Scientists Say: pH

This is a measurement of how acidic or basic a substance is

This is a pH scale showing popular items that are acidic (lemons and tomatoes) or basic (soap or bleach).

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pH (noun, “P. H.”)

This is a measure of how acidic or basic a substance is. An acid is a compound that adds positively charged hydrogen ions (H+) to a liquid. A base adds negatively charged ions called hydroxides (OH-).

The pH scale runs from zero to 14. A low pH, close to zero, is very acidic, with many free H+ ions. Lemon juice, with a pH of around two, is a good example. A high pH, close to 14, is very basic, with many free OH-ions. Basic solutions are as known as alkaline. Bleach is a very basic solution. A pH around seven is neutral. Pure water has an equal amount of hydrogen and hydroxide ions, making the pH perfectly balanced. That’s why pure water has a neutral pH of seven.

pH is an important measure. It’s a measure of water quality. Doctors use pH to help diagnose medical conditions such as kidney problems. Farmers and gardeners measure the pH of soil to find out which plants might grow best.

The pH scale is logarithmic. That means that for every step down the scale, the amount of hydrogen ions increases by a factor of 10. So a liquid with a pH of four has 10 times as many hydrogen ions as a liquid with a pH of five. In contrast, every step up the scale is equal to decrease in hydrogen ions (and an increase in hydroxide ions) by a factor of 10.

In a sentence

Carbon dioxide from climate change is acidifying oceans — from a pH of 8.2 to a pH of 8.1.

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Bethany is the staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology from Wake Forest University School of Medicine.

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