Concentrated nitric acid – sitting quietly in its glass bottle in a bed of sand – takes few prisoners. While copper atoms can be praised for their strength of bonding, thirty seconds in a 100-ml beaker of the acid is all that it takes to strip the atoms apart, one by one. “Let me see, let me see,” is the call of a few Year 6 students, squeezed behind taller students, all gawking into the fume cupboard to see the rapid change. In 30 seconds, the shiny copper sheets are gone, a noxious brown gas of nitrogen dioxide is rapidly drawn up the twin extraction chimneys and the solution turns green.
But why green? Copper nitrate solution is pale blue.
The brown gas dissolves easily in the blue solution and colours yellow/brown added to blue gives green. Eventually the gas escapes the solution and the blue copper nitrate solution is left on its own. The Year 6 students all record that a chemical reaction has occurred; there is heat involved, colour changes and the copper foil sheets have been destroyed by the nitric acid.
But why green and not blue?
Our inventive lab assistant Mrs Michelle Parcell directs the students to the brown bottles on the lab benches. She has blue solutions of copper nitrate ready for stage two. The students mix the blue solution with clear sodium hydroxide solution to provide a lumpy gel concoction of copper hydroxide. The original copper atoms are certainly on the move. More report writing for the Year 6 students. The Copper Hydroxide solution with serious heating and stirring all changes into black copper oxide, a black heavy powder layering out at the bottom of the beaker. Chem Lab session #1 draws to a close with the copper oxide lying in wait for next week.
Sulphuric acid is still a bench mark commodity for the industrial economy of a nation. The more it is used, the stronger is the economy. On the contrary for Year 6 on day 2, where its role is not so global. Copper oxide is a dormant substance but sulphuric acid can overwhelm that tendency by producing another clear blue solution, this time, copper sulphate.
And then the finale.
Getting all that copper back is achieved by adding zinc metal granules. Within minutes reddish/orange mud appears at the bottom of the beaker. The students love to try their new skills of filtering to retrieve the original copper. Our four ATC Year 6 classes did this series of experiments over two weeks. Each of their teachers- Erin Moffat, Maryanne Costi, Brett Tompson and Cam Pickering were especially pleased with the quality of their written reports and their interested questions. Year 6 students, congrats. Your imaginations have done you proud!