Acids, Bases, and Salts

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of certain salt crystals. For example, copper sulphate crystals contain five water molecules, written as CuSO₄·5H₂O. It is not simply river water, any water mixed with acid, or a gas from a base. Thus, option A gives the correct definition.

Hard water contains dissolved calcium ions, Ca²⁺, and magnesium ions, Mg²⁺, which form scum with soap. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions that precipitate these ions as insoluble calcium carbonate and magnesium carbonate. Removing them reduces hardness. Sodium and chloride ions are not the hardness-causing pair, so option A is correct.

Baking soda is sodium hydrogen carbonate, NaHCO₃. When it reacts with an acid, the hydrogen carbonate ion first forms carbonic acid, which quickly decomposes into water and carbon dioxide: NaHCO₃ + HCl → NaCl + H₂O + CO₂. Effervescence shows the gas release. The reaction does not produce hydrogen, chlorine, or oxygen, so option B is correct.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes moist and is heated, the acid reacts with sodium hydrogen carbonate and releases carbon dioxide. The gas expands and forms bubbles in the dough or batter; these bubbles make the food rise and become porous. Hydrogen, chlorine, and nitrogen are not the gases produced, so option A is correct.

On heating, baking soda, sodium hydrogen carbonate, undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The principal solid residue is sodium carbonate, while water vapour and carbon dioxide are released. Sodium chloride, calcium carbonate, and ammonium chloride are not formed in this decomposition because their required ions are absent. Therefore, option A is correct.

The chlor-alkali process is the electrolysis of brine, which is a concentrated aqueous solution of sodium chloride. At the electrodes, chloride ions are oxidised to chlorine at the anode, water produces hydrogen at the cathode, and sodium hydroxide remains in solution. Sugar solution, lime water, and vinegar are not the industrial feedstock for this process. Hence, option A is correct.

In the chlor-alkali process, concentrated brine is electrolysed. Chloride ions give chlorine gas at the anode, water is reduced at the cathode to produce hydrogen gas, and sodium ions combine with hydroxide ions in solution to form sodium hydroxide. Therefore, the three main products are chlorine, hydrogen, and sodium hydroxide. The other combinations do not describe brine electrolysis, so option A is correct.

The chlor-alkali process involves electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and are oxidised: 2Cl⁻ → Cl₂ + 2e⁻. Therefore, chlorine gas is released at the anode. Hydrogen is produced at the cathode, while sodium hydroxide remains in solution; oxygen and carbon dioxide are not the expected products of this process.

In the chlor-alkali process, brine is electrolysed. Water molecules gain electrons at the cathode and are reduced, producing hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Hence hydrogen is formed at the cathode. Chlorine is produced at the anode, whereas nitrogen and sulphur dioxide are not products of brine electrolysis. Hydrogen may be identified by its characteristic pop sound.

Tooth enamel is mainly mineral matter and can be damaged when the mouth becomes sufficiently acidic. Bacteria use food sugars and produce acids, lowering the pH around the teeth. When the pH falls below about 5.5, enamel dissolution becomes more likely and the risk of dental decay rises. A pH of 7 is neutral, while 10 and 14 are alkaline, so they do not represent the critical acidic limit asked here.

Bacteria in the mouth can convert food particles, especially sugars, into acids. These acids lower the local pH and may attack tooth enamel. Basic toothpaste contains mildly alkaline substances that react with and neutralise excess acid, helping to reduce acidity and protect enamel. It does not dissolve enamel, make the pH zero, or increase acid. Therefore option A correctly describes neutralisation.

Acidic soil has a pH lower than the range suitable for many crops. Slaked lime, calcium hydroxide, Ca(OH)₂, is a basic substance. It reacts with excess acidic substances in the soil and neutralises them, thereby raising the soil pH toward a more suitable value. It is not added to create gas or sugar, and it does not increase acidity. Thus option A correctly states its agricultural purpose.

Curd contains lactic acid and is therefore acidic. If it is stored for a considerable time in a copper vessel, the acid may react with the metal. Such a reaction can form copper compounds that contaminate the food and may be harmful when consumed in excess. Copper does not convert curd into water or sweetness, and the statement that curd has no acid is incorrect. Hence option A is the safe and chemically relevant explanation.

Laboratory chemicals may be corrosive, poisonous, or otherwise harmful, even when only a small quantity is present. Tasting an unknown acid or base can cause burns, poisoning, or injury to the mouth and digestive system, so it is never an acceptable identification method. Safe testing uses suitable indicators, pH paper, or a teacher-approved procedure. The other options are false: acids and bases are not always sweet, may be colourless, and can have strong effects.

Hydrogen chloride shows acidic behaviour only after dissolving in water. In aqueous solution it ionises, producing hydronium ions: HCl + H₂O → H₃O⁺ + Cl⁻. These ions cause blue litmus to turn red. With both the gas and the litmus paper dry, water is unavailable, so hydronium ions are not produced at the paper surface and no colour change occurs. HCl is not basic, and the absence of chlorine is not the reason.

An electric current in a solution requires mobile charged particles. When an acid is diluted with water, it ionises to form hydronium ions and corresponding negative ions; for example, HCl + H₂O → H₃O⁺ + Cl⁻. These ions move toward opposite electrodes and carry charge through the solution. Colour, sour taste, or temperature does not produce electrical conduction. Therefore the presence of mobile ions makes option A correct.

Dilution of a concentrated acid is highly exothermic: heat is released when acid particles become solvated by water. Adding acid slowly to a larger quantity of water allows the released heat to disperse through the water and permits controlled stirring, reducing the chance of sudden boiling and splashing. Adding a small amount of water to concentrated acid can create intense local heating and dangerous splashes. The acid does not freeze or become a base, so option A is correct.

The governing concept is the reaction of an active metal with an acid. Zinc reacts with a dilute acid to form a salt and hydrogen: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is identified because it burns rapidly and produces a characteristic ‘pop’ when a burning splint is brought near it. Carbon dioxide would turn limewater milky, ammonia would affect red litmus, and oxygen would rekindle a glowing splint; therefore option A is correct.

An acid reacts with a carbonate to produce a salt, water and carbon dioxide. For example, sodium carbonate with hydrochloric acid gives sodium chloride, water and CO₂. Passing this gas through limewater produces insoluble calcium carbonate, making the limewater milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Thus option A is correct. The other tests indicate acidic, basic or hydrogen-related behaviour, not carbon dioxide.

When carbon dioxide is first passed through limewater, insoluble calcium carbonate forms and causes milkiness: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. If excess CO₂ is bubbled through, the calcium carbonate reacts further with carbon dioxide and water to form soluble calcium hydrogen carbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. The precipitate dissolves, so option A is correct.