Earth Science Reference: October 2012

in addition to the class notes and power points... get the 2012 edition "let's Review Earth Science"

don't give up!

subscribe to THE EARTH STORY on Facebook

light ----> prism-----> spectrum...

Flame Tests and Spectroscopy PreLab

Last: ______________________ First: ________________ Period: ____ Group: _____ Date: / /

Group Members: ________________________, ________________________, ________________________

Failure is not an option!

Investigative Question: How can we determine the chemical composition of stars?

Materials: Bunsen burner, test tube rack, 8 test tubes, 8 nichrome wire loops, 3 gas spectral tubes, 3 spectral tube power supplies, diffraction grating glasses, spectroscopes, colored pencils

Spectral tubes gases: H₂, He, Ne

Test Tube Solution: Na⁺, K⁺, Li⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺, unknown

Safety: Goggles and aprons must be worn during the flame tests. No loose clothing. Hair is tied back. Introduction:Electrons exist in energy levels within an atom. These levels have well defined energies and electrons moving between them must absorb or emit a certain amount of energy equal to the difference between them. In optical atomic emission spectroscopy, also known as the bright-line spectrum, the energy absorbed to move an electron to a more energetic level and/or the energy emitted as the electron moves to a lower energy level is in the form of a photon (a particle of light) in a hot glowing gas. Because this energy is well-defined, an atom’s identity can be found by the energy of this transition. The characteristic colors and brightness associated with these transitions are dependent upon the specific identity of the gas and its unique electron arrangement. The wavelength of light can be related to its energy. It is usually easier to measure the wavelength of light than to directly measure its energy. Optical spectroscopy is a means of studying the properties of physical objects based on measuring how an object emits and interacts with light. It can be used to measure attributes such as an object’s chemical composition, temperature, and velocity. It involves visible, ultraviolet, or infrared light, alone or in combination, and is part of a larger group of spectroscopic techniques called electromagnetic spectroscopy. Optical spectroscopy is an important technique in modern scientific fields such as chemistry and astronomy.

In optical atomic absorption spectroscopy, also known as the dark-line spectrum, light is passed through a collection of atoms in a cooler gas. If the wavelength of the light has energy corresponding to the energy difference between two energy levels in the atoms, a portion of the light will be absorbed because the electron moves to a higher energy level. In astronomy, we use the dark lines to determine the chemical composition of stars

Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. The spectrum of a gas gives a kind of ‘finger print’ of an atom. The wavelength of the light determines the color. A wavelength of around 650 nm corresponds to red light; 500 nm, to green light; 450 nm, to blue light. The human eye responds to the wavelength range of around 400 - 700 nm.

……………………………………………………… cut and paste into lab book ……..…………………………………........... Part 1: Pre-lab Exercise

1. An emission spectrum, also known as __________________________, is formed when a ____________ gas is viewed directly. An absorption spectrum, also known as __________________________, is formed when a hot and glowing energy source (which produces a _________________ spectrum) is viewed through a _______________ gas.

2. Each element produces a unique set of emission or absorption lines. An emission spectrum involves transitions of electrons from ___________ to ______________ energy states. An absorption spectrum involves transitions of electrons from _______________ to ___________ states. These transitions occur only between discrete energy levels, and thus the lines occur only at certain wavelengths and at no others. In astronomy, dark line spectrum is usually used to _______________________________________________________________.

Lab #3:

Flame Tests and Spectroscopy: STUDENT INSTRUCTIONS

Read, Understand and Apply!

Failure is not an option!

Safety First: You must follow all written and verbal instructions.

Goggles and aprons must be worn during the flame test.
Long hair must be tied back
No loose clothing
Removal of contact lenses is advised
No leaning toward the flame or inhaling the gasses.
Before lighting the flame, make sure there is NO flammable materials near by (tie, scarf, hat, hair, loose clothing, book bag etc.) Some jewelry might present a safety hazard.
Gas valve must be in close position all the time except when it is being used specifically for the flame test
To light the burner, hold the lighted match to the side and slightly above the barrel and turn on the gas. If needed, make adjustment to the burner after the flame is lit.
Do not mix and contaminate the solutions in the test tubes. Pay attention to color coding.
The devices used in this experiment to power the gas- filled tubes use a very high voltage of electricity. Do not remove the gas-filled tubes or insert metal objects such as pens or paper clips into the power devices. The gas-filled tubes will be very hot to the touch, so be careful to not touch the surfaces.
Clean up after you are done. You must eave the lab in a very good condition for the next class.

Procedure. Record all observations in corresponding Data Tables. Cut and glue the sections as you complete them. You must follow the sequence of instructions and procedure.

Cut and glue the introduction into your lab book. Read and understand the introduction.
Part 1. After reading the introduction, complete the blank spaces.

Part 2. Flame test:

a. Obtain the elements’ names and enter them in the data table.

b. Light the burner and obtain a blue flame with two visible cones.

c. Dip the nichrome wire loop with desired solution onto top of flame’s inner cone.

Note: Pay attention to color coding. Do not cross contaminate

d. Observe the flame and write your observations in the data table. Please be advised that, the flame lasts a very short time.

e. IF advised or instructed, repeat the flame test again and use diffracting grating glasses or spectroscopes to observe the emission spectra. Use the guide below to draw the emission spectral lines for each of the solutions observed on your answer sheet provided. Label the spectra with correct solution names.

f. Turn off the gas.

g. Identify the unknown (number of element) that you tested. Justify your conclusion.

h. Clean up and leave the stations better than you found them.

Part 3. Spectral Gas Tubes

1. Record the name of the gas at the appropriate place on top of spectra in part 3. Turn on the power supply to gas tube. Note that gas tubes should be on for no more than 20 seconds.

2. Observe the color of the light and record it.

3. Put on a pair of diffraction grating glasses or use the spectroscope if it is made available to you

4. Draw one set (color and spacing with correct wavelengths) of spectral lines for each of the gases tested. If supplied, use the advanced spectroscope for one of the elements. DO NOT remove or move the advanced spectroscope.

Part 4. A schematic solar emission spectrum and absorption spectra of four elements.

The diagram represents a schematic solar emission spectrum and absorption spectra of four elements. From top to bottom: emission spectrum of the Sun, and absorption spectra of sodium, hydrogen, lithium and mercury. Determine which elements the dark lines in the solar spectrum correspond to the bright lines in the spectra of these four elements. Complete the statement in your answer sheet.

Part 5.Determine the compositions of unknown mixtures and complete the statements in your answer sheet.

Part 6. Conclusion questions. You must use complete sentences and thoughts.

1. Which is more reliable method for identifying and unknown element, a flame color or spectral lines? Explain.

2. Explain the analogy “fingerprints are to people as spectral lines are to elements”.

3. Examine the following spectra:

Artificial Solar Spectrum

Laboratory Spectrum of Iron

What is the evidence for the claim that iron exists in a relatively cool outer layer of the Sun?

4. How does the light that astronomers see from distant stars and galaxies tell them that the same atoms with the same properties exist throughout the universe? Why spectral lines are often referred to as "atomic fingerprints or bar codes"?

5. Helium was discovered in the Sun's corona during the eclipse of 1868 before it was discovered here on Earth. In 1888, traces of helium were isolated here on Earth. How could scientists determine that this was the same gas that had been identified on the Sun?

6. For a given atom, will the discrete wavelengths be different depending on whether we are viewing an emission spectrum or an absorption spectrum? Explain why or why not.

as a summary watch this video Spectrum of the stars