Chemical Property
A chemical property describes how a substance can change into a different substance, such as flammability, reactivity with acid, or tendency to rust.
Definition
A chemical property describes a substance's ability to undergo a chemical change and form new substances, such as flammability, reactivity with water or acids, tendency to oxidize or corrode, toxicity, or stability under heat, and it can only be observed by carrying out or attempting a reaction rather than by simple inspection. These properties depend on the arrangement of valence electrons, and quantitative chemical properties, including heat of combustion, standard reduction potential, acid dissociation constant, electronegativity, and ionization energy, are determined by electronic structure and bonding and follow predictable periodic trends. A chemical property is always defined relative to a specific reaction partner and set of conditions.
Example
Paper is flammable, meaning it can burn into ash and smoke, and iron can rust, both chemical properties that only reveal themselves once the change is triggered. Sodium reacts violently with water while gold does not react with water, air, or most acids at all, and hydrogen is flammable while helium, an equally light gas used in balloons, is not. The standard reduction potential of $Zn^{2+}/Zn$ is $-0.76\text{ V}$ while that of $Cu^{2+}/Cu$ is $+0.34\text{ V}$, so zinc metal spontaneously reduces copper ions, $Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu$, with a cell voltage of $1.10\text{ V}$, the basis of both the Daniell cell and the reason zinc coatings protect steel from rusting.
Key Insight
Physical properties describe what a substance is like right now, while chemical properties describe what it can become: color is what iron looks like today, and rusting is what it can turn into. Even a substance's inertness is a chemical property, since nitrogen gas resists reaction not for lack of energy but because its triple bond makes the activation energy for most reactions very high, which shows that stability is as much a statement about kinetics as about thermodynamics.