Physical properties are those that can be observed without changing the identity of the substance. The general properties of matter such as color, density, hardness, are examples of physical properties. Properties that describe how a substance changes into a completely different substance are called chemical properties.
Examples of physical properties are: color, smell, freezing point, boiling point, melting point, infra-red spectrum, attraction (paramagnetic) or repulsion (diamagnetic) to magnets, opacity, viscosity and density. There are many more examples. Note that measuring each of these properties will not alter the basic nature of the substance.
Properties that describe how a substance changes into a completely different substance are called chemical properties.
Examples of chemical properties are: heat of combustion, clammability and corrosion/oxidation resistance, reactivity with water, PH, and electromotive force.
The difference between a physical and chemical property is straightforward until the phase of the material is considered. When a material changes from a solid to a liquid to a vapor it seems like they become a difference substance. However, when a material melts, solidifies, vaporizes, condenses or sublimes, only the state of the substance changes. Consider ice, liquid water, and water vapor, they are all simply H2O. Phase is a physical property of matter and matter can exist in four phases – solid, liquid, gas and plasma.
The more properties we can identify for a substance, the better we know the nature of that substance. These properties can then help us model the substance and thus understand how this substance will behave under various conditions.
Some of the more important physical and chemical properties from an engineering material standpoint: