Covalent molecular substances consist of discrete molecules held together by weak intermolecular forces, such as London dispersion forces or dipole-dipole interactions. Because these forces are significantly weaker than the strong covalent bonds within the molecules or the ionic bonds in crystal lattices, relatively little thermal energy is required to overcome them, resulting in low melting and boiling points.

Van der Waals forces are weak, short-range electrostatic attractive forces between uncharged molecules, including London dispersion forces, dipole-dipole interactions, and Debye forces. They are significantly weaker than chemical bonds like covalent, ionic, or coordinate bonds, which involve the sharing or transfer of electrons to create stable chemical structures.

Water molecules are highly polar due to the electronegativity difference between oxygen and hydrogen. The partial positive charge on the hydrogen atom of one water molecule is attracted to the partial negative charge on the oxygen atom of a neighboring water molecule. This specific type of strong dipole-dipole interaction is known as a hydrogen bond.

Acetic acid has the highest boiling point among the listed compounds because it can form strong intermolecular hydrogen bonds and exists as a dimer in the liquid phase, requiring more energy to vaporize compared to the others.

In ice, water molecules form a rigid, open hexagonal lattice structure maintained by extensive hydrogen bonding. This structure occupies more volume than the disordered arrangement of molecules in liquid water. When ice melts, the hydrogen-bonded network partially collapses, allowing molecules to pack more closely together, which increases the density of the liquid phase compared to the solid phase.

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Macronutrients are elements required in large quantities for growth. Hydrogen is a key component of water and organic molecules, making it a macronutrient. The other options, Copper and Zinc, are micronutrients or trace elements, while Chlorine is also a micronutrient, though it's sometimes considered a macronutrient in certain contexts, Hydrogen is the clear macronutrient here.

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Methane decomposes into Hydrogen and Carbon when heated above 700⁰C without Air, a process known as thermal decomposition or cracking, producing Hydrogen Gas.

The molecules of a gas are in constant motion, and their speed is influenced by temperature. At extremely low temperatures, such as -73°C, the molecules of a gas like hydrogen (H2) are moving at their fastest pace.