Notes On Indicators - CBSE Class 7 Science
Substances can be classified as acidic, basic or neutral. Acidic Substances (Acids) The word acid is derived from the Latin word acere meaning sour. Substances that taste sour contain acids. The chemical nature of such substances is acidic. Example: Curd, lemon juice, tamarind, spinach and vinegar. Properties of Acids      •  Acids have corrosive action on skin.      •  Acids are good conductors of electricity.      •  Acids neutralize bases to form salt and water. Classification of Acids Acids can be classified as natural and mineral acids Natural acids are present in food. Example: Lactic acid, citric acid, acetic acid and tartaric acid. Common laboratory acids like hydrochloric acid, sulphuric acid and nitric acids are mineral acids or inorganic acids. Examples of some organic acids along with their sources               Organic acid                   Sources of Organic acid Acetic acid Vinegar Citric acid Oranges, Lemons Formic acid Ant sting, Nettle sting Tartaric acid Tamarind Oxalic acid Tomatoes Lactic acid Curd Basic Substances (Bases) Basic substances taste bitter. The chemical nature of such substances is basic. Bases are slippery to touch. Example: Milk of magnesia, Soaps, Lime water, Household cleaners. Properties of bases      •  Bases are bitter to taste, soapy to touch.      •  Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.      •  Bases liberate heat on dilution. Neutral substance:  Substance that does not show acid or base properties is called neutral substance. Example: Water. You have learnt about acidic, basic and neutral substances. But how would you find out whether a substance is acidic or basic? One way of doing it is tasting the substance. But it is not a safe method. Certain chemicals are used to identify the substances called indicators. Indicators Indicators are substances that are used to test whether a substance is acidic or basic ot neutral in nature. They change their color when added to a solution containing an acidic or a basic substance. Indicators are of two types - natural and artificial. Naturally occurring indicators are turmeric, litmus, china rose and red cabbage. Litmus Litmus is a natural dye extracted from lichens. It is the most commonly used natural indicator. Litmus turns acidic solutions red and basic solutions blue. Neutral solutions do not change the color of either red or blue litmus. Turmeric Turmeric gives brownish red color in basic medium and yellow in acidic medium. China Rose A solution of china rose turns green in a basic solution, and bright pink or magenta in an acidic solution. Identification of Basic Substances Soap, milk of magnesia and limewater turn red litmus paper blue. This indicates the basic nature of these solutions. Soap, milk of magnesia and limewater turn turmeric red and the china rose green. Identification of Acidic Substances Red litmus paper when dipped in lemon juice and vinegar remains red. But they turn blue litmus paper red. This indicates the acidic nature of these solutions. Lemon juice and vinegar turn turmeric yellow and china rose pink.

#### Summary

Substances can be classified as acidic, basic or neutral. Acidic Substances (Acids) The word acid is derived from the Latin word acere meaning sour. Substances that taste sour contain acids. The chemical nature of such substances is acidic. Example: Curd, lemon juice, tamarind, spinach and vinegar. Properties of Acids      •  Acids have corrosive action on skin.      •  Acids are good conductors of electricity.      •  Acids neutralize bases to form salt and water. Classification of Acids Acids can be classified as natural and mineral acids Natural acids are present in food. Example: Lactic acid, citric acid, acetic acid and tartaric acid. Common laboratory acids like hydrochloric acid, sulphuric acid and nitric acids are mineral acids or inorganic acids. Examples of some organic acids along with their sources               Organic acid                   Sources of Organic acid Acetic acid Vinegar Citric acid Oranges, Lemons Formic acid Ant sting, Nettle sting Tartaric acid Tamarind Oxalic acid Tomatoes Lactic acid Curd Basic Substances (Bases) Basic substances taste bitter. The chemical nature of such substances is basic. Bases are slippery to touch. Example: Milk of magnesia, Soaps, Lime water, Household cleaners. Properties of bases      •  Bases are bitter to taste, soapy to touch.      •  Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.      •  Bases liberate heat on dilution. Neutral substance:  Substance that does not show acid or base properties is called neutral substance. Example: Water. You have learnt about acidic, basic and neutral substances. But how would you find out whether a substance is acidic or basic? One way of doing it is tasting the substance. But it is not a safe method. Certain chemicals are used to identify the substances called indicators. Indicators Indicators are substances that are used to test whether a substance is acidic or basic ot neutral in nature. They change their color when added to a solution containing an acidic or a basic substance. Indicators are of two types - natural and artificial. Naturally occurring indicators are turmeric, litmus, china rose and red cabbage. Litmus Litmus is a natural dye extracted from lichens. It is the most commonly used natural indicator. Litmus turns acidic solutions red and basic solutions blue. Neutral solutions do not change the color of either red or blue litmus. Turmeric Turmeric gives brownish red color in basic medium and yellow in acidic medium. China Rose A solution of china rose turns green in a basic solution, and bright pink or magenta in an acidic solution. Identification of Basic Substances Soap, milk of magnesia and limewater turn red litmus paper blue. This indicates the basic nature of these solutions. Soap, milk of magnesia and limewater turn turmeric red and the china rose green. Identification of Acidic Substances Red litmus paper when dipped in lemon juice and vinegar remains red. But they turn blue litmus paper red. This indicates the acidic nature of these solutions. Lemon juice and vinegar turn turmeric yellow and china rose pink.

#### Activities

Activity 1 Iitm.ac has created an educational animation on “Acids, Bases and Salts”. From this students learn about properties of acids and bases. Students will explore indicators of acids and bases. Students can also learn more about salts, preparation and their properties. Go To Activity Activity 2 Kidsknowit.com has created an interactive learning animation on “Acids and Bases”. From this animation students learn about acids and bases and how they are measured classified. Student can also learn more about discovery of acids and bases, pH scale, behavior of acids and bases in water, etc. Go To Activity Activity 3 Unnes.ac has created an activity on “Acids and Bases”. This is quiz based activity. The questions are appropriate and stimulate the concept in student's mind. This activity will also help to improve application knowledge of students. Go To Activity

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