Here's why:

* Mixture: A mixture is a combination of two or more substances where each substance retains its individual properties. The components of a mixture can be easily separated by physical means.

* Solution: A solution is a homogeneous mixture where one substance (solute) is dissolved evenly in another substance (solvent). The solute and solvent lose their individual properties and form a new, uniform phase.

When ammonia (NH₃) dissolves in water (H₂O), it reacts with water to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). This reaction creates a new substance with different properties than either ammonia or water alone. Therefore, it is a solution, not a mixture.