When matter undergoes a chemical change, the physical properties as well as the molecular structure change. Atoms of elements can combine to form compounds having new properties. During chemical change atoms in compounds may separate and recombine to form new compounds. These chemical reactions involve energy changes. No matter how substances within a closed system interact with one another, or how they combine or break apart, the total mass of the system remains the same.
Substances have characteristic chemical properties such as pH and reactivity.
Equal volumes of different substances usually have different mass.