Every minute of every day in our physical universe, chemicals react with one another. Some of these are beneficial to humanity and save lives, while others have the potential to destroy entire empires and civilizations. A chemical reaction is a simple process whereby one or more chemicals combine in some way to create new substances. The new substances created are called products, while the chemicals used to instigate the reaction are known as reactants.
Obviously, not every chemical will react with every other one. If this statement were true, reactions would happen spontaneously without notice, and nothing in our universe would be chemically stable. Thus, we must realize that certain conditions must be met in order for a chemical reaction to take place; in order for the chemical reactants to transform into products. We have already realized that only certain chemicals will react with others, the system must also reach what is called an activation energy. This level is a certain level of energy (measured in Joules), that must be present in order for a reaction to take place. This is why one can observe liquid water at temperatures well below zero degrees Celsius or well above 100 degrees Celsius.
Knowing the concept of the activation energy, we can understand that chemical reactants do not have to be chemicals, they can be things like energy. This is why some chemical reactions are endothermic (they require and absorb energy), and others are exothermic (they release energy in the form of heat). Far too many inexperienced chemists will not include things such as energy in their chemical reactions, because this energy must also be multiplied by a coefficient when balancing an equation. If these are not balanced properly, a chemist might only burn 500 Joules worth of fuel for a reaction when many times that was required to fully complete the reaction. This would induce what is called a limiting reagent, something whose quantity affects how much of the reaction can actually take place.
Everything in our world is a potential chemical reactant. Thankfully, there are many conditions and unseen chemical reactants that also must be present for a reaction to progress, and this leads to the natural inertness of many solids. It is the ongoing obligation of chemists to understand the properties of individual chemical reactants, a task that might take decades to fully complete. Such is the nature of science, a constant journey of discovery.