Here's why:

* Hydrolysis: When aluminum sulfate dissolves in water, it undergoes hydrolysis, a reaction where water molecules break down the salt. This process produces aluminum ions (Al³⁺) and sulfate ions (SO₄²⁻).

* Aluminum Ions: Aluminum ions react with water to form hydrated aluminum ions ([Al(H₂O)₆]³⁺) and hydrogen ions (H⁺):

* Al³⁺ + 6H₂O ⇌ [Al(H₂O)₆]³⁺

* [Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺

* Increased Acidity: The release of hydrogen ions (H⁺) increases the concentration of hydrogen ions in the water, resulting in a lower pH value.

Practical Applications:

This property of aluminum sulfate is used in various applications, including:

* Water Treatment: Alum is a common coagulant in water treatment. It helps remove suspended particles by forming flocs (small clumps) that settle out, clarifying the water. The acidic nature of aluminum sulfate can also help to control the pH of the water.

* Soil Amendment: Aluminum sulfate can be used to acidify soil, which is beneficial for plants that prefer acidic conditions.

Important Note:

The extent to which aluminum sulfate affects the pH depends on several factors, including:

* Concentration: Higher concentrations of aluminum sulfate will lead to a greater decrease in pH.

* Initial pH: The initial pH of the water will also influence the final pH after adding aluminum sulfate.

* Temperature: Temperature can affect the rate of hydrolysis.

Always consult with professionals or follow safety guidelines when using aluminum sulfate.