Definition
The pH scale is a measure that uses a number to show how much hydrogen ion (H+) is dissolved in a liquid. Using numbers from 0 to 14, it lets you tell at a glance whether a solution is acidic, neutral, or basic (alkaline).
Between lemon and soap, split at 7
Touch lemon juice to your tongue and it is sour enough to sting. Get soapy water on your hands and they turn slippery. The liquids we meet every day each have completely different personalities. Chemists made the pH scale so they could line up these different liquids on a single ruler and compare them.
The exact middle of this scale, and its reference point, is the number 7 (neutral). Clean, pure water with no impurities sits right at pH 7. The further the number drops below 7, the stronger the sharp, sour acidity. The further it rises above 7, the stronger the bitter, slippery basicity.
Stomach acid in our stomachs is pH 1.5 to 3.5, and lemon juice is around pH 2, both strongly acidic. The fizzy drinks we sip every day are about pH 2 to 3. At the other end, laundry detergent is pH 9 to 10, and drain cleaner that dissolves stubborn grime is around pH 13, strongly basic.
Change the number by just 1, and it is 10 times different
There is one thing to be most careful about when reading the pH scale. When the pH number moves one step, from 1 to 2 or from 5 to 6, the amount of hydrogen ion does not change by just 1. With every one-step difference in pH, the concentration changes by a full 10 times.
For example, compared with black coffee at pH 5, tomato juice at pH 4 has 10 times as many of the hydrogen ions that make things acidic. Compare vinegar at pH 3 with coffee at pH 5, and the gap is two steps, which is 10 squared, or 100 times.
On paper it looks like a small difference of 1 or 2, but the environment inside the liquid is wildly different. Our bodies are very sensitive to this scale too. Blood has to stay in the very narrow range of pH 7.35 to 7.45, and a small slip outside it causes problems in the body.
A closer look: why does a smaller number mean stronger?
Most numbers we use in daily life get bigger as the amount gets bigger. So why does the pH number get smaller when there is more hydrogen ion? To be more precise, it is because the pH formula contains a negative logarithm (a minus sign).
Write the hydrogen ion concentration of a solution in moles, and it becomes far too small a value for everyday numbers. In one liter of neutral water, the hydrogen ion is 0.0000001 mole, with a long string of zeros after the decimal point. Writing seven zeros every time you talk about it was a real hassle.
So scientists adopted a mathematical rule: turn the number of zeros into a simple whole number, and put a minus sign in front to get rid of the negative. As a result, the fewer zeros after the decimal point, meaning the higher and stronger the hydrogen ion concentration, the smaller the pH number becomes, like 1 or 2.
Common misconceptions
pH can only be a number between 0 and 14, no matter what.
In the usual lab setting with water-based solutions, we mostly work in the 0 to 14 range. But a superacid packed with hydrogen ions can go below 0, to a negative number like -1, and a superbase can go above 14.
Where you meet it
A mildly acidic cleanser is made to match the mild acidity of the skin's surface, roughly pH 4.5 to 5.5.
People check the pH of a swimming pool or a fish tank regularly to keep the acidity comfortable for living things.
In one sentence
pH is a scale for hydrogen ion concentration. Below 7 (neutral) is acidic, above 7 is basic, and each step makes a 10-times difference.