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Monday, 29 June 2015

ACIDS, BASES AND SALTS

                                                         
 ACIDS, BASES AND SALTS
1) Acids are sour in taste, turn blue litmus red, and dissolve in water to release H+ ions e.g. HCl, H2SO4, HNO3 etc. 
2) Bases are bitter in taste, have soapy touch, turn red litmus blue and give hydroxide
ions in solution. e.g. NaOH, KOH etc.
3) A salt is a compound which is formed by neutralization reaction between an acid and base.
e.g. sodium chloride.
4) Indicators – Indicators are substances which indicate the acidic or basic nature of the solution by their colour change.
The colour of some acid – base indicators in acidic and basic medium are given below.
INDICATORS COLOUR IN
ACIDIC MEDIUM  COLOUR IN
BASIC
MEDIUM
1 Litmus solution Red Blue
2 Methyl Orange Pink Orange
3 Phenolphthalein Colourless Pink
4 Methyl red Yellow Red
5) Chemical properties of acids:
i) Acids react with active metals to give hydrogen gas.
Zn + H2SO4 ZnSO4 + H2
ii) Acids react with metal carbonate and metal hydrogen carbonate to give carbon dioxide.
NaHCO3 +HCl NaCl + H2O + CO2
iii) Acids react with bases to give salt and water. This reaction is called as neutralization
reaction.  NaOH + HCl  NaCl +H2O
iv) Acids react with metals oxides to give salt and water.
CuO + H2SO4  CuSO4 + H2O
6) Chemical properties of Bases:
i) Reaction with Metals - Certain reactive metals such as Zinc, Aluminium, and Tin react
with alkali solutions on heating and hydrogen gas is evolved.
2NaOH + Zn  Na2ZnO2 +H2
ii) Reaction with acids -Bases react with acids to form salt and water.
KOH +HCl  KCl +H2O
iii) Reaction with Non -metallic oxides – These oxides are generally acidic in nature. They
react with bases to form salt and water.
2NaOH + CO2  Na2CO3 + H2O
7) PH Scale: The concentration of hydrogen ion in solution is expressed in terms of pH. The
pH of a solution is defined as the negative logarithm of hydrogen ion concentration in moles
per litre.
pH = - log [H+]
For water or neutral solutions, pH = 7
For acidic solutions, pH < 7
For basic solutions, pH > 7
8) Some Important Chemical Compounds:
a) Common Salt (NaCl)
Sodium chloride is known as common salt. Its main source is sea water. It is also exists in the
form of rocks and is called rock salt.
Common salt is an important component of our food. It is also used for preparing sodium
hydroxide, baking soda, washing soda etc.
b) Sodium Hydroxide or Caustic Soda (NaOH)
It is prepared by passing electricity through an aqueous solution of sodium chloride also
known as brine.
2NaCl (aq) + 2 H2O (l)  2NaOH (aq) + Cl2 (g) + H2 (g)
This process is known as chlor-alkali process.
Properties:
1. It is white translucent solid.
2. Crystals of sodium hydroxide are deliquescent.
3. It is readily soluble in water and gives strong alkaline solution.
c) Bleaching Powder (CaOCl2)
Its chemical name is calcium oxychloride. It is prepared by passing chlorine gas through dry
slaked lime.
Ca (OH) 2 + Cl2 CaOCl2 + H2O
Uses –
1. For bleaching cotton and linen in textile industry.
2. For disinfecting drinking water.
3. For bleaching wood pulp in paper industry.
d) Baking Soda (NaHCO3)
Chemical name is Sodium hydrogen carbonate.
It is prepared by passing CO2 gas through brine solution saturated with ammonia.
NaCl + H2O +CO2 +NH3  NH4Cl + NaHCO3
Properties:
1. It is white crystalline solid and sparingly soluble in water at room temperature.
2. On heating it decomposes to give sodium carbonate and carbon dioxide.
3. It reacts with acids to give carbon dioxide gas.
4. It aqueous solution is weak alkaline due to hydrolysis.
Uses:
1. It is used in soda – acid fire extinguisher.
2. It acts as mild antiseptic and antacid.
3. It is used as a component of baking powder. In addition to sodium hydrogen carbonate
baking soda contains tartaric acid.
