Neutral water with pH about 7 becomes slightly acidic when aerated. This is because

Neutral water (pH 7) becomes slightly acidic when aerated because carbon dioxide (CO2) from the air dissolves into the water. When CO2 dissolves, it reacts with water molecules to form carbonic acid (H2CO3) [c1][t1]. This weak acid subsequently dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-) [t4][t8]. The release of these hydrogen ions increases the acidity of the solution, thereby lowering the pH level [c1][t3]. This phenomenon is naturally observed in rainwater, which typically has a pH of about 5.6 due to atmospheric CO2 [t2][t4]. While aeration increases dissolved oxygen, oxygen itself does not impart acidity; rather, the non-metallic oxide nature of CO2 is responsible for the acidic shift [c2]. This chemical process is also the fundamental driver behind ocean acidification as atmospheric CO2 levels rise [t6][t8].

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 18: Ocean Acidification > How it reacts? > p. 264
2. [2] https://www.epa.gov/ocean-acidification/understanding-science-ocean-and-coastal-acidification
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > 2.1.6 Reaction of a Non-metallic Oxide with Base > p. 22