Soft water gives lather with Soap.

Answer

### Step-by-Step Solution:

1. **Understanding Soft Water**:
- Soft water is defined as water that has low concentrations of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are responsible for the hardness of water.

2. **Soap and Its Function**:
- Soap is a surfactant that helps in cleaning by reducing the surface tension of water and allowing it to spread and penetrate more easily. It works effectively in soft water.

3. **Lather Formation**:
- When soap is mixed with soft water, it reacts with the water molecules and forms lather (or foam). This is because soft water does not contain high levels of calcium and magnesium ions that would otherwise react with soap to form insoluble salts.

4. **Comparison with Hard Water**:
- In hard water, the presence of calcium and magnesium ions leads to the formation of insoluble soap scum, which prevents lather formation. This is why hard water does not produce lather with soap.

5. **Conclusion**:
- Therefore, soft water is effective for lathering with soap due to the absence of significant amounts of calcium and magnesium ions, allowing the soap to function properly and create foam.