UPD8

CSI 5/11 -  Murder in the Park

Learning objectives

Students will:
•use flame test data to solve a crime scene investigation.
•model atoms as circular electron energy levels around a central nucleus in order to explain where the coloured light comes from when metal ions are heated.
•describe atoms using this model and relate features of the model to an element’s position in the periodic table.

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Curriculum link

GCSE specifications
AQA Additional Module C2: 12.1 How do sub-atomic particles help us to understand the structure of substances?
AQA Chemistry: 13.5 How do we identify and analyse substances?
Edexcel Core Topic 5: Patterns in Properties.
Edexcel Additional Topic 6: In Your Element.
OCR 21st Century Additional Module C4: Chemical Patterns.
OCR Gateway Module C3: The Periodic Table

Running the activity

Page1 sets the scene. A murder was committed during a firework display. Could firework chemicals help solve the crime? What chemicals should forensics look for? Page 2 summarises the science of fireworks and could be used to revisit a number of concepts as appropriate: combustion, what the -ides and -ates mean in compound names, oxidation and reduction.

A class experiment or demonstration could be done at this point to match up colours to the chlorides of metals in groups 1 and 2 (see practical details below). The activity leaves out toxic barium salts, and copper salts, to make modelling electronic configurations easier. If learning about electron configurations isn’t appropriate, students can skip straight to page 4.

Page 3 introduces the standard GCSE electron energy level model. At this point, the electron cards on page 5 could be used to relate features of the model to each element’s position in the periodic table. Students could be asked to arrange the symbol cards in order of atomic number and match two other cards to each symbol. What patterns can they spot? Possible questions to pose are: how can you tell how many electrons each atom has? How do the energy levels fill up? What’s the same about atoms in the same group of the periodic table?

Page 6 has an electron energy level diagram to laminate and keep, and cut out ‘electrons’ to use with it. This could be used to build the electron configurations of other atoms or model the electron transitions that let salts colour flames.

Page 4 returns to the original problem. Students use the flame colours associated with each salt to solve a murder by working out ‘who did it?’ from CCTV images. The evidence points strongly to Sue Little.

Practical details:
Clean Nichrome wire dipped in concentrated hydrochloric acid and a sample of the salt is heated strongly in a bunsen flame on the end of a cleaned nichrome wire. It is best to use chlorides. Note that sodium and potassium chlorides are low hazard but lithium chloride is harmful (Refer to CLEAPSS Hazcard 47B) and calcium chloride and strontium chloride are irritants (refer to CLEAPSS Hazcard 19A). Alternatively, and potentially less messy, use splints soaked in distilled water over night then dipped in solutions of each salt. The expected colours are: lithium - crimson; sodium - yellow; potassium - lilac; calcium - brick red and strontium – red.

It is worth noting that purple fireworks are really made using a mix of strontium (red) and copper (blue) compounds because the lilac colour from potassium is not intense enough. If students can’t see it at all because there is too much sodium around, get them to observe the flame through blue glass.

Web links

News links

chemicalconnection.org

Very accessible account of how the flame test colours arise.

chemsoc.org

Detailed account of how firework colours are produced.

Media links

How stuff works

A brief video explanation of the basic chemistry and physics of fireworks.

youtube

A 1 minute video – one of the shortest of a number of useful Youtube clips of flame tests and the metals responsible for them.

practicalchemistry.org

How to demonstrate the flame test colours using salt solutions in ethanol.

chemit

A PowerPoint presentation - filling electron energy levels for atoms and ions up to calcium.

virtualcomposer2000.com

How to use a CD in a home-made spectrometer to view the emission spectra of metal ions – good enough to see some similarity between street lamps and the flame from salt in a gas flame . . . might even spice up a disappointing firework display.

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