Multiple signs indicate a chemical transformation.
The easiest to see is that the solid product has different properties than the solid reactants. For instance, ash will partially dissolve in water while wood shavings just float, and ash can't burn like wood shavings can. This implies ash very likely has a different chemical makeup than wood, and is not merely wood that's been reduced to a powder.
Another sign is the giving off of energy in the form of heat and light. The law of conservation of energy tells us the energy in that heat and light had to come from somewhere, and releasing energy stored in chemical bonds by breaking them with a chemical reaction is a strong possibility.
More evidence includes the mass difference between the wood and the ash it leaves behind, which implies some mass was released as a gaseous product. Also, the ability to smother the fire out with a blanket or chemical extinguisher, even if there is plenty of heat and wood left to burn, which implies that something in the air may be required as a chemical reactant.
Exothermic Reaction
1.) If you wish to have the simplest and easiest definition of exothermic reaction, then note that is a chemical reaction which is followed by evolution of heat; i.e., heat is given out when this reaction takes place. You can also say that this is a reaction during which heat is lost or released. The heat or the heat energy released during these reactions is let out in the surroundings. You can take a look at the following formula to know what is an exothermic reaction:"
Reaction of Chemicals / Substances ? Products + Heat (Energy)
2. ) Now for the boiling points of pure compounds : The higher a compound's normal boiling point, the less volatile that compound is overall, and conversely, the lower a compound's normal boiling point, the more volatile that compound is overall. Some compounds decompose at higher temperatures before reaching their normal boiling point, or sometimes even their melting point. For a stable compound, the boiling point ranges from its triple point to its critical point, depending on the external pressure. Beyond its triple point, a compound's normal boiling point, if any, is higher than its melting point. Beyond the critical point, a compound's liquid and vapor phases merge into one phase, which may be called a superheated gas. At any given temperature, if a compound's normal boiling point is lower, then that compound will generally exist as a gas at atmospheric external pressure. If the compound's normal boiling point is higher, then that compound can exist as a liquid or solid at that given temperature at atmospheric external pressure, and will so exist in equilibrium with its vapor (if volatile) if its vapors are contained. If a compound's vapors are not contained, then some volatile compounds can eventually evaporate away in spite of their higher boiling points.
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Smell, heat, light, smoke, and then the ashes are the remains of the chemical change called oxidation.
Burning anything is creates a chemical change plus burning wood produces pollutants and particulate matter into the atmosphere http://www.acs.org/content/acs/en/pressr...
