LEARNING OBJECTIVES– Knowledge of acids and bases, Knowledge of natural and synthetic acid base indicators, Demonstration of  working of indicators, Understanding of neutralization reactions and salt formation, Applications of neutralization reaction in everyday life.

ACIDS(Gr. Acere= sour) -The taste of acids is sour.  They turn the blue litmus paper into red. Their pH (Potential of hydrogen ions) value is below 7. They are corrosive in concentrated form. They conduct electricity.

Those substances which are sour in taste, turn blue litmus into red and have pH value below 7 are called as acids.

Acids are natural as well as synthetic. Natural acids found in plants and animals are called as organic acids. While synthetic acids are called inorganic or mineral acids.









Acetic acid


Ascorbic acid

(Vitamin C)

Amla, Citrus fruits

Citric acid

Citrus fruits  like oranges, lemons etc.

Formic acid /Metheonic acid

Ant’s sting, bee sting

Lactic acid


Oxalic acid


Tartaric acid

Tamarind, Grapes, Unripe mangoes etc.





Nitric acid


Sulphuric acid


Phosphoric acid


Carbonic acid



BASES– The bases are bitter in taste. Bases feel soapy on touch. They turn red litmus paper into blue. The pH value of bases is more than 7 and upto 14. The liquid forms of bases are called as alkali/ alkaline solution.



Ammonium hydroxide

Window cleaner

Calcium hydroxide

Lime water

Magnesium hydroxide

Milk of magnesia

Potassium hydroxide


Sodium hydroxide


NOTE- pH –Scale-:- It is a scale to indicate the strength of an acid, base or salt. It was developed by Sorenson. On it values of these chemicals is between 0-14.

Acids have pH vale between 0-7.

Salts have pH value equal to 7

Bases have pH Value between 7-14.

ACID BASE INDICATORS- Acids are sour and bases are bitter in taste. Salts have no taste. But, it is not safe to taste any chemical substance without knowing it.  There are some substances which indicate that the given substance is an acid or a base. Those substances which indicate that the given chemical substance is an acid or a base are called as acid base indicators. There are two categories of acid-base indicators-

A. NATURAL ACID BASE INDICATORS- They are obtained from plants mainly.  e.g.-

1. LITMUS: A NATURAL DYE– It is the most common natural acid base indicator. It is extracted from ‘Lichens’ like Roccella tinctoria. Roccella tinctoria has a light purple colour in distilled water but turns red in acidic water and blue in Basic (or alkaline) water.

The Litmus strips are available in red and blue colours. The red litmus strip turns blue when dipped in basic or alkaline solution. It remains red in acidic solution. The blue litmus strip turns red when dipped in acidic solution. It remains blue in basic or alkaline solution.

Litmus Strip Colour

Colour Change on

Treatment with


Treatment with


Red Litmus Strip



Blue Litmus strip



Litmus red and litmus blue solutions also do the same work.

2. TURMERIC– It is yellow in colour. It is obtained from the roots of turmeric plants. Turmeric is used as a spice. It remains yellow on reaction with acids but turns red on reaction with bases. Soaps contain base. Therefore, the cloths on which Sabji (Vegetable Curry) containing turmeric has fallen becomes red on washing with soap.

3. CHINA ROSE (Hibiscus) – The red China rose is especially used as acid base indicators. When petals of it are rubbed on paper strips, the violet colour appears. These violet coloured strips appear pink in colour on treatment with acid. But, remain violet on treatment with base (or alkali).

B. CHEMICAL /SYNTHETIC ACID BASE INDICATOR – These are artificial or synthetic acid base indicators which are prepared in factories.

1. PHENOLPHTHALEIN –It is a synthetic acid-base indicator. It is colourless. It remains colourless in acidic solution while turns basic solution into pink colour.

2. METHYL ORANGE-It is orange coloured synthetic chemical. It becomes pink in acidic solution and become yellow in basic/alkaline solution.

NEUTRALIZATION REACTION AND SALT FORMATION– The reaction an acid reacts with a base to form salt and water is called neutralization reaction. The neutralization reactions are usually exothermic reactions that means heat is released from such reactions.

HCl + Na OH —–> NaCl + H2O + Heat  


 HCl = Hydrochloric acid

NaOH = Sodium Hydroxide

NaCl = Sodium Chloride

H2O = Water

SALTS – Salt is generally a solid substance formed as a result of neutralization reaction. They are soluble in water and conduct current in form of solution. Their pH value is 7. Salts can be- acidic salts, basic salts and neutral salts. Their melting point (m. p.) and boiling point (b. p.) are very high.

e.g.- Sodium carbonate- NaCO3

Magnesium Carbonate- MgSO4

Potassium Carbonate-KCO3etc.   



Following are the applications of neutralization reactions in our daily life-

1. Indigestion– The acid is released in stomach when excess or spicy eating is done. It causes stomachache (pain in stomach) due to acidity. It is neutralized by taking proper amount of antacid. e.g. – Digene tablets and liquid, Gelusil, Ulgel, Diovol etc. like antacids contain magnesium hydroxide (milk of magnesia), aluminium hydroxide like bases.

2.  Ant bite– The ant on biting injects formic (Metheonic) acid into the skin. It causes pain. To get the relief baking soda (or Sodium bicarbonate/NaHCO3/ Sodium Hydrogen Carbonate) is applied on the bite area. Also, Calamine solution can be applied to that place as it contains zinc carbonate. The effect of acid is neutralized.

3. Soil Treatment– The plants grow well in slight acidic, neutral or slight basic soil. Excess and continuous uses of chemical fertilizers make the soil too acidic. To neutralize the acidic soil quicklime (CaO- Calcium oxide) or Slaked lime – (CaOH)2 is used. If the soil is too basic, compost (organic matter) is used which releases acid to neutralize basic soil.

4. Factory waste – The factory waste effluents contain acidic substances. They harm the soil and aquatic organisms. So, to neutralize the affects of factory waste, basic substance like quicklime is added.


Q.1 State the differences between acids and bases.






They are sour in taste.

They are biter in taste.


Their pH value is between 0-7.

Their pH value is between 7-14.


They turn red litmus into blue colour.

They turn blue litmus into red colour.


Acids are found in curd, Vinegar, Amla, Citrus fruit  etc.

Bases are found in soap, windows cleaner, Lime water, Antacids etc


Q.2 Ammonia is found in many household products, such as window cleaner. It turns red litmus blue. What is its nature?

Ans- Liquid Ammonia (NH3) used as window cleaner is basic (alkaline) in nature.

Q.3 Name the source from which litmus solution is obtained. What is the use of this solution?

Ans- Litmus is extracted from ‘Lichens’ like Roccella tinctoria. The litmus solutions are used as acid base indicator.

Q.4 Is the distilled water acidic/basic/neutral? How would you verify it?

1 thought on “ACIDS, BASES AND SALTS Class VII Chapter 5”

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