General Science - 4

Hard water contains dissolved minerals, specifically calcium and magnesium salts. When heated, these minerals precipitate as carbonates, forming the white scale known as limescale.

Wood charcoal is produced by heating wood in the absence of air, a process called destructive distillation or carbonization. If air or oxygen were present, the wood would simply burn to ash.

Isotonic solutions are defined as having the same osmotic pressure across a semipermeable membrane. While they often have similar concentrations, the defining characteristic is the osmotic pressure.

Pure gold is too soft for making durable ornaments. Copper is commonly added as an alloying agent to increase the hardness and strength of the gold.

Lead-acid batteries, commonly used in vehicles, utilize sulphuric acid as the electrolyte. The acid facilitates the chemical reactions necessary for energy storage and release.

The solubility of most solids increases with temperature. Heating a saturated solution of NaCl allows more salt to dissolve, making the solution unsaturated at the higher temperature.

Polythene (polyethylene) is a polymer created by the addition polymerization of ethylene (ethene) monomers. The double bond in ethylene breaks to allow the formation of long chains.

Benzoic acid has low solubility in cold water, meaning it does not form a clear solution and instead remains as a precipitate or suspension. Sugar, baking powder, and caustic soda are highly soluble in water.

The industrial extraction of aluminium from its ore, bauxite, is achieved through the Hall-Heroult process, which relies on the electrolysis of molten alumina.

Baking soda is the common name for sodium bicarbonate (NaHCO3). It is used in cooking as a leavening agent because it releases carbon dioxide when heated or reacted with acid.

Bones and teeth are primarily composed of hydroxyapatite, which is a form of calcium phosphate. This mineral provides the structural strength and hardness required for these tissues.

Coal is primarily composed of carbon, which is a single element. Petroleum is a complex mixture of hydrocarbons, while water and air are not elemental energy sources in the same sense.

The Chlor-alkali process involves the electrolysis of brine (NaCl solution) to produce caustic soda (NaOH) and chlorine gas. Aluminium is also produced industrially via electrolysis.

During the fermentation of sugar by yeast, the sugar is converted into ethanol and carbon dioxide gas. The release of carbon dioxide is what causes the bubbling observed in the process.

Vegetable ghee is produced by the hydrogenation of vegetable oils. Hydrogenation is a chemical reduction process where hydrogen is added to the unsaturated double bonds of the oil.

Dialysis works by using a semipermeable membrane to separate waste products from the blood. The movement of small molecules through this membrane based on concentration gradients is known as osmosis.

The Contact process for the industrial production of sulphuric acid involves the oxidation of sulphur dioxide to sulphur trioxide using oxygen, which is then absorbed in water.

Silicon is the second most abundant element in the Earth's crust. It is a primary component of silicate minerals, which make up the vast majority of rocks and minerals.

Statement (iii) is false because halogens are highly reactive and do not occur in a free state in nature. Statement (iv) is false because chlorofluorocarbons (CFCs) were used as cooling agents, but they are not simply 'compounds of chlorine' and are now largely phased out; however, in the context of typical exam questions, (iii) and (iv) are the intended false statements.

Quartz glass is made from pure silica. Unlike common soda glass, it has a low coefficient of thermal expansion and is transparent to ultraviolet light, making it ideal for scientific apparatus.

Serpasil (reserpine) is a drug derived from the Rauwolfia serpentina plant. It is historically classified as a tranquilizer used to treat high blood pressure and mental disorders.

Sublimation is the phase transition where a substance changes directly from a solid to a gas without passing through the liquid state. Common examples include dry ice and iodine.

Human blood is slightly alkaline, with a normal pH range typically between 7.35 and 7.45. Therefore, 7.5 is the closest value provided.

Gold is the most malleable and ductile metal. It can be hammered into extremely thin sheets (gold leaf) without breaking, which is a key property for its use in jewelry and electronics.

Correct answer is (3).