While hydrogen peroxide is a well-known antiseptic, the provided answer is sodium chloride. Sodium chloride is often used in oral care for its mild antibacterial properties and ability to soothe gum tissue, though it is less potent than specialized antiseptics. The source key identifies sodium chloride, which is commonly found in various oral hygiene formulations.

The provided list contains chemical symbols for elements. Copper is Cu, Gold is Au, Platinum is Pt, and Silver is Ag. However, Mercury is represented by the symbol Hg, not Mg. Mg is the symbol for Magnesium. Therefore, the pair 'Mercury - Mg' is incorrect, making option D the correct choice.

Pearls are biological gemstones formed within mollusks. They consist primarily of aragonite, which is a crystalline form of calcium carbonate, arranged in microscopic layers. These layers are held together by an organic protein matrix known as conchiolin. The combination of these inorganic and organic materials creates the characteristic luster and durability of the pearl through a process called biomineralization.

Source answer preserved: option A ($$CaS{O_4} \cdot 2{H_2}O$$). AI attempted to change protected answer data (option_a, option_c), so this item is flagged for manual review before study use.

The density of gold is approximately 19.3 g/cm³, mercury is approximately 13.5 g/cm³, and steel typically ranges between 7.7 and 8.0 g/cm³. Therefore, the correct decreasing order of density is Gold > Mercury > Steel. This reflects the high atomic mass and packing efficiency of gold compared to the liquid state of mercury and the alloy composition of steel.

The chemical formula for sodium carbonate is Na2CO3. This molecule consists of 2 sodium atoms, 1 carbon atom, and 3 oxygen atoms. Summing these values (2 + 1 + 3) results in a total of 6 atoms per molecule.

Lithium (Li) is the lightest metal in the periodic table, with an atomic number of 3 and an atomic mass of approximately 6.94 u. While helium is lighter, it is a noble gas, not a metal. Phosphorus is a non-metal, and calcium is a heavier alkaline earth metal.

Washing soda (sodium carbonate) is widely used in water treatment to remove permanent hardness. It works by precipitating calcium and magnesium ions as carbonates, which are insoluble in water, thus softening the supply.

Slaked lime is the common name for calcium hydroxide, Ca(OH)2. It is produced by the reaction of quicklime (calcium oxide) with water, a process known as slaking. It is widely used in industrial applications, including water treatment and construction, due to its basic properties.

Mercury is used in thermometers because it remains liquid over a wide temperature range, has a high coefficient of thermal expansion, and does not wet the glass surface, allowing for accurate and consistent temperature readings.