Can we say that the burning of wood is the conversion of matter into heat?
Matter is converted into energy only during a nuclear reaction, when nuclei turn into different nuclei, and the total mass is not conserved. Burning is a chemical reaction, which means when material is burnt, its atoms are rearranged into different substances. At the same time, the nuclei stay intact, and the total amount of matter does not change. The total mass of firewood being burnt and oxygen spent for burning equals the total mass of ash and gases released from burning. Therefore, burning of wood is not a conversion of matter into heat.