Pure substances and mixtures

Although we say we can separate mixtures into pure substances, it is virtually impossible to separate mixtures into totally pure substances. No matter how hard we try, some impurities (components of the original mixture) remain in each of the pure substances.  

Virtually all of the matter around us consists of mixtures of substances. For example, if you closely observe a sample of soil, you will see that it has many types of components, including tiny grains of sand and remnants of plants. The air we breathe is a complex mixture of such gases as oxygen, nitrogen, carbon dioxide, and water vapor. Even the sparkling water from a drinking fountain contains many substances besides water. 

A mixture can be defined as something that has variable composition. For example, wood is a mixture (its composition varies greatly depending on the tree from which it originates) wine is a mixture (it can be red or pale yellow, sweet or dry) coffee is a mixture (it can be strong, weak, or bitter) and, although it looks very pure, water pumped from deep in the earth is a mixture (it contains dissolved minerals and gases). 

A pure substance, on the other hand, will always have the same composition. Pure substances are either elements or compounds. For example, pure water is a compound containing individual H2O molecules. However, as we find it in nature, liquid water always contains other substances in addition to pure water—it is a mixture. This is obvious from the different tastes, smells, and colors of water samples obtained from various locations. However, if we take great pains to purify samples of water from various sources (such as oceans, lakes, rivers, and the earth s interior), we always end up with the same pure substance—water, which is made up only of H2O molecules. Pure water always has the same physical and chemical properties and is always made of molecules containing hydrogen and oxygen in exactly the same proportions, regardless of the original source of the water. The properties of a pure substance make it possible to identify that substance conclusively. 

Mixtures can be separated into pure substances elements and/or compounds. 

The chemical model of matter, therefore, starts with the following  

A pure substance is something with a uniform and fixed composition at the nanoscopic level. As you will see, pure substances can be recognized by the unchanging nature of their properties. 

An element is a pure substance composed of only one kind of atom. 

An atom is the smallest particle of an element, and the atoms of different elements are different. 

If a pure substance is not an element, it is a chemical compound. To take our most common example, what does it mean that water is a chemical compound You probably know that water is referred to as h-2-oh even if you have never studied chemistry. The h-2-oh is a way of reading the notation that chemists use to represent water HgO. In this kind of notation, H represents the element hydrogen, and O represents the element oxygen. 

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Next we will consider the phase behavior of mixtures of two components. The petroleum engineer does not normally work with two-component systems usually mixtures consisting of many components are encountered. However, it is instructive to observe the differences in phase behavior between two-component mixtures and pure substances. These differences are amplified in multicomponent mixtures. 

A solution is a homogeneous mixture of a solute and solvent. In a classification of matter, matter is divided into mixtures and pure substances. Mixtures are divided into homogenous mixtures, solutions, and heterogeneous mixtures such as suspensions. Pure substances are divided into elements made up of atoms and compounds made up of molecules. 

All matter can be classified into two groups mixtures and pure substances. A mixture is a physical combination of two or more kinds of matter. For example, soil is a mixture of sand, clay, silt, and decomposed leaves and animal bodies. If you look at soil under a magnifying glass, you can see these different components. Figure 1.11 shows another way to see the components of soil. 

I See the Saunders Interactive General Chemistry CD-ROM, Screen 1.1 3, Mixtures and Pure Substances. 

To learn to distinguish between mixtures and pure substances... 

Sandy sediment from the bottom of a river was filtered to isolate a clear liquid, which was distilled to give a liquid compound that could not be broken down further by non-chemical means. Explain how these operations illustrate heterogeneous mixtures, homogeneous mixtures, and pure substances. 

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