Class X - NCERT
(CBSE) Science (Chemistry)
Chapter 2, Acids, Bases and Salts
Chemistry (Science) Notes
Some Points to Remember
=> Olfactory indicators are some
substances whose odour changes in acidic or basic media.
=> Acid +
Metal --> Salt + H2
=> Metals
react with bases and replace the hydrogen to form salt.
Base + Metal ---> Salt + H2
(however, all metals do not react with bases).
=> Acids react
with metal oxides to produce metal salts.
Metal oxide + Acid ---> Salt + Water.
(Since, this reaction is similar to the reaction of bases with acids so, metal
oxides are also known as basic oxides).
=>
Non-metallic oxides react with bases to produce salt and water. Since, this
reaction is similar to the reaction of acids with bases so, non-metal oxides
are also known as acidic oxides.
=> pH is
the measure of Acidity or Alkalinity of a solution. The term pH stands for
“Potential of hydrogen”.
=> If the
pH rain water drops below 5.6, it is called acid rain.
=> Acid
rain corrodes buildings made with marble such as, Taj Mahal. Marble (CaCO3)
and stone generally contains metal carbonates which react with various acids
like - Nitric Acid, Sulphuric Acid etc. present in the acid rain.
=> Neutralization Reaction: The process
of treating an acid with a base to form salt and water is called
neutralization. a neutralization reaction can be written as,
Acid + Base ---> Salt + Water + Heat
Hence, it is an exothermic process.
=> Alkalies: The bases
that dissolve in water are known as alkalies. All bases are not
alkalies, but all alkalies are bases.
=> Most
reactions in human bodies take place in a pH range of 7.0 - 7.8 Living
organisms can survive only in a narrow range of pH change.
Sample Questions
Q1: Define (i) Acid (ii)
Bases (iii) Salt
Answer: Acid: A substance which
liberates hydrogen ions in aqueous solution, sour in taste, and turns blue
litmus to red. Metals react with acids to form salts and liberate hydrogen gas.
Many acids are corrosive such as, HNO3, H2SO4,
HF, etc.
Base: A substance which reacts with an acid to form a salt and water only. If
dissolved in water, they give hydroxyl ions (OH–) ions. Bases are
bitter in taste and change the colour of red litmus to blue.
Salt: a chemical compound formed when the hydrogen of an acid has been replaced
by a metal. a salt is produced together with water, when an acid reacts with a
base. Salts are named according to the acid and the metal from which the salt
is derived. For example, ‘sodium sulphate’ is a salt derived from sodium metal
and sulphuric acid.
Q2: What is Universal
Indicator?
Answer: Universal Indicator is a mixture
of several indicators. The universal indicator shows different colours at
different concentration of hydrogen ions in a solution. Thus, with the help of
a universal indicator we can quantitatively judge how strong a given acid or
base is.
Q3: Why is it advised to
clean mouth after consuming food? or
pH change is a cause for toot decay.
Explain.
Answer: Acids are produced in the mouth
due to the degradation of sugar and food particles which remain in mouth after
eating, by certain bacteria. As a result the pH of mouth is lowered. Tooth
enamel made up of calcium phosphate is corroded when the pH in the mouth is
lower than 5.5 So, tooth decay starts when the pH of the decreases below 5.5
Therefore, it is advised to clean the
mouth after eating food with toothpastes which are generally basic.
Q4: How sodium hydroxide
is produced? or
What is ‘Chlor-alkali’ process? or
What do we obtain by electrolysis of
brine?
Answer: Sodium hydroxide is produced by
electrolysis of aqueous solution of sodium chloride (NaCl) or Brine.
electrolysis of brine results in the decomposition of NaCl and formation of NaOH.
This process is called ‘chlor-alkali’ process because the products formed - chlor for chlorine and alkali for sodium
hydroxide.
2NaCl (aq) + 2H2O(liq)
--> 2NaOH (aq) + Cl2(g) + H2(g)
Chlorine is given off at the anode
while hydrogen at cathode.
Q5: Give some uses of
sodium hydroxide.
