Classification of Matter
There are three general categories of matter: elements, compounds, and mixtures. Figure 1 shows a system to organize those categories along two dimensions:
- Homogeneous vs. heterogeneous
- Pure substance vs. mixture
Both dimensions are defined in terms of the intensive properties of matter: properties that do not change when the amount of a sample changes. Examples of intensive properties of matter include color, density, pressure, metling/freezing/boiling point, and conductivity.
A pure substance refers to a sample of matter that has a distinct set of properties that are common to all other samples of that substance. The intensive properties of a pure substance are the same in any purified sample of that same substance. A good example would be ordinary salt, sodium chloride. No matter what its source (from a mine, evaporated from sea water, or made in the laboratory), all samples of this substance, once they have been purified, possess the same unique set of properties. Thus, pure substances have fixed, stoichiometric composition, e.g., NaCl, O2, CO2, C6H12O6.
Pure substances are classified as either an element or a compound. An element is a substance that cannot be decomposed into simpler substances by chemical or physical means. It consists of atoms all having the same atomic number. Examples include carbon, iron and oxygen.
A compound is a material formed from elements chemically combined in definite proportions by mass. For example, water is formed from chemically bound hydrogen and oxygen. Any pure water sample contains 2 g of hydrogen for every 16 g of oxygen. Unlike elements, compounds can be decomposed, or broken down by simple chemical reactions.
A mixture is composed of two or more substances combined in varying proportions, each retaining its own specific properties, and exhibits a wide range of properties depending on the relative amounts of the components present in the mixture. For example, you can dissolve up to 357 g of salt in one liter of water at room temperature, making possible an infinite variety of "salt water" solutions. For each of these concentrations, properties such as the density, boiling and freezing points, and the vapor pressure of the resulting solution will be different. The components of a mixture can be separated by physical means, i.e. without the making and breaking of chemical bonds.
A heterogeneous mixture is one in which the properties and composition are not uniform throughout the sample. Examples include milk, wood, and concrete. Most matter in the natural world is heterogeneous, apart from air, fresh clear water and various minerals. However, scale is important: a 1.0 m3 sample of air will be homogeneous but the atmosphere as a whole is heterogeneous.
A homogeneous mixture is one in which the properties and composition are uniform throughout the sample. Such mixtures are called solutions, and are formed by a mixture of two or more gases, by gases dissolved in liquids, or a mixture of two or more miscible liquids. Examples of homogenous mixtures include a small sample of sir and table salt thoroughly dissolved in water.
A colloid is a heterogeneous mixture composed of tiny particles suspended in another material. The particles are larger than molecules but less than 1 µm in diameter. Particles this small do not settle out and pass right through filter paper. The particles can be solid, tiny droplets of liquid, or tiny bubbles of gas; the suspending medium can be a solid, liquid, or gas (although gas-gas colloids aren't possible). Milk is an example of a colloid.
A suspension is a mixture in which fine particles are suspended in a fluid where they are supported by buoyancy. The particles are usually larger than 1 micrometer. The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid in which the suspended particles are smaller and do not settle. In a solution, the dissolved substance does not exist as a solid and the two are homogeneously mixed. An example of a suspension would be sand in water. Mud is a very short-lived suspension, while peanut butter is a very long-lived suspension.
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