Matter is anything that has mass and takes up space. All objects are composed of matter, and each sample possesses unique characteristics. These properties allow us to identify, classify, and differentiate substances. Physical properties are the observable traits scientists use to understand the world.
Defining and Observing Physical Properties
A physical property is any characteristic of a substance that can be observed or measured without altering the substance’s chemical composition. This means the identity of the substance remains the same before and after the observation. For instance, observing that a metal is shiny does not change the metal into a different material.
These properties are often determined by the senses, such as noting the color or odor of a sample. However, many physical properties require specialized tools for accurate measurement. Devices like thermometers, balances, and rulers allow for the precise quantification of traits, such as temperature or length.
Physical properties include changes in the state of matter, such as when liquid water freezes into ice or boils into steam. In both cases, the substance is still water, only its physical form has been altered.
Physical properties are subcategorized based on whether they relate to the size of the sample being examined. This distinction helps scientists understand which properties are inherent to the material itself and which are simply a reflection of the quantity available.
Properties Independent of Amount
Physical properties that remain constant regardless of the sample size are known as intensive properties. These characteristics are inherent to the substance’s composition and structure, meaning they can be used reliably to identify an unknown material. The melting point of pure gold, for example, is always \(1064\,^{\circ}\mathrm{C}\), whether you have a small grain or a large ingot of the metal.
Density is a prime example of an intensive property, which is calculated by dividing a substance’s mass by its volume. Although both mass and volume change with the sample size, their ratio remains constant for a pure substance under the same conditions. A single drop of water has the same density as a swimming pool full of water, approximately one gram per milliliter.
Other intensive properties include the boiling point, the specific temperature at which a liquid turns into a gas. The boiling point of water is consistently \(100\,^{\circ}\mathrm{C}\) at standard atmospheric pressure, regardless of the amount being heated. Color is also an intensive property, as a substance’s light absorption and reflection pattern does not change with quantity.
Hardness, which is a material’s resistance to being scratched or indented, is another property used for identification. For instance, diamonds are known for their extreme hardness, a trait that is consistent throughout any size of the crystal.
Properties Dependent on Amount
Extensive physical properties change in direct proportion to the amount of matter present in a sample. These properties are dependent on the extent of the substance, meaning they cannot be used to identify a substance because their value is not unique to the material. If you combine two identical samples, the value of the extensive property will double.
Mass is the most common example, representing the total amount of matter in an object. A larger object will naturally have a greater mass than a smaller one made of the same material. Volume, which is the amount of three-dimensional space a substance occupies, is also extensive, increasing as more of the substance is added.
Length is an extensive property, as the measurement of a side or dimension changes if the object is cut or more material is added. Total energy, which includes internal and potential energy, is also extensive because it scales directly with the quantity of material in the system.
These properties are additive, meaning the total mass of a system is the sum of the masses of all its individual parts. While extensive properties describe the size of a sample, they are not useful for determining what the substance is, only how much of it is present.
The Difference Between Physical and Chemical Properties
While physical properties are measured without changing a substance’s core identity, chemical properties describe its ability to undergo a transformation that results in a new substance. A chemical property can only be observed when the substance participates in a chemical reaction, permanently altering its molecular structure.
Flammability is a chemical property because observing it requires the substance to burn, converting it into ash, smoke, and other combustion products, meaning the original substance is no longer present. Reactivity is another chemical property that describes how readily a substance will interact with other chemicals, such as an acid or oxygen, to form a different compound.
Toxicity is also classified as a chemical property because it involves the substance’s ability to react with biological systems in a harmful way. The resistance of iron to rust is another chemical property, as rusting is a process called corrosion where iron reacts with oxygen and water to form iron oxide, a chemically distinct substance.
The key difference is the outcome of the observation process: if the substance remains chemically the same, you have observed a physical property; if a new substance is formed, you have observed a chemical property.