Chemistry is the science of matter and the changes it undergoes. The science of matter is also addressed by physics, but while physics takes a more general and fundamental approach, chemistry is more specialized, being concerned with the composition, behavior or reaction of matter, as well as the changes it undergoes during chemical reactions.
Everything around us is related somehow to chemistry. There are chemical reactions inside our bodies, in our food, and there are also reactions that happen between our bodies and the clothes we wear. It is, therefore, not surprising that the colors that we see around us are mainly a result of chemical reactions. But have you ever thought of colors and how they exist? Are they a gift from Mother Nature or is it chemistry?
Dyes vs. Pigments
Color results from the way colorants interact with light. Dyes and pigments are colorants; substances that impart color to a material. The major difference between dyes and pigments is solubility or the tendency to dissolve in a liquid, especially water.
Dyes are usually soluble, or can be made to be soluble in water. Once a dye is dissolved in water, the material to be dyed can be immersed in the dye solution. As the material soaks up the dye and dries, it develops a color. If the material then retains that color after being washed, the dye is said to be colorfast.
Pigments, on the other hand, are generally not soluble in water, oil, or other common solvents. To be applied to a material, they are first ground into a fine powder and thoroughly mixed with some liquid, called the dispersing agent or vehicle. The pigment-dispersing agent mixture is then spread on the material to be colored. As the dispersing agent dries out, the pigment is held in place on the material.
In most cases, dyes are used for coloring textiles, paper, and other substances, while pigments are used for coloring paints, inks, cosmetics, and plastics.
Many dyes can be obtained from natural sources; such as plants, animals, and minerals. In fact, humans have known about and used natural dyes since the dawn of civilization. Red iron oxide, for example, has long been used to color cloth and pottery and to decorate the human body. Today, T-shirts dyed with naturally occurring iron oxide, known as red dirt, are popular among tourists on Hawaii's island of Kauai. Red dirt imparts a brilliant orange-red color to cloth that is almost impossible to wash out. Other natural dyes include sepia, obtained from cuttlefish.
Some natural dyes are expensive to produce, difficult to obtain, or hard to use. Royal purple got its name because it comes only from the tropical murex snail. So many snails were needed to produce even the smallest amount of dye that only royalty could afford to use it. The dye known as indigo, obtained from the Indigofera plant, imparts a beautiful blue color to material, but it is insoluble in water. It must first be converted to a different “reduced” chemical form that is yellow and is soluble in water. Once attached to a material and exposed to air, the yellow form of indigo is converted back (oxidized) to its original blue form.
A revolution in colorant history occurred in 1856, when, by sheer accident, English chemist William Perkin (1838–1907) discovered a way to manufacture a dye in the laboratory. That dye; mauve, was produced from materials found in common coal tar. Perkin's discovery showed chemists that dyes and pigments could be produced synthetically, which means that humans can produce it in a lab.
Today, the vast majority of dyes and pigments are produced synthetically. These products are easier and less expensive. In addition, their colors are more consistent from batch to batch than the various samples of natural colorants.
Mordant dyeing involves the use of a chemical that combines with the dye to form an insoluble compound. In the mordant process, the mordant is first applied to the fabric. After the mordant has dried, the dye is added. The dye sticks to the mordant, and the fabric is able to take on the color of the dye, forming an insoluble bond.
Pigments are applied to a surface as a mixture that always consists of at least two parts, the pigment itself and the vehicle, and usually many more components. The purpose of the vehicle in this mixture is to carry the pigment onto the surface, much as motor vehicles carry people and goods. A thinner is often needed because many vehicles are thick, viscous materials that are difficult to apply with a brush.
For example, a thinner such as turpentine is often added to a given mixture to make it easier to apply. One of the simplest paints that you imagine, then, might consist of red iron oxide, linseed oil, which is the vehicle, and turpentine, which is the thinner.
