### Theory:

Any substance that changes its chemical properties is known as chemical change. In this change, a new substance is formed, and these changes are irreversible (i.e. they cannot be brought back to their original form) in nature. These changes are permanent.
Burning: When the substances such as matchstick, wood and paper are burned, they turn into ashes under complete combustion. The ashes have an entirely new chemical composition than that of the unburnt substances.
Burning of a matchstick

Burning of  wood

Burning paper turning into ashes
For any substance to burn, it requires three essential elements such as air, oxygen and fuel. This is known as the fire triangle. Here the fuel vaporises and combines with oxygen to form a fire. This process is known as chemical reaction.
Note: The chemical process in which any substance reacts with oxygen to provide heat is known as combustion.
Let us now see the reaction of magnesium ribbon on burning:

Magnesium ribbon produces a bright white light and forms powdery ashes when allowed to burn. We know magnesium ($\mathit{Mg}$) is a metal. Metals on reaction with oxygen (${O}_{2}$) form its oxides. Similarly, magnesium forms magnesium oxide ($\mathit{MgO}$).

The ashes collected on the burning of the magnesium ribbon is magnesium oxide ($\mathit{MgO}$).

$\begin{array}{l}\phantom{\rule{0.735em}{0ex}}2\mathit{Mg}\phantom{\rule{0.882em}{0ex}}+\phantom{\rule{0.735em}{0ex}}{O}_{2}\phantom{\rule{0.882em}{0ex}}\to 2\mathit{MgO}\\ \mathit{Magnesium}+\phantom{\rule{0.147em}{0ex}}\mathit{Oxygen}\to \mathit{Magnesium}\phantom{\rule{0.147em}{0ex}}\mathit{oxide}\end{array}$

Thus, the above-explained changes occur when one material or substance comes in contact with the other.

We can now conclude by stating that physical contact is one of the necessary conditions for chemical changes to occur.