Acids, alkalis and pH — Science, 11–13 years
Some liquids can be corrosive, while others can neutralise them. The pH scale gives a way to compare how acidic or alkaline a solution is, and indicators make this invisible difference visible through colour.
Idea
Acids have a pH below 7, neutral substances have pH 7, and alkalis have a pH above 7. The farther a value is from 7, the more strongly acidic or alkaline the solution usually is. An indicator changes colour across this scale, so it can reveal a property that our eyes cannot see directly.
Why measure pH?
Taste is unsafe and unreliable for comparing chemicals, especially in a laboratory or a swimming pool. Scientists needed a safer, shared scale to describe how solutions behave. The pH scale solves that problem, while neutralisation explains what happens when an acid and an alkali react: their properties become less extreme.
Worked example
Universal indicator turns orange in a liquid. Step 1: compare the colour with its chart. Orange matches about pH 4. Step 2: place 4 below 7, so the liquid is acidic. Step 3: compare pH 4 with pH 6: pH 4 is farther from neutral, so it is usually the stronger acid. Never identify an unknown liquid by tasting it.
A common trap
Many people think pH 2 is only twice as acidic as pH 4 because 4 is twice 2. That is a reasonable first guess if pH is treated like an ordinary ruler. But the pH scale is logarithmic: each step represents about ten times the acidity, so pH 2 is about one hundred times more acidic than pH 4.
Outside school
pH matters in swimming pools, soil, farming, cleaning products and the human body. Pool workers adjust pH so water is safer for skin and equipment. Gardeners test soil because different plants prefer different pH ranges. Medicines for indigestion can neutralise some stomach acid, but they must be used as directed.
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