Two commonly used synthetic indicators in Chemistry are: 1. Phenolphthalein and 2. Methyl Orange. Indicators are substances that change colour in acid or base solutions, helping us determine whether a substance is acidic, basic, or neutral. Synthetic indicators are laboratory-made chemicals, unlike natural indicators such as litmus or turmeric.
Two synthetic indicators: Phenolphthalein and Methyl Orange.
Phenolphthalein: colourless in acid, pink/magenta in base.
Methyl Orange: red in acid, yellow in base, orange in neutral.
Synthetic indicators are made in labs; natural indicators come from plants (litmus, turmeric).
Phenolphthalein: best for weak acid–strong base titrations.
Methyl Orange: best for strong acid–weak base titrations.
Definition: An indicator is a dye that changes colour when added to an acid or base. Synthetic indicators are chemically synthesised in laboratories.
Two Synthetic Indicators:
Phenolphthalein: • Colourless in acidic solution • Pink/Magenta in basic (alkaline) solution • Colourless in neutral solution • Used in: titration of weak acids against strong bases • pH range of colour change: 8.2 to 10.0
Methyl Orange: • Red in acidic solution • Yellow/Orange in basic solution • Orange in neutral solution • Used in: titration of strong acids against weak bases • pH range of colour change: 3.1 to 4.4
Comparison Table:
| Feature | Phenolphthalein | Methyl Orange |
|---|---|---|
| Acidic solution | Colourless | Red |
| Basic solution | Pink/Magenta | Yellow |
| Neutral | Colourless | Orange |
| pH range | 8.2–10.0 | 3.1–4.4 |
| Best used for | Weak acid–strong base titration | Strong acid–weak base titration |
Natural vs Synthetic Indicators:
| Type | Examples |
|---|---|
| Natural | Litmus (lichens), turmeric, red cabbage, China rose |
| Synthetic | Phenolphthalein, methyl orange, bromothymol blue |
Olfactory Indicators: • Onion, clove oil, vanilla — change smell (not colour) in acid/base
Litmus Paper: • Red litmus turns blue in base • Blue litmus turns red in acid • Litmus is a natural indicator (from lichens)
NCERT Reference: Class 10 Science — Chapter 2 (Acids, Bases and Salts)
Two synthetic indicators are: 1. Phenolphthalein — colourless in acidic solution, pink/magenta in basic solution. 2. Methyl Orange — red in acidic solution, yellow in basic solution, orange in neutral. Both are used in acid-base titrations. They are synthetic (lab-made), unlike natural indicators such as litmus or turmeric.
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