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Prelab for Flame Test

What causes the color in fireworks?

Explosions:

Fireworks were first made in China over 1,000 years ago!  Gunpowder is what makes fireworks explode. Potassium nitrate is the most important part of gunpowder. This is what propels the firework into the sky. A fuse is used to light the gunpowder, which ignites to send the firework skyward. Once the firework is in the air, more gunpowder inside it causes it to explode with a BANG!

Colour and sparkles:

Colours are made by burning metal salts. Did you know that if you burn table salt (sodium chloride) it makes a yellow flame?

Burning different types of metal salts makes different coloured flames. Have a look at this:

This activity will focus on the visible portion of the electromagnetic spectrum.

Explanation of visible light at the electronic level:

What do fireworks, lasers, and neon signs have in common?  In each case, we see the brilliant colors because the atoms and molecules are emitting energy in the form of visible light.

The chemistry of an element strongly depends on the arrangement of the electrons.  Electrons in an atom are normally found in the lowest energy level called the ground state.  However, they can be "excited" to a higher energy level if given the right amount of energy, usually in the form of heat or electricity.  Once the electron is excited to a higher energy level, it quickly loses the energy and "relaxes" back to a more stable, lower energy level.  If the energy released is the same amount as the energy that makes up visible light, the element produces a color.  The visible spectrum, showing the wavelengths corresponding to each color, is shown below:

Note:  [1 Å = 0.1 nm]           

Is light a particle or a wave?

Is light composed of waves or of particles?  If light is waves, then one can always reduce the amount of light by making the waves weaker, while if light is particles, there is a minimum amount of light you can have - a single ``particle'' of light.  In 1905, Einstein found the answer: Light is both!  In some situations it behaves like waves, while in others it behaves like particles.  

This may seem odd. How can light act like both a wave and a particle at the same time? Consider a duck-billed platypus. It has some duck-like properties and some beaver-like properties, but it is neither. Similarly, light has some wavelike properties and some particle like properties, but it is neither a pure wave nor a pure particle.

A wave of light has a wavelength, defined as the distance from one crest of the wave to the next, and written using the symbol .  The wavelengths of visible light are quite small: between 400 mm and 650 nm, where 1 nm = 10-9 m is a ``nanometer'' - one billionth of a meter.  Red light has long wavelengths, while blue light has short wavelengths.

A particle of light, known as a photon, has an energy E. The energy of a single photon of visible light is tiny, barely enough to disturb one atom; we use units of “electron-volts”, abbreviated as eV, to measure the energy of photons.  Photons of red light have low energies, while photons of blue light have high energies.

The energy E of a photon is proportional to the wave frequency f,    

E = h f 

where the constant of proportionality h is the Planck's Constant, h = 6.626 x 10-34 J s.

1. A photon has a frequency (n) of 2.68 x 106 Hz or cyc/sec.  Calculate its energy.

Given: f = 2.68 x 106 Hz            h = 6.626 x 10-34 J s

Find: E in Joules

Formula:    E = h f

Solution:  

Also, the relationship between frequency and wavelength can be defined as:

f = c         

                                          λ

where c is the speed of light (3×108 m/sec).

2.  Calculate the frequency and the energy of blue light that has a wavelength of 400 nm

Part 1:

Given:    λ = 400nm        c = 3×108 m/sec

Find: f in cyc/sec

Formula: f = c   

                      λ

Solution: f = 3×108 m/sec   =

                       400 x 10-9 m

Part 2:

Given: f = ____________         h = 6.626 x 10-34 J s

Find: E in Joules

Formula:E = h f

Solution:  E = (_____ cyc/sec) (6.626 x 10-34 J s)

So photons still have a wavelength.  A famous result of quantum mechanics is that the wavelength relates to the energy of the photon.  The longer the wavelength, the smaller the energy.  For instance, ultraviolet photons have shorter wavelengths than visible photons, and thus more energy.  This is why they can give you sunburn, while ordinary light cannot.

Flame Test Lab

Background:

In this activity, you will investigate the colors of flame produced by solutions of metal salts.

A flame test is a procedure used to test qualitatively for the presence of certain metals in chemical compounds. 

When the compound to be studied is excited by heating it in a flame, the metal ions will begin to emit light.   Based on the emission spectrum of the element, the compound will turn the flame a characteristic color.  This technique of using certain chemical compounds to color flames is widely used in pyrotechnics to produce the range of colors seen in a firework display.

The normal electron configuration of atoms or ions of an element is known as the “ground state.”  In this most stable energy state, all electrons are in the lowest energy levels available.  When atoms or ions in the “ground state” are heated to high temperatures, some electrons may absorb enough energy to allow them to “jump” to higher energy levels.  The element is then said to be in the “excited state.”   This excited configuration is unstable, and the electrons “fall” back to their normal positions of lower energy (ground state).

 As the electrons return to their normal levels, the energy that was absorbed is emitted in the form of electromagnetic energy.  

Some of this energy may be in the form of visible light.  The color of this light can be used as a means of identifying the elements involved.  Such analysis is known as a flame test.

Materials:

        Set of metal chloride solutions (NaCl, CuCl2, KCl, CaCl2, SrCl2, LiCl , BaCl2)

        Bunsen Burner

        Q-tip

Safety:  Be sure to wear goggles; closed toed shoes; long pants; hair tied back; no loose clothing

Procedure:

1. Light the burner..
2. Using the Q-tip, dip it into the water and then into one of the salts and then hold it in the hottest part of the burner flame.  Observe the color of the flame.  Do not to catch the Q-tip  on fire.  Carefully record your observations in the data table.  Be accurate here - your description of the color must be accurate enough to distinguish this metal ion from the other ions tested.
3. Use a new  side of the Q-tip  for each of the other salts, check the color of their flame tests.  Record your observations.

Data table:

Draw this table into your journal.

Answer these questions in your journal

        Chemists began studying colored flames in the 18th century and soon used “ flamed tests“ to distinguish between some elements. Different elements burn with different colored flames. Although some of the flames you will be seeing will appear similar in color, their light can be resolved ( separated ) with a prism into distinctly different bands of colors on the electromagnetic spectrum (ROYGBIV). These bands of color are called atomic line spectra and they are UNIQUE to each element.

        Niels Bohr studied the line spectrum for hydrogen, and wondered what the specific line spectrum had to do with the structure of the atom. He postulated that an electron can have only specific energy values in an atom, which are called energy levels. Bohr believed that the energy levels for electrons were quantized, meaning that only certain, specific energy levels were possible.

        How does an electron move between energy levels? By gaining the right amount of energy, an electron can move, or undergo a transition, from one energy level to the next. We can explain the emission of the light by atoms to give the line spectrum.

1. An electron in a high energy level undergoes a transition to a low energy level.
2. In this process, the electron loses energy, which is emitted as a photon (a particle which behaves like a wave)
3. The energy difference between the light energy level and the low energy level is related to the frequency (color) of the emitted light.

Basically, adding heat causes the electrons to jump to higher energy level. When they drop back down, they emit energy in the form of waves that have the frequency of visible light.