lab 4

CHEM 1001 – 2020: Lab #4 – Background Page 1 of 4

CHEM 1001 Lab #4 – Radiation, Molecules, and Plants BACKGROUND INFORMATION

This lab has two parts: In Part 1, you will become familiar with chromatography and use the technique to “separate” and identify pigments found in plants. In Part 2, you investigate how different molecules react to specific wavelengths of the electromagnetic spectrum. BACKGROUND Electromagnetic radiation is commonly referred to as waves. Waves are composed of photons (according to Einstein’s particle model). Each photon has a wavelength (λ) and a frequency (ν). Wavelength and frequency are inversely proportional, as are wavelength and energy. Therefore, photons with short wavelengths have high frequencies & high energies, while photons with large wavelengths have low frequencies & low energies. The electromagnetic spectrum is a continuum of waves, organized by the size of their wavelength, Figure 1. The visible spectrum (the light that our eyes can detect) is a very small portion of the electromagnetic spectrum. We will focus on the visible spectrum in this lab.

Figure 1: The electromagnetic spectrum (top) and the visible spectrum (bottom)

Plants contain many different molecules that are directly or indirectly involved with photosynthesis, which may also impart color to the plant. Photosynthesis is the process by which green plants, algae, and some bacteria capture the energy of the sunlight to produce glucose and oxygen from carbon dioxide and water, see Figure 2. It is the process in which light energy in converted into chemical energy.

Figure 2: Reaction of photosynthesis (Image created by Sal Khan)

CHEM 1001 – 2020: Lab #4 – Background Page 2 of 4

Chlorophyll is the molecule in the chloroplast (food producer) of the plant that absorbs solar radiation. There are 2 main types of chlorophyll in green plants – “chlorophyll a” and “chlorophyll b”. Because chlorophyll traps light it can be called a photoreceptor. However, chlorophyll does not absorb all wavelengths of solar radiation, it only absorbs certain wavelengths of light (photons) within the visible region of the electromagnetic spectrum. More specifically, chlorophyll only absorbs wavelengths of visible light in the blue range (400 – 500 nm) and in the yellow-red range (600 – 700 nm). The visible light photons that are not absorbed are reflected, and appear green to the human eye, see Figure 3.

Figure 3: When visible light hits a green leaf, green photons are absorbed less and reflected more. The blue

and orange/red photons are absorbed and used in photosynthesis.

Chromatography is a technique used by scientists to separate the components of a mixture. There are many types of chromatography – each one has advantages and disadvantages. In this lab, you will use paper chromatography, which is one of the least expensive and simplest types of chromatography, see Figure 4. All types of chromatography have two parts: the stationary phase, which does not move, and the mobile phase, which – unsurprisingly – moves! In paper chromatography, the stationary phase is the chromatography paper and the mobile phase is a solvent (liquid) that is wicked up the paper through capillary action.

sunlight (full visible spectrum)

reflected green light makes the leaf appear green

a green leaf

CHEM 1001 – 2020: Lab #4 – Background Page 3 of 4

Figure 4: The basics of paper chromatography.

A) Prepared chromatography paper. B) Prepared chromatography paper just after it enters the test tube with solvent. C) Solvent is moving up the prepared chromatography paper and components of the sample (mixture) are separating. D) The paper is removed from the test tube when the solvent

front approaches the top of the paper.

Word Meaning

Baseline The line drawn with pencil near the bottom of the

chromatography paper. This is where sample will be applied to the paper.

Solvent Front

The part of the solvent (mobile phase) that has moved the farthest up the paper (stationary phase). This is the

highest part of the paper that looks wet. It can be difficult to see.

Chromatogram This is what you are left with after successfully performing chromatography. Usually a mixture has been separated

into its constituents and they are visible.

Paper chromatography separates chemical components based on their relative attraction for the stationary and mobile phases. Components (compounds) that are highly attracted to the mobile phase will travel farther along the chromatography paper (stationary phase) and will stop traveling near the solvent front. Components that are more attracted to the stationary phase will not travel far, and will remain near the baseline.

baseline (drawn with pencil)

solvent front

sample (ink, vegetable juice, etc)

test tube with solvent

A B DC

colored chemicals present in

original sample

CHEM 1001 – 2020: Lab #4 – Background Page 4 of 4

You can imagine that the chemicals being separated through chromatography are like people riding boats down a river. The river is the mobile phase and the stationary phase is the riverbank. If the people stay in their boats the whole time, they will travel a long distance from where they started. Yet, if the people stop and spend a lot of time on the riverbank, they will not move far. Chemists use something called the Rf (retention factor) to quantify the distance that a component (compound) travels up the paper. To find the Rf for a chemical on your chromatogram, measure the distance traveled by the compound (from the baseline), and the distance traveled by the solvent. The values are then placed in the equation, shown below in Figure 5, to calculate Rf for each compound in the mixture.

Figure 5: Measuring distances and calculating an Rf value for one chemical on a chromatogram

The distance traveled by the solvent front is the furthest that a compound can travel on the chromatography paper. If a chemical travels the entire distance of the solvent front it will have an Rf value of 1.0. In contrast, a compound that remains on the baseline will have an Rf value of zero.

R f = distance traveled by a chemical

distance between baseline and solvent front

solvent traveled 6.15 cm

blue chemical traveled 5.00 cm

For the blue chemical:

R f = 5.00 cm 6.15 cm

= 0.81