When performing an experiment in a combustion tube, the key observation usually pertains to the chemical reactions taking place when a metal oxide, like copper(II) oxide (CuO), is replaced with magnesium oxide (MgO).
Observation with Magnesium Oxide:
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Absence of Reduction Reaction: When magnesium oxide is heated in a combustion tube, typically combined with a reducing agent (such as carbon or carbon dioxide), there will be no reduction of magnesium oxide to magnesium metal. In contrast, copper(II) oxide can be reduced with carbon to yield copper metal (Cu).
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Stability of Magnesium Oxide: Magnesium oxide is a very stable compound and is less reactive than copper(II) oxide. In the presence of heat, magnesium oxide remains largely unchanged, while copper(II) oxide may undergo a reduction:
\[ \text{CuO (s)} + \text{C (s)} \rightarrow \text{Cu (s)} + \text{CO (g)} \]
For magnesium oxide, you would find:
\[ \text{MgO (s)} + \text{C (s)} \rightarrow \text{No significant reaction} \]
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Physical Observation: As magnesium oxide does not decompose easily, there won't be any noticeable change in the color or state of the magnesium oxide itself. In contrast, in the cupric reduction, you would see a visual change as solid copper is formed, which appears as a reddish-brown solid.
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Temperature Requirements: The combustion of magnesium would require a much higher temperature to initiate, such as that which may be available in a flame, unlike copper(II) oxide which can be reduced at lower temperatures due to its relatively lower stability.
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Product Features: If an attempt is made to further react the magnesium oxide in an oxidizing environment, it won't produce any volatile gases like CO, which might be observed in the copper case.
In summary, the main observation when replacing copper(II) oxide with magnesium oxide in a combustion tube is that magnesium oxide does not undergo reduction under normal experimental conditions, highlighting its stability, in contrast to the reduction of copper(II) oxide to copper metal.