CBSE Class 10 Science Notes Chapter 2 Acids Bases And Salts:Download PDF Here
Introduction to Acids, Bases and Salts
Classification of matter
On the basis of
a) composition – elements, compounds and mixtures
b) state – solids, liquids and gases
c) solubility – suspensions, colloids and solutions
Types of mixtures – homogeneous and heterogeneous
Types of compounds – covalent and ionic
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What Is an Acid and a Base?
Ionisable and non-ionisable compounds
An ionisable compound when dissolved in water or in its molten state, dissociates into ions almost entirely. Example: NaCl, HCl, KOH, etc.
A non-ionisable compound does not dissociate into ions when dissolved in water or in its molten state. Example: glucose, acetone, etc.
For More Information On Acids and Bases, Watch The Below Video:
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Students can refer to the short notes and MCQ questions along with separate solution pdf of this chapter for quick revision from the links below:
- Acids, Bases and Salts Short Notes
- Acids, Bases and Salts MCQ Practice Questions
- Acids, Bases and Salts MCQ Practice Solutions
Arrhenius theory of acids and bases
Arrhenius acid – when dissolved in water, dissociates to give H+ (aq) or H3O+ ion.
Arrhenius base – when dissolved in water, dissociates to give OH− ion.
Examples
Acids
- Hydrochloric acid (HCl)
- Sulphuric acid (H2SO4)
- Nitric acid (HNO3)
Bases
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Calcium hydroxide (Ca(OH)2)
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Bronsted Lowry theory
A Bronsted acid is an H+ (aq) ion donor.
A Bronsted base is an H+ (aq) ion acceptor.
Example
In the reaction: HCl (aq) + NH3 (aq) → NH+4(aq) + Cl− (aq)
HCl – Bronsted acid and Cl− : its conjugate acid
NH3 – Bronsted base and NH+4 : its conjugate acid
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Physical test
Given are two possible physical tests to identify an acid or a base.
a. Taste
An acid tastes sour whereas a base tastes bitter.
The method of taste is not advised as an acid or a base could be contaminated or corrosive.
b. Effect on indicators by acids and bases
An indicator is a chemical substance which shows a change in its physical properties, mainly colour or odour when brought in contact with an acid or a base.
Below mentioned are commonly used indicators and the different colours they exhibit:
a) Litmus
In a neutral solution – purple
In acidic solution – red
In basic solution – blue
Litmus is also available as strips of paper in two variants – red litmus and blue litmus.
An acid turns a moist blue litmus paper to red.
A base turns a moist red litmus paper to blue.
b) Methyl orange
In a neutral solution – orange
In acidic solution – red
In basic solution – yellow
c) Phenolphthalein
In a neutral solution – colourless
In acidic solution – remains colourless
In basic solution – pink
Acid-Base Reactions
Reactions of acids and bases
a) Reaction of acids and bases with metals
Acid + active metal → salt + hydrogen + heat
2HCl + Mg → MgCl2 + H2 (↑)
Base + metal → salt + hydrogen + heat
2NaOH + Zn → Na2ZnO2 + H2 (↑)
A more reactive metal displaces the less reactive metal from its base.
2Na + Mg (OH) 2 → 2NaOH + Mg
b) Reaction of acids with metal carbonates and bicarbonates
Acid + metal carbonate or bicarbonate → salt + water + carbon dioxide.
2HCl + CaCO3 → CaCl2 + H2O + CO2
H2SO4 + Mg (HCO3)2 → MgSO4 + 2H2O + 2CO2
Effervescence indicates liberation of CO2 gas.
c) Neutralisation reaction
1. Reaction of metal oxides and hydroxides with acids
Metal oxides or metal hydroxides are basic in nature.
Acid + base → salt + water + heat
H2SO4 + MgO → MgSO4 + H2O
2HCl + Mg (OH) 2 → MgCl2 + 2H2O
2. Reaction of non-metal oxides with bases
Non-metal oxides are acidic in nature
Base + Nonmetal oxide → salt + water + heat
2NaOH + CO2→ Na2CO3 + H2O
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Water
Acids and bases in water
When added to water, acids and bases dissociate into their respective ions and help in conducting electricity.
Difference between a base and an alkali
Base:
- Bases undergo neutralisation reaction with acids.
- They are comprised of metal oxides, metal hydroxides, metal carbonates and metal bicarbonates.
- Most of them are insoluble in water.
Alkali:
- An alkali is an aqueous solution of a base, (mainly metallic hydroxides).
- It dissolves in water and dissociates to give OH− ion.
- All alkalis are bases, but not all bases are alkalis.
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Hydronium ion
Hydronium ion is formed when a hydrogen ion accepts a lone pair of electrons from the oxygen atom of a water molecule, forming a coordinate covalent bond.
Dilution
Dilution is the process of reducing the concentration of a solution by adding more solvent (usually water) to it.
It is a highly exothermic process.
To dilute acid, the acid must be added to water and not the other way round.
Strength of acids and bases
Strong acid or base: When all molecules of a given amount of an acid or a base dissociate completely in water to furnish their respective ions, H+(aq) for acid and OH−(aq) for base).
Weak acid or base: When only a few of the molecules of a given amount of an acid or a base dissociate in water to furnish their respective ions, H+(aq) for acid and OH−(aq) for base).
Dilute acid: contains less number of H+(aq) ions per unit volume.
Concentrated acid: contains more number of H+(aq) ions per unit volume.
Universal indicator
A universal indicator has a pH range from 0 to 14 that indicates the acidity or alkalinity of a solution.
