The reason pure deionized water rarely reads as 7.0 is because it’s incredibly reactive. As soon as it’s exposed to air, it starts sucking up carbon dioxide like a thirsty sponge. This creates carbonic acid, which drops the pH down to around 5.5 or 5.8.
It’s not that the water is “bad” or dirty. It’s just that it’s so pure, so hungry for ions, that it grabs whatever is floating nearby. It’s like that friend who shows up to a party and instantly starts absorbing everyone’s energy.
This is why you’ll often hear scientists say that truly neutral water is a theoretical ideal. Getting a stable, perfect 7.0 reading requires a hermetically sealed, air-free environment.
