Indicators – Acids and Bases

Key Concepts

• Indicators
• Natural Indicators
• Synthetic Indicators
• Olfactory Indicators
• Action of Indicators on Acids and Bases

Introduction: Indicators – Acids and Bases

By now, we know that substances like tomatoes, lemon, pineapples taste sour because they contain acids. In contrast, substances like detergents and soaps feel bitter and slippery because they contain the substance known as base. 

How do we find it acidic or basic if we cannot taste every substance? Special types of substances are used to determine whether a given substance is acid or base. These substances are known as indicators. Indicators change colors when they contact an acid or a base. 

Let us learn more about these indicators, types and how they are used to identify the nature of substances. 

INDICATORS 

Definition: Indicators are the substances used to determine whether an aqueous solution is acidic, neutral, or basic. 

There are three types of indicators. 

NATURAL INDICATORS AROUND US 

1. Natural indicators are generally found in nature. 

2. They are mostly derived from plants. 

3. Examples : Litmus, China rose, red cabbage, turmeric, beetroot are all examples of natural indicators around us. 

1.  LITMUS AS AN INDICATOR 

1. The most commonly used indicator is litmus. 

2. It is extracted from a plant known as lichen(belonging to the thallophyte family) 

3. Litmus is purple in color in neutral solution. 

4. There are two types of litmus paper used. 

a.  Blue Litmus Paper 

Action of Acid: When blue litmus paper is dipped inside the acid solution, it changes color from blue to red. 

Action of Base: When blue litmus paper is dipped inside the base solution, it remains blue only. (colorless) 

b.  Blue Litmus Paper 

Action of Acid: It has no effect when red litmus paper is dipped inside the acid solution. 

Examples of acids: HCl, H2SO₄, citric acid 

Action of Base: When dipped in basic solution (like NaOH, KOH), it turns red to blue. 

Litmus Test 

2.  TURMERIC AS AN INDICATOR 

Turmeric is another good natural indicator. 

Turmeric is bright yellow color and is obtained from turmeric plants. 

However, when mixed with a basic solution, it turns red in color. 

Action of turmeric on Acids: It remains yellow only. No change in color.   

Action of turmeric on Bases: Turmeric turns red when in contact with basic solutions. 

On Salts: No change in color. 

Sometimes, when you have stains of curry on your clothes, applying soap turns out too red. Have you ever wondered why it happens? 

It is because of turmeric. Turmeric is a natural indicator, as soap is basic in nature, so the action of turmeric changes the color of stain into red. 

However, when it is added to lemon juice or vinegar, the color remains the same. The turmeric paper is not effective to identify the acidic solution. However, it is very effective to identify the basic solution. 

3.  CHINA ROSE AS AN INDICATOR 

1. China rose indicator is a solution that is pink color. 

2. It is extracted from the red flower of the hibiscus plant (China rose) and mixed with water. 

Action on Acids: 

 when it is added to acid, the color changes to magenta (deep pink). 

Action on Base: 

When added to a solution of the base (e.g., baking soda), the color of the indicator changes from light pink to green. 

(4) RED CABBAGE AS AN INDICATOR 

1. Red cabbage was originally purple in color. 

2. However, it changes the color when added to acid or basic solution. 

3. The pigment molecule flavin or anthocyanin present in the cabbage is responsible for changes in color. This pigment molecule changes color when an acid or base is added to it. 

Action on Acid:  

When red cabbage solution is mixed with acid solution, the color changes from violet to red. 

Action of Base: 

When base is added to red cabbage solution , the solution turns blue-green  in color 

Natural Indicators With Their Characteristic’s Colors 

SYNTHETIC INDICATORS 

1. Synthetic indicators are those indicators that are made from artificial substances. 

2. They were synthesized in the lab by chemical reactions. 

3. Some commonly used synthetic indicators are Phenolphthalein and methyl orange. 

1. Phenolphthalein as a Synthetic Indicator 

1. Phenolphthalein is colorless solution. 

2. In an acidic medium, it does not change color. 

3. When added to basic medium, the color of solution changes to pink. 

4. It also does not change the color when salt solution is added. 

Phenolphthalein:  

2. Methyl Orange as a Synthetic Indicator 

1. Methyl orange is another most common synthetic indicator. 

2. It is orange in color. 

3. In acids: When added to acid, it turns red-orange. 

4. In bases: It turns yellow in the basic medium. 

5. Salt: It turns out to be a yellow-coloured solution. 

Olfactory Indicators 

Olfactory indicators are those indicators that change their smell in the acidic or basic solution. 

For example, an onion acts as an olfactory indicator and has a sharp smell. 

Some common Olfactory Indicators 

Summary

• Indicators are the substances used to distinguish between acid and bases. They change colors when mixed with acids or bases.
• Indicators are classified into three types:
• (a) Natural Indicator.
• (b) Synthetic Indicators.
• (c) Olfactory Indicators
• Natural Indicators: Obtained from natural sources.
• Synthetic Indicators: These are prepared in the lab.
• Olfactory Indicators: These indicators change their smell in the acidic or basic solution.