acid turns blue litmus red, whereas sodium hydroxide and calcium hydroxide turn red litmus blue. Hence, option C is correct.

Plaster of Paris, CaSO₄·½H₂O, reacts with water to form gypsum, CaSO₄·2H₂O. Interlocking gypsum crystals make the material set and become hard. It does not dissolve completely or produce a gas. Exam tip: Remember that Plaster of Paris hardens on adding water, so it is used for casts and moulds.

Water of crystallisation is the definite number of water molecules chemically associated with a salt within its crystal structure. It is not simply moisture or any natural source of water. For example, washing soda is Na₂CO₃·10H₂O, where ten water molecules are associated with each formula unit. River water, rainwater and seawater are external mixtures, not crystal water. Therefore, option A is correct.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO₄·5H₂O, and their blue colour is associated with water of crystallisation. On heating, this water is removed and anhydrous copper sulphate, CuSO₄, is formed, which is white. The change is reversible when water is added again. The crystals do not become red, green or black under the stated school-level condition, so option A is correct.

Anhydrous copper sulphate, CuSO₄, is white because it contains no water of crystallisation. When water is added, hydration occurs and blue hydrated copper sulphate, CuSO₄·5H₂O, is produced. This is the reverse of the colour change seen on heating blue crystals. The substance does not become red, yellow or black in this standard reaction, so option A, blue, is correct.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Thus, carbon dioxide is released as the gas. The water formed is vapour, while sodium carbonate remains as a solid; no hydrogen, oxygen or nitrogen is produced in this reaction. Therefore, option A is correct.

Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid, such as tartaric acid. On heating or when water is added, the acid reacts with the soda to release carbon dioxide, making cakes and bread rise. Iron, chlorine and copper are not added for this purpose. Exam tip: Remember the difference between baking soda and baking powder—baking powder also contains an acid.

Sodium hydrogen carbonate in baking powder produces carbon dioxide on heating or when it reacts with an acid. Bubbles of this gas expand the batter or dough, making cakes and bread rise. Oxygen supports combustion, but it does not cause the rising. Exam tip: Remember that baking powder and baking soda release carbon dioxide.

In a soda-acid fire extinguisher, sodium hydrogen carbonate reacts with an acid to produce carbon dioxide. This gas forms a blanket over the fire and cuts off its contact with oxygen, thereby stopping combustion. Oxygen is a close distractor, but it supports burning rather than putting out a fire. Exam tip: Remember carbon dioxide as the gas that smothers fire in this extinguisher.

In the chlor-alkali process, concentrated aqueous sodium chloride solution, called brine, is electrolysed. Chlorine is produced at the anode, hydrogen at the cathode, and sodium hydroxide remains in solution. A sucrose solution is not an electrolyte, so it is not used in this process. Exam tip: Remember that brine means concentrated aqueous NaCl solution.

In the chlor-alkali process, aqueous sodium chloride (brine) is electrolysed. At the anode, chloride ions lose electrons to form chlorine gas: \(2Cl^- \rightarrow Cl_2 + 2e^-\). Therefore, chlorine is correct. Hydrogen is produced at the cathode, not at the anode, by reduction of water. Exam tip: oxidation occurs at the anode; in the chlor-alkali process, remember it by chlorine formation.

Na₂CO₃·10H₂O is washing soda, or sodium carbonate decahydrate. Its carbonate ions react with dissolved Ca²⁺ and Mg²⁺ ions responsible for permanent hardness, producing insoluble carbonates that can be removed. NaHCO₃ is baking soda, CaOCl₂ is bleaching powder, and CaSO₄·½H₂O is plaster of Paris. Therefore, the salt used for washing and softening hard water is option A.

Acidic soil contains excess acidic substances, so it is improved by adding a suitable base through neutralisation. Slaked lime, Ca(OH)₂, is a basic substance and reacts with excess acidity, helping raise the soil pH toward a suitable value for crops. Vinegar and lemon juice contain acids, and hydrochloric acid would make the soil even more acidic. Hence, slaked lime in option A is the correct choice.

The governing concept is the indicator test for acids and bases. An acidic solution contains hydrogen ions in water and changes blue litmus to red. Therefore, option A is correct. Option B describes the usual action of a base on red litmus, while options C and D do not represent the standard litmus response to an acid.

The governing concept is the use of litmus as an acid-base indicator. A basic solution produces hydroxide ions in water and changes red litmus to blue, so option B is correct. Option C is the characteristic response of blue litmus to an acid. Options A and D do not describe the normal result of testing a basic solution with red litmus.

The governing concept is the behaviour of a natural indicator. Turmeric remains yellow in an acidic or neutral medium but becomes reddish brown in a basic medium. Soap solution is generally basic because of its alkaline components, so option C correctly explains the colour change. Options A and B would not produce this characteristic reddish-brown turmeric colour.