Chapter 2 - Acids, Bases and Salts Class 10 Notes | EduRev

Science for Class 10 - Short Notes

Class 10 : Chapter 2 - Acids, Bases and Salts Class 10 Notes | EduRev

 Page 1


Chapter 2 – Acids, Bases and Salts 
Acids and Bases 
Acid - Acids are molecules or ions that are able to donate hydrogen ion in an aqueous solution.  
For example – Hydrochloric acid (HCl), Sulphuric acid (H 2SO4), Carbonic acid (H2CO3).  
Base – Bases are the molecules or ions that are able to donate a hydroxide ion in an aqueous solution. For 
example – Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH) 2), Potassium hydroxide (KOH), Calcium 
oxide (CaO) 
Bases soluble in water are also known as alkali or alkaline.  
Acid-base indicators – The substance that give a characteristic color in the presence of acid or base or both are 
known as acid base indicators. E.g. Litmus solution, turmeric, methyl orange, phenolphthalein etc. 
 
 
 
 
 
Physical properties of acids 
a. Acids are sour in taste 
b. Acids change the color of blue litmus paper to red 
c. The pH of acids is less than 7. 
Physical properties of bases 
a. Bases are bitter in taste 
b. Bases change the color of red litmus paper to blue 
c. The pH of bases is more than 7 
Note – The highest pH that is possible is 14, therefore the pH of bases can be at most equal to 14. 
 
Chemical properties of Acids and Bases 
1. Reaction of Acids and Bases with Metals 
 
Metals + Acids/Bases        Salt + Hydrogen gas 
 
Examples: 
a. Zn (s) + 2HCl (aq.)            ZnCl2 + H2 (g) 
  (Zinc) (Hydrochloric acid)                     (Zinc chloride) (Hydrogen) 
b. Cu (s) + H2SO4 (aq.)        CuSO4 (aq.) + H2 (g) 
(Copper) (Sulphuric acid)   (Copper sulphate) (Hydrogen) 
c. Zn + 2NaOH            Na2ZnO4 + H2  
(Sodium hydroxide)   (Sodium zincate) 
Indicators Color change for acids Color change for bases 
Litmus solution Red Blue 
Turmeric powder N/A Red 
Methyl orange Orange Yellow 
Phenolphthalein Colorless Pink 
Page 2


Chapter 2 – Acids, Bases and Salts 
Acids and Bases 
Acid - Acids are molecules or ions that are able to donate hydrogen ion in an aqueous solution.  
For example – Hydrochloric acid (HCl), Sulphuric acid (H 2SO4), Carbonic acid (H2CO3).  
Base – Bases are the molecules or ions that are able to donate a hydroxide ion in an aqueous solution. For 
example – Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH) 2), Potassium hydroxide (KOH), Calcium 
oxide (CaO) 
Bases soluble in water are also known as alkali or alkaline.  
Acid-base indicators – The substance that give a characteristic color in the presence of acid or base or both are 
known as acid base indicators. E.g. Litmus solution, turmeric, methyl orange, phenolphthalein etc. 
 
 
 
 
 
Physical properties of acids 
a. Acids are sour in taste 
b. Acids change the color of blue litmus paper to red 
c. The pH of acids is less than 7. 
Physical properties of bases 
a. Bases are bitter in taste 
b. Bases change the color of red litmus paper to blue 
c. The pH of bases is more than 7 
Note – The highest pH that is possible is 14, therefore the pH of bases can be at most equal to 14. 
 
Chemical properties of Acids and Bases 
1. Reaction of Acids and Bases with Metals 
 
Metals + Acids/Bases        Salt + Hydrogen gas 
 
Examples: 
a. Zn (s) + 2HCl (aq.)            ZnCl2 + H2 (g) 
  (Zinc) (Hydrochloric acid)                     (Zinc chloride) (Hydrogen) 
b. Cu (s) + H2SO4 (aq.)        CuSO4 (aq.) + H2 (g) 
(Copper) (Sulphuric acid)   (Copper sulphate) (Hydrogen) 
c. Zn + 2NaOH            Na2ZnO4 + H2  
(Sodium hydroxide)   (Sodium zincate) 
Indicators Color change for acids Color change for bases 
Litmus solution Red Blue 
Turmeric powder N/A Red 
Methyl orange Orange Yellow 
Phenolphthalein Colorless Pink 
Note – Such reactions are not possible with all the metals. The “popup” sound produced by the matchstick or 
candle near the gas produced during a reaction is an indication of the presence of hydrogen gas.  
2. Reaction of Acids with metal carbonate and metal hydrogen carbonates 
 
