Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Matter
Mixtures and Pure Substances

Objective

· To learn to distinguish between mixtures and 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 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 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 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.

Chemistry in Focus Concrete—An Ancient Material Made New

Concrete, which was invented more than 2000 years ago by the ancient Romans, has been transformed into a high-tech building material through the use of our knowledge of chemistry. There is little doubt that concrete is the world’s most important material. It is used to construct highways, bridges, buildings, floors, countertops, and countless other objects. In its simplest form concrete consists of about sand and gravel, water, and cement (a mixture prepared by heating and grinding limestone, clay, shale, and gypsum). Because concrete forms the skeleton of much of our society, improvements to make it last longer and perform better are crucial.

One new type of concrete is Ductal, which was developed by the French company Lafarge. Unlike traditional concrete, which is brittle and can rupture suddenly under a heavy load, Ductal can bend. Even better, Ductal is five times stronger than traditional concrete. The secret behind Ductal’s near-magical properties lies in the addition of small steel or polymeric fibers, which are dispersed throughout the structure. The fibers eliminate the need for steel reinforcing bars (rebar) for structures such as bridges. Bridges built of Ductal are lighter, thinner, and much more corrosion resistant than bridges built with traditional concrete containing rebar.

In another innovation, the Hungarian company Litracon has developed a translucent concrete material by incorporating optical fibers of various diameters into the concrete. With this light-transmitting concrete, architects can design buildings with translucent concrete walls and concrete floors that can be lighted from below.

Another type of concrete developed by the Italian company Italcementi Group has a self-cleaning surface. This new material is made by mixing titanium oxide particles into the concrete. Titanium oxide can absorb ultraviolet light and promote the decomposition of pollutants that would otherwise darken the surface of the building. This material has already been used for several buildings in Italy. One additional bonus of using this material for buildings and roads in cities is that it may actually act to reduce air pollution very significantly.

Concrete is an ancient material but one that is showing the flexibility to be a high-tech material. Its adaptability will ensure that it finds valuable uses far into the future.

See Problem 3.32

For example, the mixture known as air can be separated into oxygen (element), nitrogen (element), water (compound), carbon dioxide (compound), argon (element), and other pure substances.

An illustration shows separation of air into oxygen, nitrogen, water, carbon dioxide, argon and others accompanied with space filling models. The space filling model of oxygen molecule shows two identical red spheres bonded together. The space filling model of nitrogen molecule shows two identical blue spheres bonded together. The space filling model of water molecule shows a red sphere bonded to two blue spheres on either side. The space filling model of carbon dioide shows a gray sphere bonded to two red spheres on either side. The space filling model of argon shows a green sphere.

Mixtures can be classified as either homogeneous or heterogeneous. A homogeneous mixture is the same throughout. For example, when we dissolve some salt in water and stir well, all regions of the resulting mixture have the same properties. A homogeneous mixture is also called a solution . Of course, different amounts of salt and water can be mixed to form various solutions, but a homogeneous mixture (a solution) does not vary in composition from one region to another (Fig. 3.4).

Figure 3.4.A photo shows a few spoons of salt, placed on a paper, in front of two beakers, one containing water and the other containing a solution of salt and water that appears slightly cloudy and less transparent in comparison to the beaker containing water.

Photo by Richard Megna/Fundamental Photographs © Cengage Learning

When table salt is stirred into water (right), a homogeneous mixture called a solution forms (left).

The air around you is a solution—it is a homogeneous mixture of gases. Solid solutions also exist. Brass is a homogeneous mixture of the metals copper and zinc.

A heterogeneous mixture contains regions that have different properties from those of other regions. For example, when we pour sand into water, the resulting mixture has one region containing water and another, very different region containing mostly sand (Fig. 3.5).

Figure 3.5.A photo shows a few spoons of salt, placed on a paper, in front of two beakers, one containing water and the other containing a solution of salt and water that appears slightly cloudy and less transparent in comparison to the beaker containing water.

Photo by Richard Megna/Fundamental Photographs © Cengage Learning

Sand and water do not mix to form a uniform mixture. After the mixture is stirred, the sand settles back to the bottom (left).

Interactive Example 3.3. Distinguishing between Mixtures and Pure Substances

Identify each of the following as a pure substance, a homogeneous mixture, or a heterogeneous mixture.

a. gasoline

b. a stream with gravel at the bottom

c. air

d. brass

e. copper metal

Solution

a. Gasoline is a homogeneous mixture containing many compounds.

b. A stream with gravel on the bottom is a heterogeneous mixture.

c. Air is a homogeneous mixture of elements and compounds.

d. Brass is a homogeneous mixture containing the elements copper and zinc. Brass is not a pure substance because the relative amounts of copper and zinc are different in different brass samples.

e. Copper metal is a pure substance (an element).

Self-Check: Exercise 3.3

· Classify each of the following as a pure substance, a homogeneous mixture, or a heterogeneous mixture.

a. maple syrup

b. the oxygen and helium in a scuba tank

c. oil and vinegar salad dressing

d. common salt (sodium chloride)

See Problems 3.29, 3.30, and 3.31.