Detailed notes of ACIDS, BASES AND SALTS | Class 10 | Chemistry

   

        ACIDS , BASES  AND SALTS 

On the basis of their chemical properties ,all the compounds classified into three groups.
1. Acids
2. Bases
3. Salts

INDICATOR FOR TESTING ACIDS & BASES:-

- An indicator is a dye that changes color when put into acid or a base.
- It gives different colors in acid and base. Thus it tells us whether the substance we are testing is acid or base.
Ex:-some most common indicator to test for acids and bases are litmus paper, methyl orange and phenolphthalein .

1. Litmus paper:-

- It is of two types (i) Blue litmus
                                 (ii) red litmus
- An acid turns blue litmus to red.
- A base turns red litmus to blue.

If a drop of given solution turns blue litmus to red, then the given solution will be acidic in nature.
Ex:-orange juice turns blue litmus to red , so orange juice is acidic in nature i.e orange juice contains acid.

If a drop of the given solution turns red litmus to blue, then the given solution will be basic in nature.
Ex:- Sodium Hydroxide solution turns red litmus to blue, so sodium hydroxide solution turns is basic in nature(Or alkaline in nature).

NOTE:-

What is alkali ?

Ans:-  water soluble base is called an alkali.

2. Methyl orange:-

- The neutral colors of methyl orange is orange. The colors change take place in methyl orange as follows.
(i) Methyl orange indicator give red color in acidic solution.
(ii) Methyl orange indicator give yellow color in basic solution.

3. Phenolphthalein:-

- The neutral color of phenolphthalein is “colorless”. The color changes which takes place in phenolphthalein  indicator as given below
(i) Phenolphthalein indicator is colorless in acidic solution.
(ii) Phenolphthalein indicator gives pink color in basic solution.

4.Olfactory indicators:-

The term “olfactory” means to the “sense of smell”.
- Those substances whose smell changes in acidic or basic solution are  called olfactory indicators.
Ex:- Onion has a chara cterstic smell .When  basic solution like sodium hydroxide solution is added to a cloth strip treated with onions, then the onion smell can not be detected.
An acidic solution like hydrochloric acid ,however doesn’t destroy the smell of onions. This can be used a test for acids and bases.

Vanilla extract has characteristics pleasant smell .If  a basic solution like sodium hydroxide solution  is added to vanilla extract, then we cannot detect the characteristic smell of vanilla extract , An acidic solution like hydrochloric acid however does not destroy the smell of vanilla extract. This can be used as a test for acids and bases.

ACIDS

- Acids are those chemical substances which have a  sour taste.
-  Acid changes the color of blue litmus to red.
Ex:- Lemon, orange, tamarind curd etc.

On the basis of occurrence the acids classified as follows

1.Organic acids:-

- The acids present in plant material and animals are called organic acids.
e.g.- Acetic acid ,citric acid lactic acid formic acid.
- It is not harmful to eat or drink substances containing naturally occurring acids  in them.
- Organic acid are weak acids.

2.Mineral acids:-

-The acids prepared from the minerals of the earth are called mineral acids.
Ex- HCL,H₂SO₄,HNO₃
-Concentrated mineral acids are very dangerous .They can burn our hands and cloths.
-Mineral acids are strong acids.

3.Concentrated acids:-

A concentrated acid is one which contain the minimum  possible of water in it.

4.Dilute acids:- 

A dilute acid is one which contain much more of water in it.
The dilution of a concentrated acid should always be done by adding concentrated acid to water gradually with stirring and not by adding water to concentrated acid.

PROPERTIES OF ACIDS:-

1. Acids have a sour taste.
2. Acids turn blue litmus to red.
3.Acids solution conduct electricity (They are electrolyte).
4.acids react with metal to from hydrogen gas.
  Acid   +    Metal  ->   salt      +   hydrogen gas
Ex:-  H₂SO₄(aq)  +   Zn(s)  ->  ZnSO₄(aq)  + H₂(g)
In this reaction zinc metal react with sulphuric acids to form zinc sulphate and hydrogen gas is dissolved.

