Colour of RED litmus paper turns ________ when react with base.

The correct answer is 1) blue .

Key Points

• Red litmus paper is used as an indicator to test the acidity or basicity of a solution.
• When red litmus paper comes in contact with a basic solution, it changes color to blue.
• This color change occurs because bases reduce the concentration of hydrogen ions (H₊) in the solution, which affects the dye in the litmus paper.
• Examples of common bases include sodium hydroxide (NaOH) and ammonia (NH₃).

Additional Information

• Litmus Paper:
  • Litmus paper is made from wood cellulose infused with litmus, a water-soluble mixture of different dyes derived from lichens.
  • It is commonly used to test for the presence of acids and bases.
• pH Scale:
  • The pH scale ranges from 0 to 14 and measures how acidic or basic a substance is.
  • A pH less than 7 indicates an acidic substance, whereas a pH greater than 7 indicates a basic substance.
• Neutral Substances:
  • Substances with a pH of 7, such as pure water, are considered neutral.
  • Neutral substances do not change the color of red or blue litmus paper.
• Common Indicators:
  • Besides litmus paper, other indicators like phenolphthalein and methyl orange are also used to determine pH.
  • Phenolphthalein turns pink in basic solutions, while methyl orange turns yellow in basic solutions.