how can covalent solids even decompose before melting
Answers
Only thermoplastic and crystalline materials melt. Both melting and decomposition need energy to break down the crystalline structure (in melting) or the chemical structure (in decomposition). Which event takes place earlier? It depends on the amount of energy required for the process. If the amount of melting energy is smaller than decomposition energy, then the material will first melts and then decomposes (for instance, polyethylene). Whereas, when the value of melting energy is greater than that for decomposition, then the material decomposes before reaching to melting conditions (for example, cellulose). Cellulose is a natural thermoplastic linear macromolecule (polymer), so we expect to see it to melt through heating, but the weak chemical bonding of —O— are broken before it reaches to the melting point. That is why we never see cellulose to melt, because it decomposes before melting.
Why? Because of the differences in the strengths of the structures that are needed to break down in these two processes. In melting, the crystalline structure of the material must be destroyed, while in decomposition the chemical bonds (covalent, whatever) must be broken.