Metal carbonates (MCO3) and bicarbonates (MHCO3) react with acids to produce salt, carbon dioxide 
and water.  
Examples 
a. `Na2CO3 (s) + HCl (aq.)              2NaCl (aq.) + H2O + CO2(g)  
(Sodium carbonate) (Hydrochloric acid)             (Sodium chloride) 
b. NaHCO3 (s) + HCl (aq.)    NaCl (aq.) + H2O + CO2 (g) 
(Sodium hydroxide) (Hydrochloric acid)             (Sodium chloride) 
Note – Carbon dioxide is a colorless gas and can be identified by passing it through lime water or Calcium 
hydroxide. Carbon dioxide converts lime water milky.  
c. Ca(OH)2 (aq.) + CO2 (g)       CaCO3 (s) + H2O (l) 
(Limewater)         (Calcium carbonate) 
     (white precipitate) 
On passing more carbon dioxide through calcium carbonate following reaction occurs: 
CaCO3 (s) + H2O (l) + CO2 (g)     Ca(HCO3)2 (aq.) 
(Calcium carbonate)           (Calcium hydroxide) 
 
 
3. Reaction of acids and bases with each other 
 
Acids reacts with bases to form salt and water.  
Acid + Base    Salt + Water  
 
Examples –  
a. NaOH (aq.) + HCl (aq.)    NaCl (aq.) + H2O (l) 
b. Zn(OH)2(aq.) + H2SO4 (aq.)         ZnSO4 (aq.) + 2H2O (l)   
 
Similarities between acids and bases 
1. Both acids and bases conduct electricity when dissolved in water.  
Note – Pure water does not conduct electricity but when an acid or base is dissolved in it, it becomes a 
good conductor of electricity.  
 
2. Hydrolysis of acids and bases 
Acids release hydrogen ions (H
+
) in the presence of water  
 
HCl
  Water  
        H
+
 + Cl
-
 
The hydrogen ion released in the presence of water reacts with water molecule to produce a hydronium 
ion 
H
+
 + H2O           H3O
+
 
    (hydronium) 
Bases release hydroxide ions (OH
-
) in the presence of water 
Page 3


Chapter 2 – Acids, Bases and Salts 
Acids and Bases 
Acid - Acids are molecules or ions that are able to donate hydrogen ion in an aqueous solution.  
For example – Hydrochloric acid (HCl), Sulphuric acid (H 2SO4), Carbonic acid (H2CO3).  
Base – Bases are the molecules or ions that are able to donate a hydroxide ion in an aqueous solution. For 
example – Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH) 2), Potassium hydroxide (KOH), Calcium 
oxide (CaO) 
Bases soluble in water are also known as alkali or alkaline.  
Acid-base indicators – The substance that give a characteristic color in the presence of acid or base or both are 
known as acid base indicators. E.g. Litmus solution, turmeric, methyl orange, phenolphthalein etc. 
 
 
 
 
 
Physical properties of acids 
a. Acids are sour in taste 
b. Acids change the color of blue litmus paper to red 
c. The pH of acids is less than 7. 
Physical properties of bases 
a. Bases are bitter in taste 
b. Bases change the color of red litmus paper to blue 
c. The pH of bases is more than 7 
Note – The highest pH that is possible is 14, therefore the pH of bases can be at most equal to 14. 
 
