

Planning of the experiment to investigate Dalton’s law of constant composition

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Dalton in the eighteenth and nineteenth century stated that all pure samples of the same chemical compound contain the same proportions of mass

As various copper compounds are readily available for laboratory use, the decision has been made to produce copper from black copper oxide and produce black copper oxide from copper by various methods, and to analysis the findings from this to see that the law of constant composition holds true. No other method or data was found to show or prove Daltons theory was true so the decision to use copper and black copper oxide was the best immediate solution.

In the preliminary work, various ways of testing Dalton’s theory was researched and tested:

* Copper Nitrate (from which copper oxide will be made to make copper)

* Copper Hydroxide (from which copper black oxide will be made to make copper)

* Copper Carbonate (from which black copper oxide will be made to make copper)

* Copper (can be oxidized to form black copper oxide)

My variable will be different methods of making black copper oxide, and in my results, I aim to see whether this affects the mass ratio of copper to oxygen in the copper oxide from various methods of creation.

Black Copper oxide will have to be reduced using carbon or hydrogen, as they are more reactive than copper and the copper will be displaced. Both theses methods were tested in the preliminary work. As carbon was tested as a reducing agent in the experiment, I found out that due to the difficulty of separating the carbon from the copper to weigh the mass of the copper alone, I decided to use hydrogen as a reducing agent. When hydrogen was used as a reducing agent, I found that the copper oxidised when you stopped heating the copper and took it with tongs to be reweighed. I decided that to prevent this from occurring, the hydrogen must continue passing over the copper while the copper cools down to stop the copper from oxidizing.

In my preliminary work I discovered that a large amount of copper nitrate, copper hydroxide and copper carbonate needs to be heated, more than can fit in a test tube, so a large crucible must be used. Also I researched using available books and information that the gas given off copper nitrate, nitrogen dioxide, is toxic, so this must be heated in a fume cupboard.

By weighing the mass of the solid such as copper or copper oxide before and after the experiment, the mass ratio from copper to oxygen can be worked out, and we can see whether constant composition holds true by finding the lowest common mass, with the same ratio being found whichever way you make copper oxide and then make copper.

When weighing the mass in my preliminary work, each time the mass was reweighed after being reheated the mass went down, so I discovered that the copper or copper oxide must be reheated and reweighed until a constant mass is achieved. Also using these weights, the formula by working out the ratios of moles can be calculated. This will show the ratio of copper atoms to the ratio of oxygen atoms, indicating the formula. If the ratio is 1:1, the formula is CuO. If the ratio is 2:1, the formula is Cu2O. Moles for copper and oxygen are calculated as below:

Mass of element = the amount of moles

Relative Atomic mass

The amount of moles can be compared to work out the ratio of atoms from copper to oxide within the copper oxide. This can be used to check whether the copper oxide maintains a constant formula however it is made.

Safety glasses and aprons are to be worn at all times. As copper nitrate gives off the toxic gas nitrogen dioxide when heated, this must be heated in a fume cupboard.


The results will be clearly laid out a table to clearly show the mass of the apparatus the solid was weighed in, the apparatus mass and the mass of the solid before the experiment and the mass of the solid and the apparatus after the experiment. From this the table should show the mass of copper and oxygen and copper oxide.

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