Which of the following reactions is not a decomposition reaction?

Options

A. starch is converted to glucose in our body
B. proteins are digested to amino acids
C. magnesium ribbon is burnt in air
D. electricity is passed through water

Answer

To determine which of the following reactions is not a decomposition reaction, we first need to understand what a decomposition reaction is. A decomposition reaction is a type of chemical reaction where a single compound breaks down into two or more simpler substances.

### Step-by-Step Solution:

1. **Identify the Reactions**:
We need to look at the given reactions and identify which one does not fit the definition of a decomposition reaction.

2. **Analyze Each Reaction**:
- **Reaction 1**: Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.
- This is a decomposition reaction.
- **Reaction 2**: Starch decomposes into glucose.
- This is also a decomposition reaction.
- **Reaction 3**: Magnesium ribbon burns in air.
- This is a combination reaction, where magnesium combines with oxygen to form magnesium oxide. Therefore, it is not a decomposition reaction.
- **Reaction 4**: Water decomposes into hydrogen ions and hydroxide ions when electricity is passed through it.
- This is a decomposition reaction.

3. **Conclusion**:
The reaction that is not a decomposition reaction is the one where magnesium ribbon is burnt in air.

### Final Answer:
The reaction that is not a decomposition reaction is the burning of magnesium ribbon in air.