Metal oxides: Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen.

The idea

When a metal reacts with oxygen, the product is a metal oxide. For example, magnesium burns in oxygen to form magnesium oxide, and iron reacts slowly with oxygen and water to form iron oxide in rusting. These are oxidation reactions because the metal gains oxygen. In terms of particles, the metal atoms lose electrons to oxygen atoms, forming metal ions and oxide ions in an ionic compound; the metal is oxidised. The reaction can be represented by a word equation such as magnesium + oxygen → magnesium oxide, or by a balanced symbol equation such as 2Mg + O₂ → 2MgO. Oxidation is not limited to oxygen gain, but at this level the definition is the gain of oxygen by a substance. Metal oxides are often basic, and some react with acids to form salts and water.

What earns the marks

• Metals react with oxygen to produce metal oxides, for example magnesium + oxygen → magnesium oxide.
• The reaction is an oxidation reaction because the metal gains oxygen.
• Oxidation can be described as the gain of oxygen by a substance, and the metal is the substance being oxidised.
• Word equations and balanced symbol equations can represent the reaction, such as 2Mg + O₂ → 2MgO.
• Metal oxides are compounds of a metal and oxygen, and many are basic and can neutralise acids to form a salt and water.

Common mistakes and how to put them right

• Writing the product as ‘magnesium oxygen’ instead of magnesium oxide.

  Put it right: Use the naming rule that a metal plus oxygen gives the metal oxide, and write the correct formula, MgO.

• Saying that oxygen is oxidised because it gains electrons.

  Put it right: At this level, identify the metal as the substance that gains oxygen and is therefore oxidised.

• Balancing equations by changing subscripts, such as writing MgO₂ for magnesium oxide.

  Put it right: Balance by placing whole-number coefficients in front of the correct formulae, as in 2Mg + O₂ → 2MgO.

Exam tips

• State the definition of oxidation as the gain of oxygen, then apply it directly to the metal in the reaction you are describing.
• When writing a symbol equation for a metal oxide formation, check that the formula of the oxide has the correct ratio of metal ions to oxide ions before balancing.
• If asked to explain why a reaction is oxidation, refer to the metal gaining oxygen rather than giving a general statement about oxygen being present.