Class 7 , Subject : Science Chapter 2

 

Acid

An acid is a substance that has a sour taste and turns blue litmus paper red.

Examples:

  • Hydrochloric acid
  • Sulfuric acid
  • Citric acid

Base

A base is a substance that has a bitter taste and feels soapy or slippery, and turns red litmus paper blue.

Examples:

  • Sodium hydroxide
  • Calcium hydroxide

Extra (Important Point)

  • Bases that are soluble in water are called alkalis.

Indicator :-

An indicator is a substance that changes its color to show whether a substance is an acid or a base.

Examples of Indicators:

  • Litmus → turns red in acid and blue in base
  • Turmeric → turns reddish-brown in base
  • Phenolphthaleincolorless in acid and pink in base
  • Rose → In acid  dark pink / red and Green in Base

 Olfactory Indicators (Smell-based) 

These indicators change their smell in acids and bases.

Examples:

  • Onion
  • Vanilla essence
  • Clove oil

Behavior:

  • In acid → smell remains same
  • In base → smell disappears or changes

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