A neutral solution has pH=7
pH
pH=−log10[H+]
In pure water, [H+]=[OH−]=10−7 mol/L. Hence, the pH of pure water is 7.
The pH scale ranges from 0 to 14.
If pH < 7 → acidic solution
If pH > 7→ basic solution
pH scale
Importance of pH in everyday life
1. pH sensitivity of plants and animals
Plants and animals are sensitive to pH. Crucial life processes such as digestion of food, functions of enzymes and hormones happen at a certain pH value.
2. pH of a soil
The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.
3. pH in the digestive system
The process of digestion happens at a specific pH in our stomach which is 1.5 to 4.
The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.
4. pH in tooth decay
Tooth decay happens when the teeth are exposed to an acidic environment of pH 5.5 and below.
5. pH of self-defence by animals and plants
Acidic substances are used by animals and plants as a self-defence mechanism. For example, bee and plants like nettle secrete a highly acidic substance for self-defence. These secreted acidic substances have a specific pH.
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Manufacture of Acids and Bases
Manufacture of acids and bases
a) Nonmetal oxide + water → acid
SO2(g) + H2O(l) → H2SO3(aq)
SO3(g) + H2O(l) → H2SO4(aq)
4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
Non-metal oxides are thus referred to as acid anhydrides.
b) Hydrogen + halogen → acid
H2(g) + Cl2(g) → 2HCl(g)
HCl(g) + H2O(l) → HCl(aq)
c) Metallic salt + conc. sulphuric acid → salt + more volatile acid
2NaCl(aq) + H2SO4(aq) → Na2SO4(aq) + 2HCl(aq)
2KNO3(aq) + H2SO4(aq) → K2SO4(aq) + 2HNO3(aq)
d) Metal + oxygen → metallic oxide (base)
4Na(s) + O2(g) → 2Na2O(s)
2Mg(s) + O2(g) → 2MgO(s)
e) Metal + water → base or alkali + hydrogen
Zn(s) + H2O(steam) → ZnO(s)+ H2(g)
f) Few metallic oxides + water → alkali
Na2O(s) + H2O(l) → 2NaOH(aq)
g) Ammonia + water → ammonium hydroxide
NH3(g) + H2O(l) → NH4OH(aq)
For More Information On Uses of Acids and Bases, Watch The Below Video:
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Salts
Salts
A salt is a combination of an anion of an acid and a cation of a base.
Examples – KCl, NaNO3 ,CaSO4, etc.
Salts are usually prepared by the neutralisation reaction of an acid and a base.
Common salt
Sodium Chloride (NaCl) is referred to as common salt because it’s used all over the world for cooking.
Family of salts
Salts having the same cation or anion belong to the same family. For example, NaCl, KCl, LiCl.
pH of salts
A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.).
A salt of a weak acid and a strong base will be basic in nature. pH > 7.
A salt of a strong acid and a weak base will be acidic in nature. pH < 7.
The pH of a salt of a weak acid and a weak base is determined by conducting a pH test.
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Preparation of Sodium hydroxide
Chemical formula – NaOH
Also known as – caustic soda
Preparation (Chlor-alkali process):
Electrolysis of brine (solution of common salt, NaCl) is carried out.
At anode: Cl2 is released
At cathode: H2 is released
Sodium hydroxide remains in the solution.
Bleaching powder
Chemical formula – Ca(OCl)Cl or CaOCl2
Preparation – Ca(OH)2(aq)+Cl2(g)→CaOCl2(aq)+H2O(l)
On interaction with water – bleaching powder releases chlorine which is responsible for bleaching action.
Baking soda
Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO3
Preparation (Solvay process):
a. Limestone is heated: CaCO3→CaO+CO2
b. CO2 is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)+NH3(g)+CO2(g)+H2O(l)→NaHCO3(aq)+NH4Cl(aq)
Uses:
1. Textile industry
2. Paper industry
3. Disinfectant
Washing soda
Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO3
Preparation (Solvay process) –
a. Limestone is heated: CaCO3 → CaO + CO2
b. CO2 is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq) + NH3(g) + CO2(g) + H2O(l) → NaHCO3(aq) + NH4Cl(aq)
Uses
1. In glass, soap and paper industries
2. Softening of water
3. Domestic cleaner
Crystals of salts
Certain salts form crystals by combining with a definite proportion of water. The water that combines with the salt is called water of crystallisation.
Plaster of paris
Gypsum, CaSO4.2H2O (s) on heating at 100°C (373K) gives CaSO4. ½ H2O and 3/2 H2O
CaSO4. ½ H2O is plaster of paris.
CaSO4. ½ H2O means two formula units of CaSO4 share one molecule of water.
Uses – cast for healing fractures.
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Frequently Asked Questions on Acids, Bases and Salts
When dissolved in water, Arrhenius base is a compound that ……….?
An Arrhenius base is a compound that increases the concentration of OH– ions that are present when added to water.
A solution X has a pH value of 2 and another solution Y has a pH value of 1. What can be inferred regarding the difference in hydrogen ion concentration between them?
pH is a measure of hydrogen ion concentration in a solution. Higher the hydrogen ion concentration, lower is the pH. Acids which give rise to more hydrogen ions are more acidic than the acids which give less hydrogen ions. Thus, lower the pH, higher is the acidic nature of the solution. Thus, Y has more hydrogen ion concentration than X.
Dilute hydrochloric acid (HCl) reacts with metals to evolve __ gas along with the formation of corresponding metal salt.
Acids react with metals to produce the respective metal salt along with hydrogen gas. Therefore, when hydrochloric acid (HCl) reacts with metals it evolve hydrogen gas along with the formation of corresponding metal salt.
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