Grade 9 → Acids, Bases and Salts ↓
Indicators and their uses
In the world of chemistry, it is fundamental to understand the nature of substances such as acids, bases, and salts. One of the most important tools for identifying these substances is the use of indicators. Indicators are special chemicals that show different colors when exposed to acids or bases. In this lesson, we will go deep into the world of indicators, finding out what they are, how they work, and what their wide applications are.
What are indicators?
Indicators are substances that change color when exposed to an acidic or alkaline environment. They help us determine whether a substance is acidic, alkaline, or neutral. In simple terms, when you add an indicator to a solution, it will show a specific color corresponding to the pH level of that solution. The term "pH" stands for "potential of hydrogen" and is a measure of how acidic or alkaline a solution is.
pH = -log[H⁺]The pH scale ranges from 0 to 14:
- 0-6: Acidic solution
- 7: Neutral Solution
- 8-14: Basic Solutions
Types of indicators
There are many types of indicators, and they can be classified based on their origin or the range of pH in which they work. Let's take a look at some commonly used indicators:
1. Litmus
Litmus is one of the oldest and widely used indicators. It is a natural dye obtained from lichens. Litmus is available in strips of paper, known as litmus paper, or as a solution. There are two types of it:
- Red litmus: Turns blue in alkaline solutions.
- Blue litmus: Turns red in acidic solution.
If you put a strip of blue litmus paper in vinegar, which is an acidic substance, the paper will turn red, indicating acidity.
2. Phenolphthalein
Phenolphthalein is a synthetic indicator that remains colourless in acidic and neutral solutions but turns pink in alkaline solutions. This change occurs around a pH of 8.3 to 10.
In Acidic Solutions: Colorless In Basic Solutions: Pink3. Methyl orange
Methyl orange is another synthetic indicator used to determine the pH of a solution. It shows a distinct color change from red in acidic solutions to yellow in alkaline solutions, with the transition occurring in the pH range between 3.1 and 4.4.
In Acidic Solutions: Red In Basic Solutions: Yellow4. Universal indicator
Universal indicator is a mixture of different indicators that provides a full spectrum of color changes allowing more accurate determination of pH. It shows different colors on the pH scale:
- Red: highly acidic (pH 0-3)
- Orange/yellow: slightly acidic (pH 3-6)
- Green: Neutral (pH 7)
- Blue: slightly alkaline (pH 8-11)
- Purple: highly alkaline (pH 12-14)
Visual example with SVG
Below is a visual representation of how the universal indicator changes color along the pH scale:
How do indicators work?
Indicators work based on the principle of ionization and change in molecular structure. When an indicator is added to a solution, it reacts with hydrogen ions [H⁺] or hydroxide ions [OH⁻] present in the solution. For example, in an acidic solution, there are more [H⁺] ions, so the indicator shows one colour. In an alkaline solution, there are more [OH⁻] ions, and the indicator shows another colour.
Use of indicators
Indicators have many uses in laboratories and real-world applications. Here are some of the main uses:
1. Determination of the nature of a substance
The primary use of indicators is to identify whether a substance is acidic or alkaline. This is especially useful in laboratories before performing an experiment. For example, when a student is given an unknown solution, adding a few drops of phenolphthalein to it can quickly determine if it is acidic or not by observing the color change.
2. Titration
Indicators play a vital role in the titration process, which is a technique used to determine the concentration of an unknown solution. During titration, an indicator helps to identify the end point when the acid neutralizes the base or vice versa.
For example, when titrating hydrochloric acid (HCl) with sodium hydroxide (NaOH), the addition of phenolphthalein can help detect the exact moment when neutralization occurs, as it changes from colorless to pink.
3. Soil pH testing
Indicators are used to test soil pH in agriculture. The pH level of soil is important for plant growth. Some plants prefer acidic soil, while others grow well in alkaline soil. Universal indicator solutions can be used to determine the pH level of soil samples.
4. Aquatic ecosystem
In ecology, indicators are used to monitor the health of aquatic ecosystems. The pH level of water bodies is essential for the survival of aquatic plants and animals. Indicators help monitor changes in water pH that affect the overall ecosystem.
5. Food industry
In the food industry, indicators can be used for quality control purposes. They help ensure that products such as wine and cheese are at the proper acidity level for consumption. By monitoring pH levels, producers can maintain consistent quality and taste.
Natural indicators
In addition to synthetic indicators, there are also natural indicators that can be found in everyday things. Some natural indicators include:
1. Red cabbage
Red cabbage contains a pigment called anthocyanin, which changes colour in response to changes in pH. When boiled, cabbage juice turns red in acidic solutions and green/yellow in alkaline solutions, making it an excellent natural indicator.
2. Turmeric
Turmeric is a bright yellow spice that acts as a natural indicator. It turns red in alkaline solution while it remains yellow in acidic or neutral solution.
3. Beetroot
Like red cabbage, beetroot can also be used as a natural pH indicator. Its juice changes color as the pH level changes, from pinkish-red in acidic solutions to yellowish-brown in alkaline solutions.
Conclusion
Indicators play a vital role in our understanding of chemical substances. They provide a simple yet effective way to identify whether a solution is acidic, neutral or alkaline by changing colour. Apart from their use in the laboratory, indicators are also essential in various industries, agriculture, environmental studies and even in our daily lives through natural indicators. With the help of indicators, we can make more informed decisions and understand the chemical nature of the world around us.
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