Answer is: using acid-base indicator or pH indicators.
Acid-base indicators are usually weak acids or bases and
they are chemical detectors for hydrogen or hydronium cations.
Example for acid-base indicator is
phenolphthalein (molecular formula C₂₀H₁₄O₄). Phenolphthalein is colorless in acidic solutions and pink in basic solutions.
Another example is methyl orange. It is red colour in acidic solutions and
yellow colour in basic solutions.
Acid-base reactions is a type of chemical process in which acids and bases react to form salt and water. The reaction between sodium hydroxide and vinegar is a type of acid-base reaction.
The acids and bases are sensitive to indicators. The impart color, change color, or undergoes a reaction to testify their acidic or alkaline properties.
In the absence of thermometer in a classroom laboratory, the acid-base reaction can be testified using indicators such as phenolphthalein or methyl orange.
Therefore, the indicators such as methyl orange can be used to observe the acid-base reactions.
To know more about acid-base reaction, refer tot he following link:
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