Chemical properties of Acids and Bases 
1. Reaction of Acids and Bases with Metals 
 
Metals + Acids/Bases        Salt + Hydrogen gas 
 
Examples: 
a. Zn (s) + 2HCl (aq.)            ZnCl2 + H2 (g) 
  (Zinc) (Hydrochloric acid)                     (Zinc chloride) (Hydrogen) 
b. Cu (s) + H2SO4 (aq.)        CuSO4 (aq.) + H2 (g) 
(Copper) (Sulphuric acid)   (Copper sulphate) (Hydrogen) 
c. Zn + 2NaOH            Na2ZnO4 + H2  
(Sodium hydroxide)   (Sodium zincate) 
Indicators Color change for acids Color change for bases 
Litmus solution Red Blue 
Turmeric powder N/A Red 
Methyl orange Orange Yellow 
Phenolphthalein Colorless Pink 
Note – Such reactions are not possible with all the metals. The “popup” sound produced by the matchstick or 
candle near the gas produced during a reaction is an indication of the presence of hydrogen gas.  
2. Reaction of Acids with metal carbonate and metal hydrogen carbonates 
 
Metal carbonates (MCO3) and bicarbonates (MHCO3) react with acids to produce salt, carbon dioxide 
and water.  
Examples 
a. `Na2CO3 (s) + HCl (aq.)              2NaCl (aq.) + H2O + CO2(g)  
(Sodium carbonate) (Hydrochloric acid)             (Sodium chloride) 
b. NaHCO3 (s) + HCl (aq.)    NaCl (aq.) + H2O + CO2 (g) 
(Sodium hydroxide) (Hydrochloric acid)             (Sodium chloride) 
Note – Carbon dioxide is a colorless gas and can be identified by passing it through lime water or Calcium 
hydroxide. Carbon dioxide converts lime water milky.  
c. Ca(OH)2 (aq.) + CO2 (g)       CaCO3 (s) + H2O (l) 
(Limewater)         (Calcium carbonate) 
     (white precipitate) 
On passing more carbon dioxide through calcium carbonate following reaction occurs: 
CaCO3 (s) + H2O (l) + CO2 (g)     Ca(HCO3)2 (aq.) 
(Calcium carbonate)           (Calcium hydroxide) 
 
 
3. Reaction of acids and bases with each other 
 
Acids reacts with bases to form salt and water.  
Acid + Base    Salt + Water  
 
Examples –  
a. NaOH (aq.) + HCl (aq.)    NaCl (aq.) + H2O (l) 
b. Zn(OH)2(aq.) + H2SO4 (aq.)         ZnSO4 (aq.) + 2H2O (l)   
 
Similarities between acids and bases 
1. Both acids and bases conduct electricity when dissolved in water.  
Note – Pure water does not conduct electricity but when an acid or base is dissolved in it, it becomes a 
good conductor of electricity.  
 
2. Hydrolysis of acids and bases 
Acids release hydrogen ions (H
+
) in the presence of water  
 
HCl
  Water  
        H
+
 + Cl
-
 
The hydrogen ion released in the presence of water reacts with water molecule to produce a hydronium 
ion 
H
+
 + H2O           H3O
+
 
    (hydronium) 
Bases release hydroxide ions (OH
-
) in the presence of water 
 
NaOH (s) 
  Water  
Na
+
 (aq.) + (OH)
-
 (aq.) 
 
Note – The ions released by dissolving acids or base in water helps in the conduction of electricity. 
 
 
3. Neutralization reaction 
 Acid + Base    Salt + Water 
 
 H X + M OH     MX + H2O 
  
 H
+
 + OH
-
           H2O 
 
The hydrogen from the acid neutralizes the hydroxide from the base to for water molecule, which is 
neutral (neither acid nor base) and therefore this reaction is known as neutralization reaction. 
Note – Dissolving acids into water is an exothermic process. So, acid must be slowly added to water with 
constant shaking and not vice versa. The acid may splash out of the test tube if water is added to acid and not 
vice versa, causing burns to the person.  
Dissolving acid or base into water results in decrease in the number of hydrogen and hydroxide ions 
respectively leading to their dilutions.  
 
