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Physical & Chemical Properties
Every substance, material, or object that we might encounter in our universe is considered “matter”. One of the basic functions of chemistry is to describe matter based on its properties, or characteristics. Matter is usually described in one of two ways, either by its physical properties or by its chemical properties. Physical properties refer to characteristics that can be observed or measured without changing the chemical identity of the substance. They may describe the object’s physical appearance or its interactions with its physical environment; for example, its color, its ductility (ability to bend or stretch), its conductivity (ability to transmit electrical or thermal energy), its hardness, its magnetic field, its melting & boiling point, or its viscosity (resistance to flow). Chemical properties, on the other hand, describe how the substance reacts chemically with its environment to form new or different substances. They can only be observed through a chemical reaction, where the substance’s chemical identity is changed. Examples of chemical properties include flammability (ability to burn), toxicity (ability to cause harm to organisms), reactivity (ability to react with other substances), or pH (acidity).
Mass, Volume, & Density
The classic, or “Newtonian”, definition of matter is anything that has mass and takes up space. The amount of space an object takes up is called its volume. The term “mass” can be generalized, in this case, to mean the amount of matter in the object.
It is important to note the distinction between mass and weight, which are often used incorrectly as synonyms. Where mass refers to the amount of matter in an object, weight refers to the gravitational force exerted on the object, which depends partly on the object’s mass. While both mass and weight would be considered physical properties of an object, its mass would remain consistent while its weight may vary depending on its location (for example, on Earth versus on the Moon).
[endif]--We can combine the concepts of mass and volume to describe the density of a substance. If you consider the amount of matter (mass) that is contained in a certain amount of space (volume), you get a picture of how closely packed the particles are in a substance. Density refers to an object’s mass per unit volume, or how many particles exist in a certain volume of the substance. It can be calculated by dividing an object’s mass by its volume, giving the observer a measurement in g/mL or g/cm3.
While a substance’s density can change depending on the temperature and pressure, it will be consistent for a substance independent of the amount. In other words, two samples of the same substance will have the same density even if the samples are different sizes because the ratio of mass to volume will be the same. As a result, density can be used in many cases to identify substances because each substance will have its own characteristic density regardless of the sample size. For example, gold has a density of 19.32 g/cm3. This will be true of a tiny fleck of gold dust as well as an 11 kg gold bar. So, prospectors can calculate the density of a sample to determine if it is actual gold or not, depending on if its density equals 19.32 g/cm3! ![endif]--
Identifying Substances
Just as two different-sized samples of the same substance will have the same density, many other physical properties are independent of the amount of a substance. For example, a small iron block and a large iron rod will have similar properties such as thermal conductivity, magnetism, and boiling and melting points. For this reason, these properties can be used to identify substances, regardless of how much of the substance is being observed.