In Class 10 Chemistry (Acids, Bases and Salts), to identify whether a solution is acidic, basic, or neutral, we use indicators. The most powerful and informative indicator is the Universal Indicator.
Neutral pH: 7 → Green.
Acid: pH < 7 → Red/Orange/Yellow.
Base: pH > 7 → Blue/Violet.
pH Scale Range: 0 to 14.
Invented By: Robert Boyle first used indicators; the Universal Indicator mixture was developed by J.L. Gay-Lussac's successors.
A Universal Indicator is a mixture of several different chemical indicators that shows a wide range of colours for different pH values (from 1 to 14). Unlike litmus (which only shows red or blue), the Universal Indicator gives a unique, specific colour for each pH value.
| pH Value | Colour | Nature |
|---|---|---|
| 1–3 | Red / Dark Red | Strong Acid |
| 4–6 | Orange / Yellow | Weak Acid |
| 7 | Green | Neutral |
| 8–10 | Blue / Cyan | Weak Base |
| 11–14 | Violet / Dark Blue | Strong Base |
Memory Trick: Red → Orange → Yellow → Green (neutral) → Blue → Violet (just like ROYGBIV of the rainbow, but reversed intensity for bases).
The Universal Indicator is a solution containing a mixture of indicators like phenolphthalein, methyl red, bromothymol blue, and thymol blue. Each component changes colour at a different pH, and the combined result gives the specific colour for each pH level.
A Universal Indicator is a mixture of chemical dyes that shows a wide range of distinct colours corresponding to different pH values from 1 to 14.
In a strong acid (pH 1-3), it turns red. In a weak acid (pH 4-6), it turns orange or yellow.
At neutral pH 7, the Universal Indicator turns green.
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