lab report chem 120(most basic)

EXPERIMENT7
Laurie LeBlanc
Emission of Light
Light is a form of electromagnetic radiation. Electromagnetic energy includes radio waves, microwaves,
infrared rays, visible light, ultraviolet rays, X-rays and gamma rays (see Figure 7.1). Visible light includes
energy that the human eye is capable of detecting. White light is visible light that comes from the sun or
an incandescent light bulb.
Increasing
Energy
Frequency
Decreasing
400 nanometers
Cosmic and
gamma rays
X-rays
figure
700 nanometers
Ultraviolet
Visible to
human eye
Infrared
Heat
7.1
Electromagnetic spectrum.
Radio waves
When white light is passed through a prism (diffraction grating and droplets of rain serve the same
purpose), the light is separated into the colors of the rainbow: red, orange, yellow, green, blue, indigo and
violet (ROY G BIV). White light is produced by the combination of all of these individual colors in the
visible spectrum. Red light is the lowest in energy and violet light is the highest.
The current model of the structure of the atom includes the presence of a nucleus surrounded by
electrons. Experimental evidence has led to the hypothesis that electrons in an atom exist at only certain
allowable locations away from the nucleus, corresponding to certain energy levels.
Electrons can absorb energy from a flame or electric discharge, but only in packets that contain the
exact amount of energy necessary to allow the electron to move farther from the nucleus into a higher
energy level. When this occurs, the electron is said to be in an “excited state.” When the electron returns
to a lower energy level, it emits the previously absorbed energy in the form of packets of light called photons,
each carrying an amount of energy equal to the differences between two energy levels.
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This phenomenon can be observed as the emission of light from
heated objects (such as the tungsten filament in incandescent light bulbs),
flames (such as the flame tests we will do in the lab) and emission tubes
(as we will observe). An emission tube is a sealed glass tube with metal
plates at both ends. The emission tube is filled with a pure, elemental
gas. By applying a high voltage to the metal plates, the electrons of the
gas can be excited and give off light. The emission tube is similar in design to neon lights (although the glass of neon light is painted to produce
a variety of colors) and fluorescent lights (although the inside of the
glass of fluorescent lights is coated with a fluorescent material).
In an atom, many excited states are possible due to the existence of
a number of energy levels and possible electron transitions. Therefore,
when light is emitted by a large collection of excited atoms, a variety of
energies are released as the electrons return to their lowest energy. When
these photons pass through a prism or a diffraction grating, they produce
a line spectrum, where each line corresponds to photons of a particular
energy.
Each element exhibits its own characteristic line spectrum due to
differences in the energy levels of atoms of different elements. When elements are heated in a flame, the characteristic color produced and seen by
figure 7.2
Flame Test for K .
the naked eye is due to the combined effect of the individual spectral lines.
Since hydrogen produces four spectral lines in the visible region – you may
only be able to see three through your diffraction grating – one line is violet, one line is turquoise and one spectral
line is red, the color of the hydrogen emission tube seen by the naked eye is purple (the sum of all the spectral colors).
Experimental Procedure
Part I (to be done in a darkened lab)
White Light Emission
1 Obtain a diffraction slide and flashlight. Observe the white light through the diffraction slide and note the
continuous visible spectrum.
2 Draw the spectrum with colored pencils in your lab notebook.
Emission Tubes
3 Note and draw the color of the hydrogen emission tube at the narrowest part of the tube as it appears to the
naked eye.
4 Look at the tube through a diffraction slide. Note the three spectral lines.
5 Draw their approximate positions and color in your lab notebook. Use black between the emission lines where
there is no color.
6 Label the drawing with the appropriate element name.
7 Repeat the procedure with the emission tubes of mercury, neon and helium.
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Chemistry 120
Part II (to be done in a lighted lab)
Elemental Flame Tests
8 Obtain a platinum wire. Be sure that the tip of the wire is coiled in the shape of a small circle. When not in use,
the rod should be kept wire up in your test tube rack. (Care should be taken to avoid bending the wire. It is very
soft.)
9 In order to thoroughly clean the wire, dip the tip of the wire into a 6 M HCl solution (being careful not to
break it) and heat the wire in the hottest part of the flame of a Bunsen burner until a blue color remains
constant.
10 Place 3 to 5 drops of each of the following saturated solutions into individually, labeled test tubes: lithium nitrate,
copper(II) chloride, potassium chloride, barium chloride, strontium nitrate, calcium chloride and sodium chloride.
(You will be observing the characteristic color emitted of the above metal ions when they are burned in a flame.)
11 Dip the loop of the cleaned wire into the first test tube and then hold the loop in the hottest part of the flame.
Note the color of the resulting flame. (It is the INITIAL color that you are looking for.) Draw the flame color
carefully in your lab notebook. Label with the element name.
12 Clean the platinum wire loop with hydrochloric acid, burn it off, and continue observing all known solutions
in the flame. You will use the drawings of your known solutions to identify your unknown solutions.
13 It may be necessary to periodically replace the 6M HCl(aq) if it appears to be contaminated.
14 Obtain samples of two unknown metal ion solutions, which will be assigned to you, and record their codes in
your lab notebook. Perform flame tests on both unknowns, drawing the flame colors of each carefully in your
notebook. Determine the identities of your two unknowns using your observations from the flame tests of the
previous metal ions.
15 Dispose of all waste in the proper waste container. Return your CLEAN platinum wire to your instructor.
Cuyamaca College
Experiment 7 n Emission of Light
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**This lab report is to be done entirely in your lab notebook.
EXPERIMENT 7
POST-LAB
QUESTIONS
1 (a) Explain what causes light to be emitted (on an atomic level) from the emission tubes in Part I.
(b) How does the process differ when light is emitted from the heated aqueous metallic solutions in Part II?
2 (a) Which unknowns were you assigned in Part II?
(b) What were the metals present in your unknowns?
3 Research the phenomenon of light emission and explain three practical applications. To receive credit, you must
explain clearly how each works to the best of your ability. Make your writing understandable; define any technical
terms used. Cite all sources used.
Formal Report Check List
Cover (5%)
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Name
Date exp started
Exp name
/5
Abstract (10%)
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Only 3 sentences, 1 paragraph
Answers why, how, your results
No pronouns
/10
Background (35%)
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5 paragraph essay.
3 Concepts relating to the experiment/lecture, defined, with examples
No comments about the experiment or materials used in the experiment
Not restating the procedure
How much effort was put unto this section
/35
Lab Notebook (30%)
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All parts in notebook
Observations do not have graphs, calculations, or opinions
Has a Data Analysis section for graphs, math, etc.
Prelab and Postlab questions as needed.
Did the student have the lab notebook ready for the lab?
/30
Conclusion (20%)
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3 major errors inherent in the experiment.
Effects of these errors on the data.
How to prevent, minimize these errors.
/20
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