Chapter 2 · Question 1

What are acid-base indicators? Give examples of natural and synthetic indicators, and explain how an olfactory indicator works with the help of an activity.

Answer Revealed

Direct Answer:

Acid-base indicators are dyes or mixtures of dyes that show different colours in acidic and basic media, helping identify the nature of a solution. Natural indicators include litmus (a purple dye from lichen) and turmeric (which turns reddish-brown in a base). Synthetic indicators include methyl orange and phenolphthalein. Olfactory indicators are substances whose odour changes in acidic or basic media. For example, NCERT tests cloth strips soaked with onion extract using dilute HCl and dilute NaOH, then compares the odour after rinsing. Vanilla essence and clove oil are similarly tested by adding them separately to dilute HCl and dilute NaOH and observing whether their smell changes.

Simple Explanation

Indicators are like chemical detectives: they change colour or smell to tell you if something is acidic or basic. Litmus paper turns red in acid and blue in base. Turmeric (haldi) stain turns reddish-brown when soap (a base) touches it. Methyl orange and phenolphthalein are lab-made indicators that show specific colour changes. Some natural substances like onion, vanilla, and clove oil are called olfactory indicators because their smell changes in acidic or basic solutions.

Exam-Ready Structure

Acid-base indicators are substances that exhibit a characteristic colour change depending on whether they are in an acidic or basic medium, and they form the foundational tool for identifying solutions before more quantitative methods are employed. 1. Definition: Indicators are substances that show different colours in acidic and basic solutions, and they include both naturally occurring and synthetic dyes. 2. Natural indicators: (a) Litmus — a purple dye extracted from lichen (a Thallophyta plant). It turns red in acidic solutions and blue in basic solutions. When the solution is neutral, litmus remains purple. (b) Turmeric — turns reddish-brown when exposed to a base. The classic example: a turmeric (curry) stain on a white cloth turns reddish-brown when washed with soap (basic), and turns yellow again after rinsing with plenty of water. (c) Other natural sources: red cabbage leaves, Hydrangea flowers, Petunia petals, and Geranium flowers can also act as indicators. 3. Synthetic indicators: These are prepared in laboratories and include (a) methyl orange and (b) phenolphthalein; phenolphthalein turns pink in basic solutions and remains colourless in acidic and neutral solutions. 4. Olfactory indicators: These are substances whose characteristic odour changes in acidic or basic media. They are established through Activity 2.2: (a) Onion extract — cloth strips are kept with finely chopped onions overnight, then treated separately with dilute HCl and dilute NaOH, rinsed, and checked for odour changes. (b) Vanilla essence and clove oil are tested by adding them separately to dilute HCl and dilute NaOH and recording whether their odour changes. 5. The importance of indicators extends beyond the laboratory into everyday life. For instance, the turmeric stain test is a simple household method to check whether a cleaning substance is basic.

Key Points

Acid-base indicators are substances that show different colours in acidic and basic media
Natural indicators: litmus (purple dye from lichen), turmeric (turns reddish-brown in base)
Synthetic indicators: methyl orange, phenolphthalein (lab-made dyes)
Olfactory indicators: substances whose odour changes in acid/base (onion, vanilla, clove oil)
Litmus: red in acid, blue in base, purple when neutral
Red cabbage, Hydrangea, Petunia, and Geranium petals can also act as natural indicators

What are acid-base indicators? Give examples of natural and synthetic indicators, and explain how an olfactory indicator works with the help of an activity.