science, art, and technology > lesson plans > THE CONNECTION BETWEEN PIGMENT AND LIGHT COLORS, PART I

The Connection Between Pigment and Light Colors, Part I

Teacher: Mae Eubanks-Love
School: Flower High School
Suggested Grade Level: 9-12
Title: The Connection Between Pigment and Light Colors, Part I
Subjects: Physics and Chemistry
Time: 30 minutes

State Goals and Chicago Academic Standards

Most pigments are made of atoms or molecules (two or more atoms bonded together) that selectively absorb, transmit, or reflect certain wavelengths of light. Basic pigment colors are red, yellow, and blue. They are identified as primary pigments because no other colors can be mixed to produce them. A primary pigment’s color depends on the color of light it reflects.

Explain to students that the relative mass in grams for any element contains the same number of atoms. This number of atoms, 6.02 X 1023, is called a mole. Have students use a periodic table to find the relative weights of all the elements in the molecules CuSO4 and K2CrO4: copper, sulfur, oxygen, potassium, and chromium, respectively. For each of the solutions in this lesson 0.1 moles of molecules are needed. A 0.1 M solution contains 0.1 moles of a substance dissolved in 1.0 liter of a solvent.

Students should note that the subscripts in a molecular formula represent the number of atoms in a molecule. Since a molecule of CuSO4 has four oxygen atoms, the relative mass of oxygen must be multiplied by four and added to the relative mass of one copper atom and one sulfur atom to find the relative mass of a mole of CuSO4 (copper sulfate) molecules. Class will calculate the mass of one mole of CuSO4 and convert it to 0.1 moles (multiplying by 0.1). Students will then calculate the mass of one mole of K2CrO4 and then convert it to 0.1 moles.

1. Explain the mole concept and use this concept to prepare chemical solutions of particular molarities
2. Use colored chemical solutions to determine the outcome of mixing particular colors of pigments

CuSO4 copper sulfate
K2CrO4 potassium chromate
Distilled water
Periodic table
Three small test tubes for each group of three students
Red food color
Three small flashlights
Red, green, and blue transparency paper or filters
white paper
centigram balance
two one-liter volumetric flasks

First, divide the responsibilities among the three students per group:
Project manager—reads the procedure and keeps group on task
Equipment manager—gathers the necessary material; weighs the chemicals
Recording manager—records observations

1. Equipment manager should prepare three flashlights by covering the lens of one flashlight with red transparency paper, the lens of another with green paper, and the lens of the third with blue paper.
2. Equipment manager helps the teacher prepare 0.1 M and 0.1 M solutions using the following combinations: 15.9g CuSO4/liter H2O; 19.4g K2CrO4/liter H2O.
3. Recording manager records the relative masses of each of the elements in the solutions on Table 1. He/she also records how many atoms of each element are in K2CrO4 in Table 2. Classmates double check answers.
4. The CuSO4 solution will appear blue and the K2CrO4 solution will appear yellow. The equipment manager places a sample of each solution into a small test tube. In addition, the equipment manager half-fills a small test tube with distilled water and add red food color to attain a bright red solution. Next, student places a sheet of white paper behind each test tube, then focus a beam of light through each solution as follows:
Red light through the blue solution (CuSO4), blue light through the yellow solution (K2CrO4), and green light through the red solution.
5. Students observe the white paper. The Recording manager writes the observations in Table 3.

Assess students’ abilities to work together in a group, on class discussion, and on the completion of their worksheets.

State Goals and Chicago Academic Standards

State Goal 12/Chicago Academic Standard C
Chicago Framework Statements 2, 10

Conceptual Statement–Chicago Program of Study
Energy can be transferred by a series of vibrations known as waves. This is true of sound and electromagnetic radiation.

Supporting Ideas
Light is electromagnetic energy in the form of transverse waves. Within a certain range of frequencies, these waves produce the sensation of sight. Light is described in terms of colors, frequency, wavelength, and intensity. Because light waves are transverse, they may be polarized. Light waves do not require a medium to travel.

The color of an object is dependent on the frequencies of light reflected or transmitted to the eye. Different combinations of frequencies produce different colors.

State Goal 12/Chicago Academic Standard C
Chicago Framework Statements 1, 4, 6

Conceptual Statement–Chicago Program of Study
Chemical reactions occur when atoms, molecules, and ions interact. These interactions follow definite patterns of change with measurable rates.

The most commonly used unit of concentration is molarity (moles/liter). Molarity is used to express the degree of acidity/basicity of an aqueous solution in terms of hydronium ion concentration.

Supporting Ideas
Stoichiometric relationships can be expressed in terms of ratios: mole-mole, mass-mass, volume-volume, etc.

A solution is a homogeneous mixture consisting of a solvent and a solute. It is described in terms of molarity (the number of moles of solute per liter of solution).

Auto-ionization occurs in water when water reacts with itself to form hydronium (H3O+) and hydroxide (OH-) ions. The addition of an acid or base to a solution causes a shift in the equilibrium of the solution. pH is a measure of hydronium ion concentration.

Various theories explain the observed properties of compounds classified as acids and bases. An Arrhenius acid produces hydrogen (H+) and hydronium (H3O+) ions in aqueous solution. The Bronstead-Lowry theory defines an acid as a proton donor. An Arrhenius base produces hydroxide (OH-) in an aqueous solution. The Bronstead-Lowry theory defines a base as a proton acceptor.

Neutralization describes the reaction of an acid and base to produce a salt and water. Titration is an experiment procedure used to determine the concentration of one reactant from the amount and concentration of the other. A buffer solution resists change in pH.