NOTE:- Curd and other sour food stuffs such as vinegar, lemon juice and orange juice etc. should not be kept in metal vessel .Because they can react with metal of vessel to form  poisonous metal compounds which can cause food poisoning and damage our health.

5.  Acids react with metal carbonate and metal hydrogen carbonate to form carbon dioxide gas.
Metal carbonate + Acid  ->    salt  +     CO2  +H2O
When dilute HCL react with sodium carbonate then sodium chloride ,carbon dioxide and water are formed.
NaHCO₃(s) + HCl(aq) -> NaCl(aq) + CO₂(g) + H₂O(l)

6.Acids react with bases to form salt and water.
        Acid       +    Base   ->  Salt     +   water
When an acid is treated with base, the base neutralize the acid and destroys its acidity. Since acid and base neutralize each other , so the reaction between acid and base to form salt and water is called neutralization.
Ex:-When hydrochloric acid react with sodium hydroxide , then a neutralization  reaction take place to form sodium chloride and water.
 NaOH(aq) + HCl(aq) -> NaCl(aq)   +  H₂O(l)

7.Acids react with metal oxides to from salt and water.
Metal  oxide   +  Acid   ->  Salt     +    water
Ex:-Copper(II) oxide is a metal oxide , reacts with dilute Hydrochloric acid to Copper (II) chloride and water.
  CuO(s)  + 2HCl(aq) ->CuCl₂(aq) +  H₂O(l)

8 . Acids have corrosive nature :- The mineral acids cause  severe burns on the skin and attack and eat up the material like cloth , wood , metal structures and stone work.

What do all acids have in common :-

-Hydrogen present in acids is such that when acids is dissolved in water , it separate out as positively charged hydrogen ions(H⁺).
-We can now define an acid on the basis hydrogen ions (H⁺) as  "An acid is a substance which dissociate on dissolving in water to produce hydrogen ions (H⁺)" .
Ex:- An aqueous solution of hydrochloric acid  dissociate to hydrogen ions.
HCl(aq)  ->   H⁺(aq)   +    Cl^-(aq)

-The aqueous solution of glucose and alcohol do not show acidic character because their hydrogen does not separate out as hydrogen ions (H⁺)on dissolving in water.
- From this we conclude that though all the acids contains hydrogen but all the hydrogen containing compounds are not acids.

STRONG ACIDS:-

- An acid which is completely ionised in water and thus produces a large amount of hydrogen ions is called strong acid
Ex:- HCl, H₂SO₄, HNO₃ etc.
-Strong acid react very rapidly with other substance.

WEAK ACIDS:-

-An acid which partially ionized in water and thus produces a small amount of hydrogen ions is called a weak  acid.
Ex:-CH₃COOH, H₂CO₃,H₂SO₃
-Weak acid react quit slowly with other substances.

USE OF MINERAL ACIDS IN INDUSTRY:-

-Sulphuric acids is used in the manufacturing of fertilizers, paints, dyes chemicals, detergents, explosives etc.
-Hydrochloric acid is used for removing scale deposits from inside the boiler. It is also used in dye stuff, textile food and leather industries.

BASE

-Bases are those chemical substances which have a bitter in taste.
-Base change the color of red litmus  to blue.
-Base is a chemical substance which can neutralize an acid.
Ex: NaOH,CaO,NH₄OH,Na₂CO₃.
-A base is a soluble in water is called alkali.
-A base substance which dissolve in water to produce hydroxide ions(OH^-)in solutions.
         NaOH(s)  ->    Na⁺(aq)  +  OH⁺(aq)
   

STRONG BASE:

A base which completely ionised in water and thus produces a large amount of hydroxide ion (OH^-)
e.g.  NaOH ,KOH ,etc.

WEAK BASE:

A base which is partially ionised in water and thus produces a small amount of hydroxide ions(OH^-) is called a weak base.
Ex-NH₄OH , Ca(OH)₃ etc.