Potenz (power, potential) of hydrogen or pH 
pH is defined as the negative log of concentration of hydrogen present in a solution or liquid.  
pH = -log10[H
+
] or log10 1 
  [H
+
] 
Concept of log – Log1010 = 1; Log10 100 = Log10 10
2
 = 2; Log10 1000 = Log10 10
3
 = 3 
Log10 – here 10 is the base and the solution to log is the power of 10 that will give the value for which we are 
calculating log. For e.g. in the above example’s log of 1000 is 3 because 10 raised to the power 3 = 1000. 
The pH of the solutions can be a maximum of 14. A solution with the pH of 7 is said to be neutral. Pure water 
has a pH of 7. 
? The pH of bases is above 7 
? The pH of acids is below 7  
? The lesser the pH of a substance the more acidic it is. HCl having pH 1 is more acidic than dilute HCl 
with pH 4. 
? Lesser pH means higher number of hydrogen ions are present in the solution. A solution with pH 1 will 
have more hydrogen ions than a solution with pH 4  
? The higher the pH of a substance the more basic it is. NaOH with pH 13 is more basic than diluted 
NaOH with pH 8. 
? Higher pH means more concentration of number of hydroxide or lesser number of hydrogen ions are 
present in the solution. 
Measuring the pH 
Page 4


Chapter 2 – Acids, Bases and Salts 
Acids and Bases 
Acid - Acids are molecules or ions that are able to donate hydrogen ion in an aqueous solution.  
For example – Hydrochloric acid (HCl), Sulphuric acid (H 2SO4), Carbonic acid (H2CO3).  
Base – Bases are the molecules or ions that are able to donate a hydroxide ion in an aqueous solution. For 
example – Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH) 2), Potassium hydroxide (KOH), Calcium 
oxide (CaO) 
Bases soluble in water are also known as alkali or alkaline.  
Acid-base indicators – The substance that give a characteristic color in the presence of acid or base or both are 
known as acid base indicators. E.g. Litmus solution, turmeric, methyl orange, phenolphthalein etc. 
 
 
 
 
 
Physical properties of acids 
a. Acids are sour in taste 
b. Acids change the color of blue litmus paper to red 
c. The pH of acids is less than 7. 
Physical properties of bases 
a. Bases are bitter in taste 
b. Bases change the color of red litmus paper to blue 
c. The pH of bases is more than 7 
Note – The highest pH that is possible is 14, therefore the pH of bases can be at most equal to 14. 
 
Chemical properties of Acids and Bases 
1. Reaction of Acids and Bases with Metals 
 
Metals + Acids/Bases        Salt + Hydrogen gas 
 
Examples: 
a. Zn (s) + 2HCl (aq.)            ZnCl2 + H2 (g) 
  (Zinc) (Hydrochloric acid)                     (Zinc chloride) (Hydrogen) 
b. Cu (s) + H2SO4 (aq.)        CuSO4 (aq.) + H2 (g) 
(Copper) (Sulphuric acid)   (Copper sulphate) (Hydrogen) 
c. Zn + 2NaOH            Na2ZnO4 + H2  
(Sodium hydroxide)   (Sodium zincate) 
Indicators Color change for acids Color change for bases 
Litmus solution Red Blue 
Turmeric powder N/A Red 
Methyl orange Orange Yellow 
Phenolphthalein Colorless Pink 
Note – Such reactions are not possible with all the metals. The “popup” sound produced by the matchstick or 
candle near the gas produced during a reaction is an indication of the presence of hydrogen gas.  
2. Reaction of Acids with metal carbonate and metal hydrogen carbonates 
 
Metal carbonates (MCO3) and bicarbonates (MHCO3) react with acids to produce salt, carbon dioxide 
and water.  
Examples 
a. `Na2CO3 (s) + HCl (aq.)              2NaCl (aq.) + H2O + CO2(g)  
(Sodium carbonate) (Hydrochloric acid)             (Sodium chloride) 
b. NaHCO3 (s) + HCl (aq.)    NaCl (aq.) + H2O + CO2 (g) 
(Sodium hydroxide) (Hydrochloric acid)             (Sodium chloride) 
Note – Carbon dioxide is a colorless gas and can be identified by passing it through lime water or Calcium 
hydroxide. Carbon dioxide converts lime water milky.  
c. Ca(OH)2 (aq.) + CO2 (g)       CaCO3 (s) + H2O (l) 
(Limewater)         (Calcium carbonate) 
     (white precipitate) 
On passing more carbon dioxide through calcium carbonate following reaction occurs: 
CaCO3 (s) + H2O (l) + CO2 (g)     Ca(HCO3)2 (aq.) 
(Calcium carbonate)           (Calcium hydroxide) 
 
 
3. Reaction of acids and bases with each other 
 
Acids reacts with bases to form salt and water.  
Acid + Base    Salt + Water  
 
Examples –  
a. NaOH (aq.) + HCl (aq.)    NaCl (aq.) + H2O (l) 
b. Zn(OH)2(aq.) + H2SO4 (aq.)         ZnSO4 (aq.) + 2H2O (l)   
 
Similarities between acids and bases 
1. Both acids and bases conduct electricity when dissolved in water.  
Note – Pure water does not conduct electricity but when an acid or base is dissolved in it, it becomes a 
good conductor of electricity.  
 
