Is the law of conservation of mass valid for nuclear reactions ?

Answer

### Step-by-Step Solution:

1. **Understanding the Law of Conservation of Mass**:
- The law states that in a closed system, mass cannot be created or destroyed during a chemical reaction. This principle was established by Antoine Lavoisier, the father of modern chemistry.

2. **Application to Chemical Reactions**:
- In typical chemical reactions, the total mass of reactants equals the total mass of products. This validates the law of conservation of mass.

3. **Nuclear Reactions Overview**:
- Nuclear reactions involve changes in the nucleus of atoms, which can lead to the transformation of one element into another and the release or absorption of large amounts of energy.

4. **Mass-Energy Conversion**:
- In nuclear reactions, a portion of the mass is converted into energy according to Einstein's equation \(E=mc^2\). This means that the mass before the reaction does not equal the mass after the reaction because some mass has been transformed into energy.

5. **Conclusion**:
- Therefore, the law of conservation of mass does not hold true for nuclear reactions. Instead, we say that the total mass and energy combined are conserved.

### Final Statement:
The law of conservation of mass is not valid for nuclear reactions because mass can be converted into energy, which alters the total mass present in the reaction.

---