Q.Solid $CO_2$ is an example of

Crystalline solids are further split into four families by what holds their constituent particles together, and each family's bonding directly sets its melting point, hardness and conductivity:

• Ionic solids (e.g. NaCl): cations and anions bound by strong electrostatic attraction. High melting point; hard but brittle; non-conducting as a solid (ions are fixed), but conducts well molten or in solution (ions become mobile).
• Covalent (network) solids (e.g. diamond, SiC): atoms bound into one continuous 3D network purely by covalent bonds. Very hard, very high melting point, usually poor thermal/electrical conductors -- though graphite is a notable exception (strong bonding within 2D sheets, weak forces between sheets, giving it softness, slipperiness and in-plane conductivity).
• Molecular solids (e.g. CO2, ice, naphthalene): neutral molecules held by weak van der Waals forces. Soft, low melting point, non-conducting. Sub-divided by the strength of that force: non-polar (dispersion forces only, e.g. naphthalene) < polar (dipole-dipole, e.g. solid CO2) < hydrogen-bonded (e.g. ice, glucose) in order of increasing melting point. …