Acids, Bases and Salts (Indicators) : Science CBSE Notes

Many substances that you encounter in your daily life have different properties. One major difference between substances is whether they are acidic or basic. 

Indicators, NCERT Notes, CBSE Notes, CTET Exam Notes

  • Acidic substances taste sour. Some examples are curd, lemon juice, tamarind, spinach and vinegar. These contain natural acids.  
  • Basic substances taste bitter and feel slippery to the touch. Some examples are baking soda and milk of magnesia.

Natural indicators:

There are special substances that are used to test whether something is acidic or basic. These substances are called indicators. Indicators change their colour when added to an acidic or basic solution. Some natural indicators are litmus, turmeric and china rose petals. 
  • Litmus is a natural dye extracted from lichens. It is the most commonly used natural indicator.A solution of litmus turns red if it is acidic, and blue if it is basic.
Litmus has a purple colour in a neutral medium, red colour in an acidic medium, and blue in a basic medium.When a drop of freshly prepared lemon juice is put on red litmus, it remains red, indicating the acidic nature of lemon juice. 
If blue litmus is used instead, then it turns red, again indicating the acidic nature of lemon juice.
  • Turmeric changes colour to indicate acidic and basic substances. It is yellow in acids and reddish brown in basic solutions.
Turmeric paper changes colour in a basic medium.
In an acidic medium, it remains yellow, while in a basic medium, it turns reddish brown.
When a drop of soap solution is put on turmeric paper, it turns red. This indicates the basic nature of soap solution.
S. NoTest solutionEffect on turmeric paper
1Lemon juiceYellow
2Orange juiceYellow
3Lime waterReddish brown
4Milk of magnesiaReddish brown
S. NoName of acid /baseEffect on litmus paperEffect on turmeric paper
1Hydrochloric acidBlue litmus turns redRemains yellow
2Sulphuric acidBlue litmus turns redRemains yellow
3Sodium hydroxideRed litmus turns blueTurns reddish brown
4Potassium hydroxideRed litmus turns blueTurns reddish brown
  • China rose is also a natural indicator. A solution of china rose turns green in a basic solution, and bright pink or magenta in an acidic solution.
China rose is another indicator extracted from China rose petals. In an acidic medium, it exhibits dark pink (magenta), while in a basic medium, it exhibits green colour.
S. NoName of the acidEffect on China rose
1Hydrochloric acidDark red
2Sulphuric acidDark red
3Sodium hydroxideGreen
4Potassium hydroxideGreen

Acid rain:

Acid rain is rain that contains a lot of acid. This acid is created when pollutants in the air, such as carbon dioxide, sulphur dioxide and nitrogen dioxide, dissolve in rainwater, and form acids like carbonic acidsulphuric acid and nitric acid
This rain causes a lot of damage to buildings, plants and animals. This is one important reason why we must avoid polluting our atmosphere.

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