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To check if a substance is an acid or an alkali (base), you can use litmus paper or a pH indicator. Litmus paper turns red in acidic solutions and blue in alkaline solutions. pH indicators, like universal indicator, provide a range of colors to indicate the strength of acidity or alkalinity on the pH scales.

Detailed Explanation:

1. Litmus Paper Test:

   • Blue litmus paper: Turns red in acidic solutions. 

   • Red litmus paper: Turns blue in alkaline solutions. 

   • Neutral: Neither litmus paper changes

     
     

   2. pH Indicator (Universal Indicator):

   • A pH indicator, like universal indicator, can be used in liquid form or as strips of paper. 

   • It changes color based on the pH of the solution. 

   • Acidic: Solutions with a pH below 7 will turn the indicator red, orange, or yellow. 

   • Alkaline: Solutions with a pH above 7 will turn the indicator blue or purple. 

   • Neutral: Solutions with a pH of 7 (like pure water) will turn the indicator green. 

     
     

Key Concepts:

• Acids:

  Substances that release hydrogen ions (H+) when dissolved in water. They have a pH below 7. 

  
  

• Alkalis (Bases):

  Substances that release hydroxide ions (OH-) when dissolved in water. They have a pH above 7. 

• pH Scale:

  A scale from 0 to 14 that measures the acidity or alkalinity of a solution. 

  • 0-6: Acidic. 

  • 7: Neutral. 

  • 8-14: Alkaline (Basic).