2. Hydrolysis of acids and bases 
Acids release hydrogen ions (H
+
) in the presence of water  
 
HCl
  Water  
        H
+
 + Cl
-
 
The hydrogen ion released in the presence of water reacts with water molecule to produce a hydronium 
ion 
H
+
 + H2O           H3O
+
 
    (hydronium) 
Bases release hydroxide ions (OH
-
) in the presence of water 
 
NaOH (s) 
  Water  
Na
+
 (aq.) + (OH)
-
 (aq.) 
 
Note – The ions released by dissolving acids or base in water helps in the conduction of electricity. 
 
 
3. Neutralization reaction 
 Acid + Base    Salt + Water 
 
 H X + M OH     MX + H2O 
  
 H
+
 + OH
-
           H2O 
 
The hydrogen from the acid neutralizes the hydroxide from the base to for water molecule, which is 
neutral (neither acid nor base) and therefore this reaction is known as neutralization reaction. 
Note – Dissolving acids into water is an exothermic process. So, acid must be slowly added to water with 
constant shaking and not vice versa. The acid may splash out of the test tube if water is added to acid and not 
vice versa, causing burns to the person.  
Dissolving acid or base into water results in decrease in the number of hydrogen and hydroxide ions 
respectively leading to their dilutions.  
 
Potenz (power, potential) of hydrogen or pH 
pH is defined as the negative log of concentration of hydrogen present in a solution or liquid.  
pH = -log10[H
+
] or log10 1 
  [H
+
] 
Concept of log – Log1010 = 1; Log10 100 = Log10 10
2
 = 2; Log10 1000 = Log10 10
3
 = 3 
Log10 – here 10 is the base and the solution to log is the power of 10 that will give the value for which we are 
calculating log. For e.g. in the above example’s log of 1000 is 3 because 10 raised to the power 3 = 1000. 
The pH of the solutions can be a maximum of 14. A solution with the pH of 7 is said to be neutral. Pure water 
has a pH of 7. 
? The pH of bases is above 7 
? The pH of acids is below 7  
? The lesser the pH of a substance the more acidic it is. HCl having pH 1 is more acidic than dilute HCl 
with pH 4. 
? Lesser pH means higher number of hydrogen ions are present in the solution. A solution with pH 1 will 
have more hydrogen ions than a solution with pH 4  
? The higher the pH of a substance the more basic it is. NaOH with pH 13 is more basic than diluted 
NaOH with pH 8. 
? Higher pH means more concentration of number of hydroxide or lesser number of hydrogen ions are 
present in the solution. 
Measuring the pH 
? The pH of a substance can be measured using a universal pH indicator (in form of a paper strip), which 
contains a mixture of several dyes that produce a characteristic color at a particular pH. 
For example, a bright red color indicated pH 1, a bright blue color represents pH14 and green color 
indicates pH7. The colors and their respective pH are given on the cover of the pH indicator for 
reference.  
? pH meter is an electronic device that measures the concentration of hydrogen ions in a solution and 
converts it into pH to show that on a digital screen. 
 
Importance of pH 
1. Animals and plants can survive at a very narrow range of pH. Increase or decrease in pH can cause 
serious complications.  
2. If the pH of rain decreases less than 5.6 it is known as acid rain. Acid rain can cause various skin 
diseases and kill aquatic organisms by reducing the pH of the water bodies.  
3. A particular pH of the soil helps the microorganisms and plants to survive. 
4. Hydrochloric acid present in our stomach reduces the pH there and helps in digestion of food. Acidity is 
a condition that arises due to increase in the acid in the stomach. Antacids (Eno) or Bases are given to a 
person suffering from acidity, which neutralized the excessive acid.  
5. Reduction in pH of our teeth due to the production of acid by microorganisms leads to tooth decay. Acid 
corrodes the layer of Calcium phosphate that makes the enamel (outermost white layer) of the teeth.  It 
is recommended to brush your teeth after having food because toothpastes are basic and helps to 
neutralize the acid and clean the teeth of microbes.  
6. Several insects such as bees use acid (methanoic acid) in their stings as a defense mechanism against 
other animals. That is why it is recommended to rub the stung area by a base or metal to neutralize the 
acid present in the sting.  
 
