Early chemists, known at the time as alchemists, had a difficult time understanding the Law of Conservation of Mass. Using the burning of wood as an example, what property of matter made this law difficult for early scientists to understand?

The law of conservation of mass states that, for any closed system, the mass of the system must remain constant.
So, in other words, mass cannot be created nor destroyed.
In a burning log, the initial mass of the wood and the ashes that are left behind after it burns are different, which would seem to violate the law of conservation of mass.

Non metal is an element or substance that is not a metal metal is a solid material that is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivitymetalloid is an element (e.g., germanium or silicon) whose properties are intermediate between those of metals and solid nonmetals. they are electrical semiconductors.

Your question is not clear but if you can explain yourself i would be glad to .  just trying to : ))