Answer: Sodium hydroxide is a very useful
substance. It is used in the manufacturing of paper, soaps, detergents, and
synthetic fibers. It is also useful in the manufacturing of house-hold bleaches
and dyes.
Q6: How bleaching powder
is formed? Give some uses of bleaching powder.
Answer: Chlorine gas obtained by the
electrolysis of brine is used for the manufacturing of bleaching powder.
Bleaching powder is produced by the action of chlorine on dry slaked lime
[Ca(OH)2] as per the following equation -
Ca(OH)2 + Cl2 -->
CaOCl2 + H2O
Uses of Bleaching Powder
1. Disinfecting drinking water to make
it free from germs.
2. As an oxidizing agent in many
chemical industries.
3. As a bleaching agent in textile
industry, paper industry, etc.
Q8: Give the chemical
equation for the production of baking soda.
Answer: The chemical name of baking soda
is Sodium Hydrogen Carbonate (NaHCO3). It is produced using sodium
chloride as one of the raw materials as per the given equation -
NaCl + H2O + CO2
+ NH3 --> NH4Cl + NaHCO3
Q9: What is baking
powder?
Answer: Baking Powder is a mixture of
baking soda and a mild edible acid such as tartaric acid. When baking powder is
heated or mixed in water, the following reaction takes place -
NaHCO3 + H+
--> CO2 + H2O + sodium salt of acid.
Carbon dioxide produced during the
reaction causes bread or cake to rise making them soft and spongy.
Q10: What happens when we
heat sodium hydrogen carbonate (baking soda)?
Answer: By heating sodium hydrogen
carbonate we get sodium carbonate (Na2CO3).
The sodium carbonate after
recrystallization produces washing soda.
Na2CO3 + 10H2O
--> Na2CO3.10H2O
Q11: What is ‘water of
crystallization’?
Answer: Water of recrystallization is a
fixed number of water molecules present in one formula unit of salt. For
example, in washing soda (Na2CO3.10H2O) there
are 10 water molecules present as water of crystallization.
The water of crystallization remain in
chemical combination with crystal. It is necessary for the maintenance of
crystalline properties of the crystal. It can be removed by sufficient
heat.
Q12: What are hydrated
salts? Give an example.
Answer: Salts containing a fixed number of
water molecules in their crystal structure are called hydrated salts.
A molecule of sodium carbonate (Na2CO3.10H2O)
contains ten molecules of water. This is known as hydrated salt of sodium
carbonate.
Q13: Why does an indicator
change its colour in an acidic or basic solution?
Answer: An indicator is itself a weak acid
or base. When added to an acidic or basic solution, it accepts or loses protons
de[pending on the conditions, and this rearranges the distribution of electrons
of its molecules. The colour of the indicator changes because of this
redistribution.
Q14: Name some chemicals
obtained by using sodium chloride (common salt) as a raw material.
Answer: Sodium hydroxide (NaOH), Baking
soda (NaHCO3), Washing soda (Na2CO3.10H2O),
Bleaching powder (CaOCl2), etc.
Q15: What are strong and
weak acids?
Answer: acids that give rise to more H+
ions are said to be strong acids and acids that give less H+
ions are said to be weak acids. a strong acid has pH value closer to zero while
acids with higher pH values or closer to 7 are weak acids.
Q16: What is ‘Plaster of
Paris’?
Answer: On heating gypsum at 3730 K,
it loses water molecules and becomes calcium sulphate hemihydrates (CaSO4.1/2
H2O). This is called ‘Plaster of Paris’.
Further Study on Chapter 2, Acids, Bases and
Salts
- 10th CBSE Science (Chemistry) | Chapter 2, Acids, Bases and Salts | NCERT Science Textbook Exercise Solution [Read]
- CBSE Xth Science (Chemistry) | Acids, Bases and Salts | NCERT Solutions for In-Text Questions [Read]
- CBSE Class 10 NCERT Science (Chemistry) - Multiple Choice Questions (mcq) | Chapter 2, Acids, Bases and Salts [Read]
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