All substances are known to have properties by which they can be identified. These properties can be classified into two basic types; namely, physical and chemical. Physical Properties ◼ These are the properties that are observable and that describe a substance without changing its identity. That is, the composition of matter does not change. Common examples of physical properties of a substance include color, texture, odor, melting point, boiling point, density, solubility, polarity, etc. ◼ In this case, the substance may undergo a physical change but not a chemical change. By physical change, we mean the changes in the state or phase of matter. Matter is known to exist in four phases, which are solid, liquid, gas, and plasma. When a substance undergoes this kind of a change (such as melting, freezing, condensation, etc.), it does not become a new substance. An example is the melting of ice. ◼ Let's take the example of calcium. It is a silvery-white metallic solid and its crystalline structure is cubic. What we just described were some of calcium's physical properties (those which are observable without changing its identity). ◼ Physical properties can further be classified into two types, based on the changes in the amount of matter. They are intensive and extensive. ◼ Intensive properties are those in which the physical property is not altered by the size or mass of the sample. Some examples of this type are color, luster, density, malleability, etc. ◼ Extensive properties are those in which the physical property is altered with a change in the amount of matter in the sample. Some examples of this type are mass, volume, and length. ◼ One may have come across terms like isotropic and anisotropic properties. They are related to physical properties. Physical properties become isotropic or anisotropic based on their dependency upon the orientation of a sample. That is, they are considered as anisotropic properties if they depend on the direction from which the sample is observed and they are isotropic if this does not apply. Chemical Properties ◼ These are the properties which are observable or measurable only after the substance undergoes a chemical change. Here the chemical identity of the substance is changed. Common examples of chemical properties include toxicity, chemical stability, heat of combustion, enthalpy of formation, flammability, reactivity with other chemicals, etc. ◼ Here, the substance undergoes a chemical change. By chemical change, we mean changes in the molecular structure or a change that involves the making and breaking of atoms, thus, forming a new chemical substance. Examples include rusting of iron or milk going sour. ◼ Let's continue with the example of calcium. It is used as a deoxidizer in steel (property: oxidation). It ignites when heated in air or oxygen (property: flammability). And it reacts with acids (property: reactivity). Here, we could describe the properties with the help of chemical activities involving calcium. These were some of calcium's chemical properties (those which are evident only after a substance undergoes a chemical change). Here is a table of comparison, that will help in better understanding of the differences between the physical and chemical properties of a substance. Both the physical and chemical properties are important as they enable us to know or study the nature of a particular substance. Furthermore, the substance can be modeled and its behavior under various conditions can be understood, studied, and used in further applications.