After the pigment/vehicle/thinner mixture has been applied to a surface, two changes occur. First, the thinner evaporates leaving the pigment/vehicle mixture evenly spread on the surface. Next, the vehicle slowly undergoes a chemical change, known as oxidation that converts it from a thick liquid to a solid. Since the pigment particles are trapped in the hardened vehicle, a thin tough skin of colored material becomes attached to the surface.
Nearly every industry uses colorants in one way or another. About 7,000 different dyes and pigments exist and new ones are patented every year. Dyes are used extensively in textile and paper industries. Leather and wood are also colored with dyes, so are petroleum-based products; such as waxes, lubricating oils, polishes, and gasoline. On the other hand, plastics, resins, and rubber products are usually colored by pigments.
Food is often colored with natural or synthetic dyes that have been approved by a federal agency and proven safe for human consumption. Dyes are also used to stain biological samples, fur, and hair.
Seasonal Changes
Every fall we hope for beautiful shades of oranges, yellows, reds and purples to send ripples of color through the landscape. Some years we have a great show of fall colors, but other years we do not. The intensity of fall color can vary quite a bit from year to year.
Have you ever wondered exactly what happens to cause these beautiful colors? Chemical processes in trees that occur this time of the year cause the leaves to change color. Let's first take a look at what happens in the leaf structure during the spring and summer.
Leaves of trees during summer are factories producing sugar from carbon dioxide and water by the action of light on chlorophyll; a process known as photosynthesis. Water and nutrients flow from the roots, through the branches, and into the leaves. The sugars produced by photosynthesis flow from the leaves to other parts of the tree, where some of the chemical energy is used for growth and some is stored.
Chlorophyll is a green pigment that masks other colors during the growing season. It absorbs red and blue light from the sunlight that falls on leaves. Therefore, the light reflected by the leaves is diminished in red and blue and appears green.
Xanthophyll is a yellow pigment and carotene is an orange pigment. For example, squash have yellow xanthophyll pigments and carrots have orange carotene pigments. However, we do not see these colors during the spring and summer because chlorophyll hides them.
On the other hand, Anthocyanin is a pigment that produces reds and purples in many types of trees; it is responsible for the red skin of ripe apples and the purple of ripe grapes. Anthocyanins are formed by a reaction between sugars and certain proteins in cell juice. This reaction does not occur until the concentration of sugar in the juice is quite high. It also requires light; this is why apples often appear red on the side facing the Sun and green on the shaded side.
The shortening days and cool nights of autumn trigger changes in the tree as leaves stop making food. One of these changes is the growth of a corky membrane between the branch and the leaf stem, which interferes with the flow of nutrients into the leaf. Because the nutrient flow is interrupted, the production of chlorophyll in the leaf declines, and the green color of the leaf fades. If the leaf contains carotene, as do the leaves of birch and hickory, it will change from green to bright yellow. Chemical changes occur to produce even more vivid colors in the fall.
The range and intensity of autumn colors is greatly influenced by the weather. Low temperatures destroy chlorophyll, and if they stay above freezing, they promote the formation of anthocyanins. Bright sunshine also destroys chlorophyll and enhances anthocyanin production. Dry weather, by increasing sugar concentration in leaf juice, also increases the amount of anthocyanin. So the brightest autumn colors are produced when dry, sunny days are followed by cool, dry nights.
The right combination of tree species and likely weather conditions produce the most spectacular displays; that is why fall is a truly beautiful time of the year.
It is clear that the science of chemistry is like the sea; as we swim deeper we unravel amazing, almost magical, secrets. As we analyze things that surround us, we end up realizing that it is all connected to chemistry one way or another. This only highlights the importance of this core science that shapes everything in our life. It is, therefore, not another detached academic science; it is the essence of our life and the heart of our universe. If there is no chemistry, there is no life at all.
References
http://scifun.chem.wisc.edu/chemweek/fallcolr/fallcolr.html
http://msucares.com/lawn/garden/coast/01/011027.html
http://www.scienceclarified.com/Di-El/Dyes-and-Pigments.html
http://en.wikipedia.org/wiki/Chemistry
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