Salts 
Salts are formed by the reaction of an acid and a base. 
? When a strong acid (low pH) reacts with a strong base (high pH) reacts with each other a neutral salt is 
produced (pH – 7) 
? When a strong acid reacts with a weak base an acidic salt (pH < 7) is produced.  
? When a weak acid reacts with a strong base a basic salt (pH > 7) is produced.   
 
Common Salt (NaCl) 
Common salt is used in everyday food items and is obtained from the sea. It is also produced from the 
reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH). 
Solution of sodium chloride is also known as brine. 
 
Chemicals produced from common salt 
1. NaOH (Sodium hydroxide)   
Page 5


Chapter 2 – Acids, Bases and Salts 
Acids and Bases 
Acid - Acids are molecules or ions that are able to donate hydrogen ion in an aqueous solution.  
For example – Hydrochloric acid (HCl), Sulphuric acid (H 2SO4), Carbonic acid (H2CO3).  
Base – Bases are the molecules or ions that are able to donate a hydroxide ion in an aqueous solution. For 
example – Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH) 2), Potassium hydroxide (KOH), Calcium 
oxide (CaO) 
Bases soluble in water are also known as alkali or alkaline.  
Acid-base indicators – The substance that give a characteristic color in the presence of acid or base or both are 
known as acid base indicators. E.g. Litmus solution, turmeric, methyl orange, phenolphthalein etc. 
 
 
 
 
 
Physical properties of acids 
a. Acids are sour in taste 
b. Acids change the color of blue litmus paper to red 
c. The pH of acids is less than 7. 
Physical properties of bases 
a. Bases are bitter in taste 
b. Bases change the color of red litmus paper to blue 
c. The pH of bases is more than 7 
Note – The highest pH that is possible is 14, therefore the pH of bases can be at most equal to 14. 
 
Chemical properties of Acids and Bases 
1. Reaction of Acids and Bases with Metals 
 
Metals + Acids/Bases        Salt + Hydrogen gas 
 
Examples: 
a. Zn (s) + 2HCl (aq.)            ZnCl2 + H2 (g) 
  (Zinc) (Hydrochloric acid)                     (Zinc chloride) (Hydrogen) 
b. Cu (s) + H2SO4 (aq.)        CuSO4 (aq.) + H2 (g) 
(Copper) (Sulphuric acid)   (Copper sulphate) (Hydrogen) 
c. Zn + 2NaOH            Na2ZnO4 + H2  
(Sodium hydroxide)   (Sodium zincate) 
Indicators Color change for acids Color change for bases 
Litmus solution Red Blue 
Turmeric powder N/A Red 
Methyl orange Orange Yellow 
Phenolphthalein Colorless Pink 
Note – Such reactions are not possible with all the metals. The “popup” sound produced by the matchstick or 
candle near the gas produced during a reaction is an indication of the presence of hydrogen gas.  
2. Reaction of Acids with metal carbonate and metal hydrogen carbonates 
 
Metal carbonates (MCO3) and bicarbonates (MHCO3) react with acids to produce salt, carbon dioxide 
and water.  
Examples 
a. `Na2CO3 (s) + HCl (aq.)              2NaCl (aq.) + H2O + CO2(g)  
(Sodium carbonate) (Hydrochloric acid)             (Sodium chloride) 
b. NaHCO3 (s) + HCl (aq.)    NaCl (aq.) + H2O + CO2 (g) 
(Sodium hydroxide) (Hydrochloric acid)             (Sodium chloride) 
Note – Carbon dioxide is a colorless gas and can be identified by passing it through lime water or Calcium 
hydroxide. Carbon dioxide converts lime water milky.  
c. Ca(OH)2 (aq.) + CO2 (g)       CaCO3 (s) + H2O (l) 
(Limewater)         (Calcium carbonate) 
     (white precipitate) 
On passing more carbon dioxide through calcium carbonate following reaction occurs: 
CaCO3 (s) + H2O (l) + CO2 (g)     Ca(HCO3)2 (aq.) 
(Calcium carbonate)           (Calcium hydroxide) 
 