PROPERTIES OF BASE :

I.Bases have bitter taste.
II.Bases feel soapy to touch.
III.Bases turn red litmus to blue.
IV.Bases conduct electricity in solution (They are electrolyte).
V.Base react with some metals to form hydrogen gas.
   Ex: When sodium hydroxide solution is heated with zinc , then sodium zincate and hydrogen gas are formed.
    2NaOH +Zn (s)  ->   Na₂ZnO₂(aq)+H₂(g)

VI. Bases react with acids to form salt and water
Ex : When sodium hydroxide react with sulphuric acid then sodium sulphate and water are  formed.
2NaOH +  H₂SO₄    ->      Na2SO4  +  2H₂O

VII. Bases react with non metal oxide to form salt and water.
Ex: Calcium hydroxide react with Corban dioxide to form calcium carbonate and water.
Ca(OH)₂ +  CO₂   ->    CaCO₃+H₂O

USES OF BASES:

-Sodium hydroxide is used in the manufacture of soap, paper and a synthetic fiber called rayon.
-Calcium hydroxide is used in manufacturing of bleaching powder.

STRENGTH OF ACID AND BASE SOLUTIONS : PH SCALE

-In 1909 Sorenson devised a scale (PH scale )on which the strength of acide solution as well as basic solution could be represented by making use of the hydrogen ion concentration in them.
-The pH of a solution is inversely proportional to the concentration of hydrogen ions in them.
-A solution having high concentration of hydrogen ions have a low pH value .On the other hand, a solution having low concentration of hydrogen ion has High pH value.
-In the term 'pH', letter 'p' stands for a German word "potenz" which means 'power' and letter 'H' stands for hydrogen ions concentration (H⁺).
a) Neutral substances have a pH of exactly 7. Ex - Pure water, sugar solution etc.
b) Acids have a pH of less than 7.
Lower pH, stronger the acid.
Ex-Solution of pH  1 is more acidic than another solution of pH 4.
c)  Bases have  a pH  between 7 and 14.
Ex-solution of pH 14 will be much more basic than another solution of pH 10.

Importance of pH in every day life :

1.pH In our digestive system:

           Dil. Hydrochloric acid helps in digesting out food . The  excess acid in stomach  cause indigestion. Being basic in nature , antacids react with excess acid in stomach and neutralize it.

2. pH change as the cause of tooth decay:

The bacteria present in mouth break down the sugar to form acids. Tooth decay start when the pH of acid prevent tooth decay is to clean the mouth thoroughly after eating food.

3.Self defence by the animal and plant through the chemical warfare:

•When a honey bee stings a person ,it injects an acidic liquid into  the skin .which causes immenes pain and irritation.
•An ant is sting inject nethanoic acid into the skin of a person causing burning pain.

             SALTS

-A salt is a compound formed from an acid by the replacement of the hydrogen in the acid by a metal.
     e.g.- Hydrochloric acid is HCl. if we replace the hydrogen (H) of this acid by a metal  atom, say a sodium atom (Na), then we will get a salt NaCl (sodium chloride).                                                 Salts are formed when acids react with bases. In a way, the name of a salt consists of two parts: the first part of the name of salt is derived from the name of base, and the second part of the name of the salt comes from the name of acid. 
  e.g.- The name of a salt called 'sodium chloride comes from sodium hydroxide base an hydrochloric acid.

Family of Salts

The salts having the same positive ions (or same negative ions) are said to belong to a family of salts. For example, sodium chloride (NaCl) and sodium sulphate (Na₂SO₄) belong to the same family of salts called 'sodium salts' (because they both contain the same positively charged ions, sodium ions Na+.

The pH of salt solution

 The aqueous solutions of many salts are neutral having a pH of 7, but some salts produce acidic or basic solutions (alkaline solution) when dissolved in water.

