The pH scale is a scale from 0 to 14 that measures how acidic or alkaline a solution is, based on the concentration of hydrogen ions.
Malay: Skala pH · Chinese: pH值
The pH scale is a scale running from 0 to 14 that measures how acidic or alkaline a solution is, based on the concentration of hydrogen ions, H⁺, in it. A pH below 7 is acidic, a pH of exactly 7 is neutral, and a pH above 7 is alkaline. The higher the concentration of hydrogen ions, the lower the pH; the higher the concentration of hydroxide ions, the higher the pH.
A worked example anchors the scale. Dilute hydrochloric acid, rich in H⁺, sits near pH 1. Pure water, where H⁺ and OH⁻ are equal, sits at pH 7. Sodium hydroxide solution, rich in OH⁻, sits near pH 14. You can read an approximate pH with universal indicator, which shows a colour for each region, red for strongly acidic through green at neutral to purple for strongly alkaline, or measure it precisely with a pH meter.
The confusion to avoid is thinking pH measures the strength of an acid. pH responds to the concentration of hydrogen ions actually present, not to whether the acid is strong or weak by nature. Diluting a strong acid raises its pH toward 7 even though it stays a strong acid, and a concentrated weak acid can share a pH with a dilute strong acid. A second common slip is direction: lower pH means more acidic, so students who assume “bigger number, more acidic” get it backwards.
Within this chapter the pH scale turns the acid and alkali definitions into a measurable quantity, and it is the tool used to follow a neutralisation. Our teachers link each pH value back to the hydrogen ion concentration behind it, so a number on the scale always means something you can explain.
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