 
3. Reaction of acids and bases with each other 
 
Acids reacts with bases to form salt and water.  
Acid + Base    Salt + Water  
 
Examples –  
a. NaOH (aq.) + HCl (aq.)    NaCl (aq.) + H2O (l) 
b. Zn(OH)2(aq.) + H2SO4 (aq.)         ZnSO4 (aq.) + 2H2O (l)   
 
Similarities between acids and bases 
1. Both acids and bases conduct electricity when dissolved in water.  
Note – Pure water does not conduct electricity but when an acid or base is dissolved in it, it becomes a 
good conductor of electricity.  
 
2. Hydrolysis of acids and bases 
Acids release hydrogen ions (H
+
) in the presence of water  
 
HCl
  Water  
        H
+
 + Cl
-
 
The hydrogen ion released in the presence of water reacts with water molecule to produce a hydronium 
ion 
H
+
 + H2O           H3O
+
 
    (hydronium) 
Bases release hydroxide ions (OH
-
) in the presence of water 
 
NaOH (s) 
  Water  
Na
+
 (aq.) + (OH)
-
 (aq.) 
 
Note – The ions released by dissolving acids or base in water helps in the conduction of electricity. 
 
 
3. Neutralization reaction 
 Acid + Base    Salt + Water 
 
 H X + M OH     MX + H2O 
  
 H
+
 + OH
-
           H2O 
 
The hydrogen from the acid neutralizes the hydroxide from the base to for water molecule, which is 
neutral (neither acid nor base) and therefore this reaction is known as neutralization reaction. 
Note – Dissolving acids into water is an exothermic process. So, acid must be slowly added to water with 
constant shaking and not vice versa. The acid may splash out of the test tube if water is added to acid and not 
vice versa, causing burns to the person.  
Dissolving acid or base into water results in decrease in the number of hydrogen and hydroxide ions 
respectively leading to their dilutions.  
 
Potenz (power, potential) of hydrogen or pH 
pH is defined as the negative log of concentration of hydrogen present in a solution or liquid.  
pH = -log10[H
+
] or log10 1 
  [H
+
] 
Concept of log – Log1010 = 1; Log10 100 = Log10 10
2
 = 2; Log10 1000 = Log10 10
3
 = 3 
Log10 – here 10 is the base and the solution to log is the power of 10 that will give the value for which we are 
calculating log. For e.g. in the above example’s log of 1000 is 3 because 10 raised to the power 3 = 1000. 
The pH of the solutions can be a maximum of 14. A solution with the pH of 7 is said to be neutral. Pure water 
has a pH of 7. 
? The pH of bases is above 7 
? The pH of acids is below 7  
? The lesser the pH of a substance the more acidic it is. HCl having pH 1 is more acidic than dilute HCl 
with pH 4. 
? Lesser pH means higher number of hydrogen ions are present in the solution. A solution with pH 1 will 
have more hydrogen ions than a solution with pH 4  
? The higher the pH of a substance the more basic it is. NaOH with pH 13 is more basic than diluted 
NaOH with pH 8. 
? Higher pH means more concentration of number of hydroxide or lesser number of hydrogen ions are 
present in the solution. 
Measuring the pH 
? The pH of a substance can be measured using a universal pH indicator (in form of a paper strip), which 
contains a mixture of several dyes that produce a characteristic color at a particular pH. 
For example, a bright red color indicated pH 1, a bright blue color represents pH14 and green color 
indicates pH7. The colors and their respective pH are given on the cover of the pH indicator for 
reference.  
? pH meter is an electronic device that measures the concentration of hydrogen ions in a solution and 
converts it into pH to show that on a digital screen. 
 