 The acidic nature and basic nature of some salt solutions can be explained on the basis of hydrolysis of salts.
(i) The salts of strong acids and strong bases give neutral solutions (having pH = 7).
e.g. - Sodium chloride salt (NaCl) is formed from a strong acid hydrochloric acid (HCI), and a strong base sodium hydroxide (NaOH). Since sodium chloride is formed from a strong acid and a strong base, therefore, an aqueous solution of sodium chloride is neutral.
(ii) The salts of strong acids and weak bases give acidic solution (having pH less than 7).
e.g- Ammonium chloride (NH₄Cl) is the salt of a strong acid hydrochloric acid (HCI), and a weak base ammonium hydroxide (NH₂OH), so an aqueous solution of ammonium chloride is acidic in nature.
(iii) The salts of weak acids and strong bases give basic solutions (having pH more than 7).
e.g.- Sodium carbonate, Sodium carbonate (Na₂CO₃) is the salt of a weak acid carbonic acid (H₂CO₃) and a strong base sodium hydroxide (NaOH), so an aqueous solution of sodium carbonate will be basic in nature or alkaline in nature.

COMMON SALT (SODIUM CHLORIDE)

*The chemical name of common salt is sodium chloride (NaCl)
* Common salt (or sodium chloride) is a neutral salt.
* Sodium chloride can be made in the laboratory by the combination of sodium hydroxide and hydrochloric acid.

Use of Common Salt (or Sodium Chloride)

Some of the important uses of common salt (or sodium chloride) are
1. Common salt (sodium chloride) is used as a raw material for making a large number of useful chemicals in industry such as : sodium hydroxide (caustic soda), sodium carbonate (washing soda).
2. Common salt (sodium chloride) is used in cooking food. Sodium chloride is required by our body for the working of nervous system, the movement of muscles, and the production of hydrochloric acid in the stomach for the digestion of food.
3. Common salt (sodium chloride) is used as a preservative in pickles, and in curing meat and fish .
4. Common salt (sodium chloride) is used in the manufacture of soap.
5. Common salt (sodium chloride) is used to melt ice which collects on the roads during winter in cold countries.

CHEMICALS FROM COMMON SALT

The preparation and uses of some of the important chemicals (chemical compounds) which are     obtained from common salt (or sodium chloride) by various methods.

1. Sodium hydroxide- 

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide.
 The process is called the chloro-alkali process because of the products formed–chloro for chlorine and alkali for sodium hydroxide.
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl2(g) + H2(g)

Uses of Sodium Hydroxide

i. Sodium hydroxide is used for making soaps and detergents.
ii. Sodium hydroxide is used for making artificial textile fibers (such as rayon).
iii. Sodium hydroxide is used in the manufacture of paper.

2. Bleaching powder-

 Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂].
Bleaching powder is represented as CaOCl₂.
        Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Use of Bleaching powder

i. for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper     factories and for bleaching washed clothes in laundry.
ii. As an oxidising agent in many chemical industries.
iii. To make drinking water free from germs.

3. Baking soda 

The chemical name of the compound is sodiumhydrogencarbonate (NaHCO₃). Sodiumhydrogencarbonate is produced by reacting a cold and concentrated solution of sodium chloride (called brine) with ammonia and carbon dioxide.
 NaCl + NH₃ + CO₂ + H₂O      NaHCO₃ + NH₄Cl

Use of Baking soda

i. Sodiumhydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
ii. It is also used in soda-acid fire extinguishers.

4. Washing soda

 Sodium carbonate can be obtained by heating baking soda, recrystallization of sodium carbonate gives washing soda.
          Na2CO3 + 10H2O -> Na₂CO₃.10H₂O

Uses of washing soda

i. Sodium carbonate (washing soda) is used in glass, soap and paper industries.
ii. It is used in the manufacture of sodium compounds such as borax.
iii. Sodium carbonate can be used as a cleaning agent for domestic purposes.
iv. It is used for removing permanent hardness of water.

5. Plaster of Paris-

 On heating gypsum at 373 K, it loses water molecules and becomes
calcium sulphate hemihydrate (CaSO₄.1/2H₂O). This is called Plaster of Paris.

Uses of plaster of paris

i. Doctors use as plaster for supporting fractured bones in the right position.
ii. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.