pH and Indicators

Definition

The pH scale measures how acidic or basic a solution is, and indicators are substances that change colour to reveal that acidity or basicity.

Key points

• The pH scale runs from 0 to 14: below 7 is acidic, exactly 7 is neutral (pure water), and above 7 is basic (alkaline); each whole-number step represents a tenfold change in acidity.
• Litmus turns red in acid and blue in base; methyl orange turns red or pink in acid and yellow in base; phenolphthalein is colourless in acid and pink in base.
• Universal indicator gives a range of colours mapping to the whole pH scale, allowing an approximate numerical reading.
• Natural indicators include litmus (from lichens) and the pigment in red cabbage and turmeric (which turns red in base); turmeric and china rose are common natural indicators.
• pH matters in daily life: blood is slightly basic (about 7.4), soil pH affects crops, and acidic stomach contents are neutralised by antacids (mild bases).

Why it matters for CAPF

The 0 to 14 scale with 7 as neutral, the colour changes of litmus, methyl orange, and phenolphthalein, and natural indicators are standard chemistry facts, and pH links to soil, blood, and pollution questions.

Common confusion

A higher pH means less acidic (more basic), so do not read high pH as strong acid; pH 7 is neutral, not the midpoint of acidity in a simple sense. Phenolphthalein is colourless in acid and pink in base, which is the reverse of how some assume an indicator behaves.

Related concepts

concept ph scale, concept acids bases and salts, concept catalysts

Parent note

chemistry everyday