Final answer:
The reactions 2Mg + O₂ → 2MgO (a synthesis or composition reaction) and CaO + H2O → Ca(OH)₂ (a combination reaction) are not decomposition reactions.
Step-by-step explanation:
The student is questioning which of the listed reactions are not decomposition reactions. A decomposition reaction is a type of chemical reaction in which one compound breaks down into two or more simpler substances. The reactions provided can be categorized based on this definition:
- 2Mg + O₂ → 2MgO: This is not a decomposition reaction; it's a synthesis or composition reaction, as magnesium and oxygen combine to form magnesium oxide.
- CaO + H2O → Ca(OH)₂: This is also not a decomposition reaction; it is a combination reaction, resulting in the formation of calcium hydroxide from calcium oxide and water.
- NaOH → Na₂O + H₂O: This is a decomposition reaction, where sodium hydroxide decomposes into sodium oxide and water.
- KClO₃ → KCl + O₂: This is another example of a decomposition reaction, where potassium chlorate breaks down into potassium chloride and oxygen gas.
According to these examples, 2Mg + O₂ → 2MgO and CaO + H2O → Ca(OH)₂ are not decomposition reactions, but rather are examples of composition or combination reactions.