Letters - July 2009

Copper sulfate

In his recent letter (see Educ. Chem., 2009, 46 (2), 41) Peter Borrows asks, what is so special about copper sulfate that students ought to know its colour?

My answer is that copper(II) salts are important in chemistry and in everyday life. For example: 

• a test for water is that it turns anhydrous copper sulfate blue; 
• a test for reducing sugars is that they precipitate red Cu₂O from Fehling's solution, a blue reagent made from copper sulfate; 
• copper sulfate is used as a fungicide. Farmers and gardeners know they have the right chemical from its colour. Farmers also use the chemical in foot baths to harden the hooves of cattle; 
• copper(II) salts are used to preserve wood. A builder knows that timber has been treated if it is bluish green; 
• metallic copper corrodes in moist air to CuCO₃·Cu(OH)₂ and other basic salts. If you find a bluish green powder in your home you have copper corroding somewhere; 
• copper and copper alloys can leave a bluish green mark on prolonged contact with moist skin. If your 'gold' ring leaves such a mark you do not have the gold you may have imagined. 

I think students ought to know the properties of important substances. 

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Alan Dronsfield, University of Derby

In his review of Teaching secondary how science works, Mike Shipton suggests that it would be helpful if worksheets supplied on the CD-ROM that accompanies the book could be customised for student use (Educ. Chem., 2009, 46 (3),95). It is implied that because the material is presented in Portable Document Format (PDF), this is difficult.  

However there are several programs available on the Internet, many of which are free, that can help with this. I use the freeware A PDF text extractor, which is available to download. To extract and edit text from a PDF file: 

Readers are advised to consider any copyright implications before copying and circulating published materials. 

Recent endpoints

Michael Laing, Durban, South Africa

I was greatly impressed by the Endpoints in two recent issues. In Practically minded published in March, Anthony Hardwicke used phrases such as 'enemies', and 'greedy exam boards', and 'foist new initiatives onto schools', and 'latest fad' (see Educ.Chem., 2009, 46 (2), 64). In the May issue's Target setting fails education, Nick Barker used the phrases 'soulless ideology' and 'obsession with assessment' (see Educ. Chem., 2009, 46 (3), 96). Quite remarkably these are the sorts of comments that I heard from our local teachers of physical science at a recent conference.  

In South Africa we have had too many new curricula and their revised versions in the past 10 years. Most recent was the addition of VSEPR theory to the Grade 11 (GCSE equivalent) syllabus. The basic philosophy of teaching is OBE - outcomes-based education - which was the answer I got when I asked one teacher what he taught. (I had expected 'optics' or 'bonding'.) Worse still, every pupil now has a 'portfolio', which contains every piece of paper relating to the student: exams, projects, tests etc, anything that can be assessed with a number. As for practical work, I recently visited a school of 1300 boys, all of whom are required to study physical science for five years. The school employed one science technician.  

It seems that we share similar problems. What is the solution? My usual answer is 'step boldly back 40 years' to pre-CHEM Study and pre-Nuffield courses, and learn a few simple facts. Let 'moral implications' come later, after 'understanding' has been built on a foundation of facts.  

Catalyst for discussion

I would like to add a few notes to Adrian Guy's description of the platinum-catalysed ammonia oxidation demonstration (Educ. Chem., 2009, 46 (3), 76). Although the platinum wire will last for many years, the quoted cost of ca £100 for 20 cm may well cause teachers to abandon the idea, having higher priorities for their limited budgets. In fact, 10 cm is adequate and can be bought from educational suppliers for £25 to £30. 

Provided visibility is not too restricted, and students can gather around the fume cupboard, the experiment can be scaled down. Since very little of the ammonia is consumed, the demonstration works just as well with ca 10 cm³ of warm saturated (0.880) ammonia solution in a 100 cm³ beaker. (Warming helps by increasing the concentration of ammonia gas in the beaker.)  

When the demonstration is performed in a fume cupboard there is no need for the wooden lid. Simply hang the coiled wire from the glass rod supported on the beaker rim. This also allows air to enter the beaker more easily, assisted by convection currents caused by the hot wire. 

That the wire continues to glow red hot shows that the catalytic oxidation of ammonia is highly exothermic (ΔH = -950 kJ mol^-1). If the reaction stops, it can be restarted by re-heating the wire. 

Teachers (and maybe some brighter students) will realise that the nitrogen monoxide produced should spontaneously oxidise in air to form nitrogen dioxide. However, I have been doing this demonstration for 25 years and have never noticed any brown gas. You might challenge your students to offer explanations for this (lack of) observation. 

Logical responses include: 

• very little nitrogen monoxide, and thus dioxide, is produced - too little to be noticeable;   
• the products are dispersed in air by convection;   
• the nitrogen dioxide reacts with the co-product, water, forming nitric acid (and nitrogen monoxide again). 

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