Devise three or more methods of labelling samples correctly using chemicals and apparatus in the laboratory
Explore this two part problem and explore dilute acids and the reduction of metal oxides.
There are two parts to the problem; the first part asks students to distinguish between carbon and copper(II) oxide; and the second asks them to decide which of the three given samples is a mixture. Each part can be tackled separately or consecutively.
Teachers who have not used the problems before should read the section Using the problems before starting.
Properties of dilute acids, reduction of metal oxides and reactivity series. A detailed knowledge is unnecessary as students are encouraged to consult textbooks and data books during the exercise.
Data books and inorganic textbooks should be available for reference.
- For part 1 unlabelled, but numbered, samples of powdered carbon and copper(II) oxide should be provided.
- For part 2 unlabelled, but numbered, samples of powdered carbon and copper(II) oxide are needed and a mixture of roughly equal amounts of the two mixed together are needed.
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.
- On heating in air, copper(II) oxide is unchanged, carbon burns away.
- On warming with dilute acid the copper(II) oxide sample gives a blue solution (filtration is probably needed).
- On reduction, the copper(II) oxide sample releases copper. Ethanol vapour passed over the heated solids would cause similar reactions to occur.
- Flame tests.
- Solid phase displacement with a more reactive metal such as magnesium, iron or zinc also produces copper. These reactions are quite unpredictable and are not recommended.
The easiest method of discriminating between the three solids is to heat them; the one that gives copper metal is the mixture. For this to work, students will have to take care not to oxidise away all the carbon. It is sometimes necessary to cool the hot mixture quickly under a slowly running water tap to prevent reoxidation of the copper.
Zinc powder can be a particular problem – some samples react well, others explosively, perhaps because the different samples are oxidised to different extents.
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