Question

Is wood burning a physical or chemical change?

Answer 1

Hint: In a physical change, the molecules of a substance are rearranged but the composition of the substance remains the same. Physical change is easily reversible, i.e., the original substance can be easily recovered. On the contrary, in a chemical change, the molecular composition of a substance is entirely changed. This gives rise to a new substance. Chemical changes are irreversible which means that the original cannot be recovered.

Complete answer:
The burning of wood is a chemical change as it leads to the formation of new substances like carbon dioxide gas, water vapour, ash, etc. these substances have entirely different chemical composition than that of wood.
Wood contains approximately \[50\% \] carbon, \[40\% \] oxygen, \[8\% \] hydrogen, \[1\% \] nitrogen, and \[1\% \] other elements like calcium, potassium, magnesium etc. When wood is set on fire, the carbon present in the form of methane or butane combust in the presence of oxygen to form water and carbon dioxide. The burning of these gases releases heat, having temperatures of around \[260^\circ C\]. At this high temperature, other substances present in the wood also start to react with oxygen which forms carbon dioxide, water. There are some substances which do not undergo complete combustion or do not react at all. These substances form the fire ash.

Note:
In case you are not able to understand the existence of methane and butane gases at the beginning of the combustion reaction of wood, simply treat the whole carbon composition as cellulose \[({C_{14}}{H_{26}}{O_{11}})\]. Now comprehend the combustion reaction as the oxidation reaction of cellulose as shown below.
\[{C_{14}}{H_{26}}{O_{11}}(s) + {O_2}(g) \to C{O_2}(g) + {H_2}O(l) + \Delta \]