Detecting copper in black solids

Give learners a choice of black solids, and see if they can discover what the solids are, using a range of experiments

Introduction

Teachers who have not used the problems before should read the section Using the problems before starting.

Prior knowledge

Properties of dilute acids, the reduction of metal oxides and the reactivity series. A detailed knowledge is unnecessary as students are encouraged to consult textbooks and data books during the exercise.

Equipment

Data books and inorganic textbooks should be available for reference. 

• A mixture of roughly equal amounts of powdered carbon and copper(II) oxide should be provided at the start of the exercise.

Students can request apparatus and chemicals during the practical session, and these should be issued if they are safe to use eg flame test equipment will probably be required but it should not be on view.

Possible methods

The students’ starting point is to find out what solids are black. They might recall this; otherwise they should look at chemicals on the shelves and refer to textbooks. They should identify carbon and metal oxides such as CuO and NiO as possibilities, but are unlikely to consider that the unknown could be a mixture. They may try experiments such as those listed below, however, they will find anomalies in their results if they assume that the mixture is a pure solid. Success will depend upon how the group deals with these anomalies.

1. A flame test gives blue/green colour, showing that copper ions are present.
2. Warming with dilute acid gives a blue solution showing that copper(II) ions are present. The reaction between CuO and warm dilute acid is slow, and filtration is necessary to separate the unreacted CuO as well as the carbon.
3. On heating in air, the mixture should give copper metal.
4. On reduction, for example with natural gas, there will be two reductants – natural gas and carbon. Copper will be produced but the carbon may mask its colour.
5. Solid phase displacement with a more reactive metal such as magnesium, iron or zinc, will produce copper. These reactions can be unpredictable and are not recommended

Notes

• Heating the solid mixture will give copper metal. For this to work, students will have to heat the solid, but not for too long because this will burn off all the carbon and the copper produced will re-oxidise. It is sometimes necessary to cool the hot mixture quickly under a slowly running water tap to prevent this re-oxidation.
• Zinc powder can be a particular problem – some samples react well, others explosively, perhaps because the different samples are oxidised to different extents.