Here's why:

* Sand is a heterogeneous mixture: This means it consists of different components. While the majority is sand particles, there might also be small amounts of other materials like:

* Dissolved minerals: These can be picked up from the source of the water or the sand itself.

* Organic matter: Decomposing plant and animal material can be present.

* Bacteria and microorganisms: These might be present in the water or on the sand.

* Filtration is a physical separation: It removes the larger sand particles but doesn't eliminate smaller dissolved substances or microorganisms.

Therefore, even after filtration, the water might still contain impurities, making it not perfectly pure.

To get pure water, additional purification methods like boiling, distillation, or using specific filters are required.