Question

What is meant by a universal indicator? State the colours given by a universal indicator to the solutions of following \(\mathrm{pH}\) values. (i) \(\mathrm{pH}=1\) (ii) \(\mathrm{pH}=8\) (iii) \(\mathrm{pH}=5\) (iv) \(\mathrm{pH}=12\)

Short answer

Answer: A universal indicator shows the color red for a pH level of 1, green for a pH level of 8, yellow for a pH level of 5, and violet or blue for a pH level of 12.

Step-by-step solution

Definition of a Universal Indicator

A universal indicator is a mixture of different pH indicators that displays a range of colors across the pH scale from 0 to 14. The colors change as the acidity or basicity of a solution changes, which helps to indicate the pH level of the solution.

Color at pH = 1

At pH = 1, the solution is strongly acidic. A universal indicator shows the color red in such a solution.

Color at pH = 8

At pH = 8, the solution is slightly basic. A universal indicator shows the color green in a solution with this pH level.

Color at pH = 5

At pH = 5, the solution is somewhat acidic. A universal indicator shows the color yellow in a solution with this pH level.

Color at pH = 12

At pH = 12, the solution is strongly basic. A universal indicator shows the color violet or blue in a solution with this pH level.

Key concepts

pH scale

The pH scale is a numeric scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14.
The scale is logarithmic, which means each whole number on the scale represents a tenfold change in acidity or basicity.

• A solution with a pH less than 7 is acidic.
• A solution with a pH of 7 is considered neutral, like pure water.
• A solution with a pH greater than 7 is basic.

Understanding the pH scale is critical in chemistry since it helps determine how substances will interact.

acidic solutions

An acidic solution is one that has a lower pH value, typically less than 7. These solutions contain a higher concentration of hydrogen ions (\( ext{H}^+\)), which contribute to their acidity.
Common examples of acidic solutions include lemon juice and vinegar. Here are some characteristics of acidic solutions:

• They taste sour.
• They can turn blue litmus paper red.
• They react with metals to produce hydrogen gas.

It is the prevalence of hydrogen ions that imparts these properties to acidic solutions.

basic solutions

Basic solutions, also called alkaline solutions, have a pH greater than 7. They contain a higher concentration of hydroxide ions (\( ext{OH}^-\)), which influence their basic nature.
Typical examples are baking soda and soap solutions. Basic solutions have distinct properties:

• They feel slippery to touch.
• They taste bitter.
• They can turn red litmus paper blue.

Recognizing basic solutions is important since they can counteract acids in neutralization reactions.

color change in indicators

Universal indicators and other pH indicators show a distinct color change in response to different pH levels. These color changes are very useful for identifying the acidity or basicity of a solution.
The principle behind this is the indicator's chemical structure, which changes depending on the pH of the solution it's in.

• For a pH of 1, the indicator typically turns red, indicating strong acidity.
• At pH levels around 5, it turns yellow, showing mild acidity.
• Neutral solutions with pH around 7 appear green.
• For a basic pH of 8, it might also show as green, indicating slight alkalinity.
• At pH 12, indicative of strong basicity, the color shifts to violet or blue.

These color transitions provide a visual cue for determining a solution's pH value effectively.