The pH of pure water is 7 at 25 °C.
However, this is a very specific scenario. In practical settings, pure water is rarely encountered because it readily interacts with its environment. This interaction can significantly alter its pH. Here's a breakdown:
Understanding pH and Pure Water
- pH Scale: The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. A pH of 7 is considered neutral. Values below 7 indicate acidity, and those above 7 indicate alkalinity (basicity).
- Pure Water (H₂O): Theoretically, pure water consists solely of H₂O molecules, and at 25°C, it has an equal concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻), resulting in a neutral pH of 7.
The Impact of Carbon Dioxide
The real world is not sterile, and pure water, when exposed to the atmosphere, changes quickly.
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CO₂ Dissolution: Carbon dioxide (CO₂) from the air dissolves in water.
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Carbonic Acid Formation: This dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃).
CO₂ (gas) + H₂O (liquid) <=> H₂CO₃ (aqueous) -
pH Change: Carbonic acid is a weak acid, which increases the concentration of hydrogen ions (H⁺) in the water, lowering its pH. As a result, the pH of water in contact with air will shift to approximately 5.2.
Summary
Here is a table illustrating the key pH values:
| Condition | pH |
|---|---|
| Pure Water (25°C, ideal) | 7 |
| Water in contact with air (approx.) | 5.2 |
Key Takeaway
While pure water ideally has a pH of 7, it's essential to note that in real-world conditions, the interaction with atmospheric carbon dioxide will typically cause the pH to drop to around 5.2.
