Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Indicators are special substances used to test whether a substance is acidic or basic. They change their color when added to a solution containing an acidic or a basic substance.
Turmeric is a natural indicator obtained from the plant Curcuma longa.
In an acidic medium, turmeric remains yellow (no change in color).
In a neutral medium, turmeric remains yellow (no change in color).
In a basic medium, turmeric changes its color from yellow to reddish-brown.
Turmeric paper can be prepared by applying turmeric paste on blotting paper/filter paper and drying it.
A common observation is that a turmeric stain on a white shirt turns red when washed with soap, because soap solutions are basic in nature: .
📐Formulae
💡Examples
Problem 1:
A student puts a drop of sodium hydroxide () solution on a strip of turmeric paper. What color change will be observed?
Solution:
The turmeric paper will turn reddish-brown.
Explanation:
Sodium hydroxide () is a strong base. Since turmeric indicator turns reddish-brown in the presence of a basic solution, the yellow paper changes its color.
Problem 2:
What happens when dilute hydrochloric acid () is added to a turmeric indicator?
Solution:
There is no change in color; the indicator remains yellow.
Explanation:
Turmeric does not change its color in acidic solutions. Since is an acid, the yellow color of turmeric is maintained.
Problem 3:
A turmeric stain on a white cloth turns red when soap is applied. If we then pour lemon juice over this red stain, what will happen?
Solution:
The red stain will turn back to yellow.
Explanation:
Soap is basic, which turns the turmeric red. Lemon juice contains citric acid (). The acid neutralizes the base and makes the medium acidic/neutral again, causing the turmeric to return to its original yellow color.