Importance of pH 
1. Animals and plants can survive at a very narrow range of pH. Increase or decrease in pH can cause 
serious complications.  
2. If the pH of rain decreases less than 5.6 it is known as acid rain. Acid rain can cause various skin 
diseases and kill aquatic organisms by reducing the pH of the water bodies.  
3. A particular pH of the soil helps the microorganisms and plants to survive. 
4. Hydrochloric acid present in our stomach reduces the pH there and helps in digestion of food. Acidity is 
a condition that arises due to increase in the acid in the stomach. Antacids (Eno) or Bases are given to a 
person suffering from acidity, which neutralized the excessive acid.  
5. Reduction in pH of our teeth due to the production of acid by microorganisms leads to tooth decay. Acid 
corrodes the layer of Calcium phosphate that makes the enamel (outermost white layer) of the teeth.  It 
is recommended to brush your teeth after having food because toothpastes are basic and helps to 
neutralize the acid and clean the teeth of microbes.  
6. Several insects such as bees use acid (methanoic acid) in their stings as a defense mechanism against 
other animals. That is why it is recommended to rub the stung area by a base or metal to neutralize the 
acid present in the sting.  
 
Salts 
Salts are formed by the reaction of an acid and a base. 
? When a strong acid (low pH) reacts with a strong base (high pH) reacts with each other a neutral salt is 
produced (pH – 7) 
? When a strong acid reacts with a weak base an acidic salt (pH < 7) is produced.  
? When a weak acid reacts with a strong base a basic salt (pH > 7) is produced.   
 
Common Salt (NaCl) 
Common salt is used in everyday food items and is obtained from the sea. It is also produced from the 
reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH). 
Solution of sodium chloride is also known as brine. 
 
Chemicals produced from common salt 
1. NaOH (Sodium hydroxide)   
The aqueous solution of sodium chloride (NaCl + water) produces sodium hydroxide (NaOH), chlorine 
and hydrogen gas when electricity is passed through it. The process is also known as chlor-alkali 
process.  
 
2NaCl (aq.) + 2H2O(l)
 
    
Electricity  
  2NaOH (aq.) + Cl2 (g) + H2(g) 
 
Hydrogen gas is released at the cathode or negative terminal of the setup while chlorine is released at the 
anode or positive terminal of the setup.       
 
? Hydrogen is used as a fuel in rockets and production of fertilizers 
? Chlorine is used in the treatment of water of swimming pools, disinfectant, PVCs (poly vinyl 
chloride pipes), CFCs and pesticides.  
? Sodium hydroxide is used to make soaps and detergents, degreasing, making paper and 
producing artificial fibers.  
? Hydrogen and chlorine are used to make hydrochloric acid 
? Sodium hydroxide and chlorine are used to make bleaching agents. 
 
 
2. Bleaching powder (CaOCl2) 
The chlorine gas produced during electrolysis of brine (aqueous solution of NaCl) is used in the 
production of bleaching powder 
 
Ca(OH)2 + Cl2      CaOCl2 + H2O 
(Calcium hydroxide)    (Bleaching powder) 
 
? Bleaching powder is used in the textile industry for bleaching of wood pulp to produce fibers.  
? It is used to bleach clothes after washing. 
? It is used as an oxidizing agent in chemical factories. 
? It is used as a disinfectant to kill microbes in water.  
 
3. Baking soda (sodium bicarbonate) (NaHCO3) 
It is produced by the reaction of brine with carbon dioxide and ammonia. The aqueous solution of 
sodium chloride reacts with carbon dioxide and ammonia gas to produce ammonium chloride and 
sodium bicarbonate.  
 
NaCl + H2O + CO2 + NH3     NH4Cl + NaHCO3 
          (Ammonia)     (Ammonium chloride) (Sodium bicarbonate) 
? Sodium bicarbonate is a mild based and is used as an antacid to treat acidity.  
? The carbon dioxide produced on mixing a mild edible acid to the baking soda is used to make the bread 
and cake fluffy.  
 NaHCO3 + H
+ 
     CO2 + H2O + Sodium salt of acid (depends upon the acid) 
Note – Hydrogen ions in the above reaction comes form the edible acid mixed with the baking soda.  
? It is used in fire extinguishers. The carbon dioxide produces while reacting with an acid is used to 
extinguish fire.  
 
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