Properties can be classified as physical or chemical, depending on whether the change causes any modifications in the composition of the substance or not. If no modification is there, such as in melting, the property or change is termed physical. In comparison, chemical properties or changes involve changes in the composition of the substance.
The ability to react with oxygen is a chemical property. This is because in order to observe this property, the composition of the substance changes. For example, hydrogen gas reacts with oxygen to form water:
`2H_2 + O_2 ---> 2H_2O`
Similarly, sodium reacts with oxygen to form a mixture of sodium oxide and sodium peroxide.
`4Na + O_2 -> 2Na_2O`
`2Na + O_2 -> Na_2O_2`
In each of these reactions, the reactant undergoes a change in its chemical composition and new substances are formed. Thus, reactivity with oxygen is a chemical property.
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