Acids, bases and pH — Chemistry, 11–13 years
The pH scale compares how acidic or alkaline a solution is. Indicators make this invisible difference visible through colour.
A scale for solutions
Some solutions are acidic, such as lemon juice, and others are alkaline, such as soapy water. The pH scale describes this difference: lower numbers mean more acidic, higher numbers mean more alkaline, and 7 is neutral at room temperature. Indicators change colour to show the range.
Why test pH?
Taste and touch are unsafe and unreliable ways to compare chemicals. A pH test gives a safer clue about how a solution may react with materials or living things. This matters because soil, swimming pools, medicines and body fluids each need a suitable pH range.
Testing three liquids
Add universal indicator to lemon juice, pure water and soapy water. Lemon juice turns red-orange, suggesting about pH 2; pure water turns green, about pH 7; soapy water turns blue-purple, about pH 10. These are estimates, not exact readings.
pH is not a danger score
It is tempting to think that a lower pH always means a more dangerous substance. The pH tells how acidic or alkaline a solution is, but danger also depends on concentration, amount, temperature and what the substance can do. A dilute acid may be safer than a concentrated one.
Keeping systems balanced
pH is checked in swimming pools so the water is comfortable and disinfectants work properly. Farmers test soil pH because plants absorb nutrients differently in acidic and alkaline soils. Water treatment workers also adjust pH before releasing cleaned water.
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