ACIDS, BASES AND SALTS Class VII Chapter 5

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.

TYPE OF ACID	NAME OF ACID	SOURCE/FOUND IN
ORGANIC ACIDS	Acetic acid	Vinegar
	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	Curd
	Oxalic acid	Spinach
	Tartaric acid	Tamarind, Grapes, Unripe mangoes etc.
MINERAL /INORGANIC/ LABORATORY ACIDS	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.

NAME OF BASE	FOUND IN
Ammonium hydroxide	Window cleaner
Calcium hydroxide	Lime water
Magnesium hydroxide	Milk of magnesia
Potassium hydroxide	Soap
Sodium hydroxide	soap

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  Acid	Treatment with  base/Alkali
Red Litmus Strip	Red	Blue
Blue Litmus strip	Red	Blue

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  

Where,

 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.   

         

APPLICATIONS OF NEUTRALIZATION REACTION IN DAILY LIFE-

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.

SOLVED EXERCISE QUESTIONS-

Q.1 State the differences between acids and bases.

Ans-

S.N.	Acids	Bases
1	They are sour in taste.	They are biter in taste.
2	Their pH value is between 0-7.	Their pH value is between 7-14.
3	They turn red litmus into blue colour.	They turn blue litmus into red colour.
4	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?