unequal number of atoms in reactants and products.

Options

A. chemical change
B. rearrangement of atoms
C. formation of new substances
D. all of these.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the implications of having an unequal number of atoms in the reactants and products of a chemical equation.

2. **Identifying Types of Changes**:
- There are two main types of changes:
- **Physical Change**: No new substances are formed, and the number of atoms remains the same. For example, changing the state of water (H2O) from liquid to gas does not alter the number of H and O atoms.
- **Chemical Change**: New substances are formed, and the properties of these substances differ from the reactants. In chemical changes, the number of atoms can differ between reactants and products.

3. **Chemical Change Characteristics**:
- In a chemical change, atoms are rearranged to form new substances.
- The properties of the new substances are different from those of the original reactants. For example, calcium carbonate (CaCO3) can decompose to form calcium oxide (CaO) and carbon dioxide (CO2), which have different properties.

4. **Conclusion**:
- An unequal number of atoms in the reactants and products indicates that a **chemical change** has occurred.
- This change involves the **rearrangement of atoms** and the **formation of new substances**.

5. **Final Statement**: Therefore, the unequal number of atoms in the reactants and products signifies that a chemical change has taken place, characterized by the rearrangement of atoms and the formation of new substances.