The Fundamental Chemistry of pH
pH, which stands for "potential of hydrogen," is a scale used in chemistry to measure how acidic or basic an aqueous solution is. The scale runs from 0 to 14, with 7 being neutral. A pH below 7 indicates acidity, while a pH above 7 indicates alkalinity or basicity. This scale is logarithmic, meaning each whole number change represents a tenfold change in acidity or basicity. A pH of 6, for instance, is ten times more acidic than a pH of 7.
The Purest Form of Water
To understand the difference, it is essential to first consider pure water, which consists of only H2O molecules. At 25°C, pure water theoretically has an equal concentration of hydrogen ions ($H^+$) and hydroxide ions ($OH^-$), resulting in a perfectly neutral pH of 7. However, achieving and maintaining this state is practically impossible in a real-world setting.
The Role of Distillation
Distillation is a purification process where water is boiled into vapor and the resulting steam is condensed back into a liquid, leaving most contaminants behind. While this process removes minerals, salts, and other impurities, it creates a unique vulnerability to atmospheric gasses.
Why Distilled Water is Acidic
As soon as distilled water is exposed to the air, it begins to absorb carbon dioxide ($CO_2$). This dissolved $CO_2$ reacts with the water to form a weak carbonic acid ($H_2CO_3$) through the following reaction:
$CO_2(aq) + H_2O(l) \rightleftharpoons H_2CO_3(aq) \rightleftharpoons H^+(aq) + HCO_3^-(aq)$
This small amount of carbonic acid is enough to lower the pH of distilled water to a slightly acidic range, often between 5.8 and 7, depending on how long it has been exposed to the air. Because distilled water has no buffering capacity—meaning no minerals to counteract the change—its pH is highly susceptible to this effect.
The Complex Nature of Tap Water
In contrast, tap water is not pure H2O. Its pH is highly variable and depends on a multitude of factors, including its source (groundwater, reservoirs, rivers), the surrounding geology, and the specific treatment processes used by a municipality.
What Influences Tap Water pH
- Dissolved Minerals: Tap water contains a variety of dissolved minerals like calcium and magnesium, which give it a buffering capacity. These minerals react with carbonic acid, preventing the pH from dropping as dramatically as it does in distilled water.
- Treatment Chemicals: Water treatment plants often add chemicals like lime or sodium carbonate to adjust the pH into a target range, typically 6.5 to 8.5, to minimize pipe corrosion.
- Source Water Geology: If the source water passes through granite formations, it might be more acidic, while passing through limestone tends to make it more alkaline.
- Dissolved Gasses: Like distilled water, tap water can also absorb $CO_2$ from the air, but the presence of minerals buffers this effect.
Tap Water pH Variations
Tap water leaving a treatment plant might have a pH between 7 and 8, but this can fluctuate as it travels through a distribution network. For example, studies have shown that tap water can vary from 7.2 to 7.8 in some locations. This variability is a key distinction from the more consistently acidic nature of distilled water after air exposure.
Tap Water vs. Distilled Water pH: A Comparison
| Feature | Tap Water | Distilled Water |
|---|---|---|
| Purity | Contains dissolved minerals, salts, and other impurities. | Highly purified, with most contaminants and minerals removed. |
| Typical pH Range | Highly variable, often regulated to be between 6.5 and 8.5 by municipal water suppliers. | Slightly acidic, typically between 5.8 and 7, due to dissolved $CO_2$. |
| Causes of pH | Minerals, treatment chemicals, and source geology buffer the pH. | Absorbs carbon dioxide from the air, forming carbonic acid. |
| Buffering Capacity | High buffering capacity due to dissolved minerals. | Poor buffering capacity, making it susceptible to pH changes. |
| Consistency | Varies significantly depending on location and treatment process. | Tends toward a slightly acidic pH once exposed to air. |
How to Test the pH of Your Water
Testing the pH of your water is straightforward and can be done with a few different methods:
- Litmus Paper or pH Test Strips: These are inexpensive and readily available. A strip of paper is dipped into a water sample and changes color, which is then compared to a color chart to determine the approximate pH.
- Electronic pH Meter: For more accurate and precise results, an electronic pH meter with a probe can be used. These meters must be calibrated periodically using solutions of a known pH to ensure accuracy.
- Water Testing Kits: More advanced kits or professional lab testing can provide a comprehensive analysis of pH and other contaminants in your water.
Conclusion: The Key Difference is in the Composition
Ultimately, the difference in pH between tap water and distilled water boils down to their chemical composition. While theoretically pure water is neutral, distilled water’s lack of mineral buffers and its high reactivity with atmospheric carbon dioxide causes it to become slightly acidic upon contact with air. In contrast, tap water's dissolved minerals and managed treatment processes give it a higher, more stable, and often more alkaline pH within a regulated range. Therefore, the pH difference is not just an arbitrary number but a reflection of the fundamental chemical properties and impurities present in each type of water.
For more detailed information on the properties of water, you can refer to the U.S. Geological Survey's (USGS) water science resources.
Common pH Levels for Different Water Types
- Alkaline Water: pH 8-10
- Ocean Water: pH 8-8.1
- Spring Water: pH 6.5-8.5
- Rainwater: pH 6.5-7.5
- Sparkling Water: pH 4.5-6
These examples illustrate that a wide variety of pH levels are found in nature and in commercial products, emphasizing that a pH of 7 is a rarity outside of controlled laboratory conditions.
Why is Distilled Water Not Always a Perfect 7?
Because maintaining a perfectly neutral pH of 7 in distilled water is extremely difficult. Even storing it in an airtight container for a short period will result in slight absorption of $CO_2$ from the surrounding air during the sealing process. The moment it is opened, the contamination begins. The key to understanding this is recognizing that pure water is naturally unstable and will always seek to reach equilibrium with its environment. The only way to measure a true pH of 7 is to use a special in-line electrode for ultra-pure water, and even then, maintaining it requires significant effort.
pH and Its Impact on Plumbing and Appliances
For most household applications, understanding pH is crucial. Water that is too acidic (below 6.5) can corrode metal pipes, causing heavy metals like lead and copper to leach into the drinking water. Conversely, highly alkaline water (above 8.5) can lead to scale buildup in pipes and appliances, decreasing their efficiency over time. This is why municipal water treatment facilities work to maintain a regulated pH range for public safety and infrastructure protection.
Comparing pH: A Summary
The fundamental difference is chemical composition: tap water contains buffering minerals, while distilled water does not. This contrast dictates their respective pH values and stability, making tap water's pH more resilient to atmospheric influence and distilled water's pH naturally more acidic after air exposure.
Beyond pH: Other Water Quality Indicators
While pH is an important indicator, it is not the sole determinant of water quality. Other factors like total dissolved solids (TDS), hardness, and the presence of specific contaminants also play a significant role. For specialized applications such as laboratories or medical equipment, the high purity and low conductivity of distilled water make it essential, regardless of its mildly acidic pH. For general drinking purposes, tap water's mineral content and regulated pH typically make it a safe and balanced choice.
