Atomic Emission Spectra

Aim

This experiment is designed to give you a qualitative introduction to the spectra emitted by some s-block elements when their atoms are excited by heating in a Bunsen flame.

Introduction

You will be using a hand spectroscope to observe the difference between a continuous emission spectrum; produced by a fluorescent and a line spectrum; produced by the excitation of electrons in various elements.

Requirements

• electric lamp with tungsten filament or energy saving light bulb
• hand spectroscope
• Bunsen Burner and heat proof mat.
• flame-test wire
• hydrochloric acid, HCI, 1 mol dm^-3 (LOW HAZARD)
• spatulas
• watch-glasses
• at least three of the following:
• barium chloride, BaCl_{2 (s) (TOXIC)}
• calcium chloride, CaCl_{2 (s) (IRRITANT)}
• lithium chloride, LiCl _{(s) (HAZARD)}
• potassium chloride, KCl_{(s) (LOW HAZARD)}
• sodium chloride, NaCl_{(s) (LOW HAZARD)}
• strontium chloride, SrCl_{2 (s) (IRRITANT)}
• safety glasses

Procedure

1. Switch on the lamp and look at the bulb through the Look for a series of colours, one running into the next. This is a continuous spectrum.
2. Hold the spectroscope up to a window which does not face the You must NEVER point the spectroscope directly at the sun. This could result in permanent damage to your eyes.
3. You should see the continuous spectrum of visible light.
4. Light the Bunsen burner – adjust it to get a roaring blue flame.
5. Dip the flame test wire into concentrated hydrochloric acid, then hold it in the hottest part of the Repeat the process until there is little or no colour from the flame test wire in the flame. You may have to repeat this step several times, especially towards the end of the experiment.
6. Add a small amount of the salt to be tested and mix with a little hydrochloric acid on a watch Be careful here – use just enough of the acid to give you a semi-solid ‘mush’ of crystals.
7. Dip the cleaned nichrome wire into the mush of the salt to be tested and place the end of the wire into a blue flame and observe with the spectroscope.
8. Repeat steps 2-6 with the salts of at least two other
9. If available use the spectroscope on any vapour lamps which may be available, and compare their spectra.

Question

1. What is the difference between a continuous spectrum and a line emission spectrum?
2. Every element produces a unique line spectra, what is the reason behind this?