Acids, bases and pH — Chemistry, 14–17 years
The pH scale describes how acidic or basic an aqueous solution is. It turns a chemical property into a number that can be tested, compared and used safely.
What pH tells us
In water-based solutions, acids increase the amount of hydrogen ions, while bases reduce it or increase hydroxide ions. A pH below 7 is acidic, 7 is neutral at ordinary conditions, and above 7 is basic; the scale is logarithmic, not a simple score.
Why measure acidity
People noticed that substances with a sharp taste or a slippery feel could react differently, but taste and touch are unsafe tests. A numerical scale lets scientists control soil, food, blood and industrial mixtures without relying on guesses or dangerous contact.
Comparing pH 3 and pH 5
Suppose lemon juice has pH 3 and a drink has pH 5. The difference is two pH units. Because each unit means a tenfold change in hydrogen-ion concentration, the lemon juice has 10 × 10 = 100 times more hydrogen ions, so it is much more acidic.
pH is not a normal ruler
A common mistake is to say pH 4 is twice as acidic as pH 2 because 4 is twice 2. That is reasonable if you treat pH like an ordinary scale, but it is logarithmic: pH 2 has 100 times the hydrogen-ion concentration of pH 4.
Keeping systems in range
Swimming pools are tested so disinfectants work and swimmers' skin and eyes are not irritated. Farmers and water-treatment workers also adjust pH because plants, microbes and aquatic life need particular ranges; the correct value depends on the system.
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