A substance's pH is a measure of its acidity. A pH value below 7 implies an acidic substance, while a pH above 7 means the material is alkaline. Water is often thought of as "neutral," which means it has a pH of 7 and is neither acid nor alkaline. However, this is only true for pure water and only at a specific temperature. As temperatures move away from this specific temperature, pH will change, albeit very slightly.
The pH scale measures a substance's acidity. When working with a liquid, chemists use the concentration of hydrogen ions to calculate pH levels. The higher the concentration of hydrogen ions, the lower the pH, and the higher the acidity. Conversely, low concentrations of hydrogen ions mean that the substance has low acidity and a correspondingly higher pH.
Although an absolutely accurate measurement of pH levels requires advanced equipment, a rough measurement can be made with pH test strips. A staple of chemistry labs all around the world, these inexpensive strips are simply dipped into the liquid to gauge its pH level. The color of the strip is then compared to the colors on the test-strip packaging, where each shade is assigned a pH number.
The pH scale varies between 0 and 14, with the value 7 indicating a substance that is neither an acid nor a base; in other words, a neutral substance. Water is a common example of a pH-neutral substance. Pure water, however, is surprisingly hard to find. The water that comes out of the faucet or flows in a clean river will have many impurities and its pH will rarely be 7.
Effect of Temperature
Temperature will have a measurable yet very slight effect on the pH of water. In fact, pure water has a pH of exactly 7 only at 25 degrees Celsius, or 77 degrees Fahrenheit. As the water temperature goes up, pH goes down. The converse is also true: colder water has a higher pH value. At 60 degrees Celsius or 140 degrees Fahrenheit, pure water registers a pH value of 6.96. In other words, the change is very slight and cannot be registered with crude measurement techniques such as pH test strips.
The reason temperature affects water's pH is that water molecules have a slight tendency to break down into their constituents, hydrogen and oxygen, as temperature increases. As temperatures increase, a larger proportion of water molecules break up, releasing a few more hydrogen ions, which then decreases the pH of the water.