Chemistry 151

Chemistry and Color

(from an elementary school perspective)

Electromagnetic radiation

•Light is a form of energy that travels through space like a wave and is characterized by its wavelength.

Electromagnetic radiation

•Only certain wavelengths of light can be detected by the human eye.

•“White” light is actually composed of all colors in equal amounts.

White Light

•Specialized cone cells in our eyes respond to visible light and signal color to the brain.

Color Perception

Color Perception

•There are three different kinds of human cone cells varying in their sensitivities to different colors of light: blue, green, and red.

•Leads to trichromatic color vision. •Sometimes called long,

medium and short cones.

Color Perception

•Birds have four different kinds of cones (one in the UV).

•Dogs have only two (no green).

Color Perception

•Birds have four different kinds of cones (one in the UV).

•Dogs have only two (no green).

The Bird’s View

•A defect in one of the three types of cones leads to “color blindness,” difficulty in distinguishing colors.

Color Perception

•Objects appear colored when they reflect light of that color.

Color Perception

•Red cones respond, signalling to the brain that the apple is “red”.

•Adding red, green, and blue light in equal amounts leads to white light.

Red, green, and blue are the primary colors of light.

Color Perception

•Adding red, green, and blue light in unequal amounts leads to all possible colors of light.

•Pigments are molecules that absorb certain colors of light and reflect the others.

•The brain integrates the responses of the cone cells to perceive the pigment’s color.

Color Perception

The yellow filter absorbs blue light; reflects red and green.

•A pigment generally absorbs light that is complementary to that observed.

•Absorbance data can be correlated to the identity of pigment.

Absorbance of Light

•For example: a purple pigment absorbs yellow-green light.

Absorbance of Light

Yellow: 425 nm (absorbs violet)

Red: 522 nm(absorbs cyan)

Blue: 610 nm(absorbs orange)

Absorbance of Light

Food Coloring Data:

Absorbance of Light

• The absorbance of light can be measured with a spectophotometer.

•The three primary colors of pigments arise from subtracting out one of the three primary colors of light.

Magenta pigment absorbs green light.

Absorbance of Light

Magenta pigment absorbs green light.

Yellow pigment absorbs blue light.

•The three primary colors of pigments arise from subtracting out one of the three primary colors of light.

Absorbance of Light

Magenta pigment absorbs green light.

Yellow pigment absorbs blue light.

Cyan pigment absorbs red light.

•The three primary colors of pigments arise from subtracting out one of the three primary colors of light.

Absorbance of Light

•Adding magenta, cyan, and yellow pigments in equal proportions leads to black.

Magenta, cyan, and yellow are the primary colors of pigments.

Color Perception

•Adding magenta, cyan, and yellow pigments in unequal proportions leads to all possible colors of pigments.

•Equally adding adjacent pigments leads to the development of color wheels.

Primary Colors of Pigments

primary colors secondary colorstertiary colors

•Colors opposite each other on color wheels are complementary colors.

Complementary colors

cyan red

yellow

blue magenta

green

•Pigments of complementary colors add in equal amounts to give black.

Complementary colors

Yellow and blue

Red and cyan Magenta and green

2012: Color Wheels2012: Color Wheels

Primary and secondary colors are introduced to show that some colors are made when mixing other colors together. Students make their own color wheels using homemade paint.

•Paints are a homogeneous mixture of a pigment, which provides color, and a binder, which cements the pigment to the solid support after drying.

Colors in art

Pigments in nature

•Early artists had primarily earth tones in their palettes, many mineral-based.

charcoal brown ochre

hematite (Fe2O3 · nH2O) is a source of red, yellow, and brown ochre

cinnabar (HgS) is a source of the red pigment vermilion

azurite (2 CuCO3 · Cu(OH)2)

Mineral-Based (Inorganic) Pigments

2010: Alien Rocks2010: Alien Rocks

Students analyze rock samples from the newly discovered planet of Zircondrion to determine if they are similar to Earth minerals. Analysis includes:

• Color• Iron? (absorbance of light)

Properties such as magnetism and fluorescence

• Chemical reactions

Plant & Animal-Based (Organic) Pigments

Indigo

Madder lake

(alizarin)

Tyrian purple

Common Sources of Pigments

Marigolds Grape Juice

Walnut hulls Kool-Aid

Beets Cochineal Beetles

CH112: Color Activity CH112: Color Activity

Learning Goals:

•Investigate the interaction of dyes with different fabrics.

•Identify ionic and polar groups in dyes and fabrics.

•Identify the different types of bonding interactions between dyes and fabrics.

Pigment/Metal Dates in useAzurite/Cu until 1800Ultramarine/Al until 1828Prussian Blue/Fe post-1704Synthetic Ultramarine/Al post-1828Cerulean Blue/Co, Sn post-1850Manganese Blue/Mn post-1935

History of blue pigment use

The chemistry of fraud detection

Last Friday, a priceless masterpiece was stolen from the little-known Waterville Museum of Old Weird Art. The painting was the masterpiece of the great Italian finger painter, Alfredo Fettuccini. However, yesterday the caretaker told police he forgot he had taken the painting home to clean it, and found it in his bedroom the next morning. An original painting by Fettuccini and the painting produced by the caretaker have been confiscated, packaged, and sent to the Colby Chemistry Lab for analysis.

Alfredo Fettuccini1600 - 1650

2008: Art Fraud Activity 2008: Art Fraud Activity

You suspect that the caretaker, Ebenezer Wheezingeezer, stole the real painting and replaced it with a fake. You will investigate the pigments used in the paintings and then look for clues on the paintings themselves. The Waterville Museum of Old Weird Art has granted you special permission to handle these rare paintings. The police are standing by, awaiting your judgment.

The case of the missing masterpiece

Ebenezer Wheezingeezer1950 - ????

Experiment 1:

Absorbance spectra of the following pigments:

Pigment from the “Masterpiece” Ebenezer found

Pigment from a modern painting

Pigment found on the floor of the museum

Pigment from a preserved brush of Fettuccini

Pigment stain found on Ebenezer’s overalls

The case of the missing masterpiece

Experiment 2:

Chemical reactivity of the pigments:

pH

Reaction with aspirin

Reaction with potassium hexacyanoferrate(II)

The case of the missing masterpiece

Tests for Fe

Experiment 3:

Examination for possible restoration:

Fluorescent paint detected with UV light

The case of the missing masterpiece