e) Washing Soda (Na2CO3.10 H2O)
Chemical name is sodium carbonate decahydrate.
It is prepared by heating baking soda. Recrystallisation of sodium carbonate gives washing
soda.
2NaHCO3  Na2CO3 +H2O +CO2
Na2CO3 +10 H2O  Na2CO3 . 10 H2O
Uses:
1. It is used for removing permanent hardness of water.
2. It is used in glass, soap and paper industries.
3. It can be used as a cleaning agent for domestic purposes.
14
f) Plaster of Paris (CaSO4.1/2H2O)
Its chemical name is calcium sulphate hemihydrates. It is obtained by heating Gypsum upto
373K.
CaSO4.2H2O  CaSO4.1/2H2O + 11/2H2O
On treatment with water it is again converted into gypsum and sets as a hard mass.
CaSO4.1/2H2O+11/2H2OCaSO4.2H2O
Uses:-
1. It is used by doctors for setting fractured bones.
2. It is used for making statues, models and other decorative materials.
SUGGESTED ACTIVITIES
1. Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning
2. Preparation of carbon dioxide gas by sodium carbonate and sodium hydrogen carbonate
and passing through calcium hydroxide solution.
3. Acid solution in water conducts electricity.
4. Testing of pH values of given solutions.
5. Removing water of crystallisation from crystals of copper sulphate.
HOTS QUESTIONS
Q.1. In one of the industrial process used for manufacture of sodium hydroxide, a gas ‗X‘ is formed
as by – product. The gas ‗X‘ reacts with lime water to give a compound ‗Y‘ which is used as a
bleaching agent in chemical industry. Identify ‗X‘ and ‗Y‘ giving the chemical equation of the
reaction.
Ans. In the manufacture of sodium hydroxide, hydrogen gas and chlorine gas (X) are formed as by
products. When chlorine gas (X) reacts with lime water, it forms calcium oxy chloride
(bleaching powder) Y.
2NaCl (aq) + 2 H2O (L) 2NaOH (Aq) + Cl2 (G) + H2 (G)
‗X‘  Cl2 gas
Ca(OH)2 + Cl2 CaOCl2 + H2O
Q.2. Dry hydrogen chloride gas does not turn blue litmus, whereas hydrochloric acid does. Why?
Ans. In the dry state, hydrogen chloride (HCl) does not release H+ ions. Therefore, it cannot behave
as an acid. When dissolved in water, it forms hydrochloric acid. It dissociates to give H+ ions in
solution and behaves as an acid.
Q.3. Acid when react with metals release hydrogen gas but there is one acid which when reacts with
metals does not release hydrogen except for two metals. Prove this statement.
Ans. Acid + Metal  Salt + Hydrogen
e.g. 2HCl + 2Na  2NaCl + H2
H2SO4 + 2Na  Na2SO4 + H2
HNO3 + Na  No hydrogen gas.
Because nitric acid is strong oxidising agent. Nitric acid reacts only with Mg and Mn to give
hydrogen gas.
Mg + 2HNO3 Mg(NO3)2 + H2
Mn + 2HNO3Mn(NO3)2 + H2

Q. 4 Name the properties responsible for the following uses of baling powder. (i) Baking industry (ii)
As an antacid (ii) As soda-acid fire extinguisher.
Q. 5 What is meant by water of crystallisation of a substance? What is its importance?
Q. 6 What effect does an increase in concentration of ‗H‘ ions in a solution have on the pH of a
solution
Q. 7 Fresh milk has a pH Of 6. When it changes to curd, will its pH value increase or decrease?
Why?
Q. 8 How does the flow of acid rain water into a river make the survival of aquatic life in a river
difficult?
Q. 9 Arrange in the increasing order of their pH values: NaOH solution, Blood, Lemon juice,
Q. 10 Two solutions A and B have pH values of 5 and 8 respectively. Which solution will be basic in
nature?
Q. 11 Give reason  an aqueous solution of acid conduct electricity?
Q. 12 write difference between alkali and base ?



Chemistry for class 10....
acids-and-alkalis
carbon-and-its-compound-notes
periodic-classification-of-elements
unsaturated-hydrocarbon
metal-and-non-metals-notes
chemical-reaction-and-equations-notes-2
periodic-classification-of-elements
sharma sir,scceducation,scc,ssc,free cbse notes,free study material,maths,mathematics,science,9718041826,

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