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Class X
CH -2 Acid, Base and Salt
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Syllabus of Acids, bases and salts: Their definitions in terms of furnishing of H+
and OH- ions, General properties, examples and uses, concept
of pH scale(Definition relating to logarithm not required), importance of pH
in everyday life; preparation and uses of sodium hydroxide, Bleaching
powder, Baking soda, Washing soda and Plaster of Paris.
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1. Acids - Acids are
sour in taste and change the colour of blue litmus to red, whereas, bases are
bitter in the test and change the colour of the red litmus to blue.
2. Litmus - Litmus is a natural indicator, turmeric is another natural indicator for the base.
Another examples of natural indicators like litmus, red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia, and Geranium, which indicate the presence of acid or base in a solution.
3. The stain of
curry on a white cloth becomes reddish-brown when soap, which is basic in
nature, is scrubbed on it. It turns yellow again when the cloth is washed with
plenty of water.
4. Acid Base Indicator - Red litmus
solution, Blue litmus solution, Phenolphthalein solution - Methyl orange
solutions are the indicators that tell us whether a substance is acidic or
basic by a change in colour.
5. Synthetic Indicator - Some synthetic
indicators are methyl orange and phenolphthalein used to test for
acids and bases.
6. Olfactory indicator - There are some substances whose odour changes in acidic or in basic media. These are called olfactory indicators. Onion, Vanilla, and clove are the olfactory indicators.
Onion: It loses its smell in Basic solution but not in acidic solution.
Vanilla: It loses its smell in basic solution but not in acidic solution and
Cloves: It loses its smell in basic solution but not in acidic solution.
7. Alkali - An alkali is
a base that dissolves in water. They are soapy to touch, bitter and corrosive.
8. The similarity in properties of acid and base-
i. Acids and some bases generate hydrogen gas on reacting with metals.
ii. Both are electrolytes.
iii. Both forms ion in the presence of water.
9. Glucose and Alcohol do not conduct electricity- electric
current is carried through the solution by ions. Glucose and alcohol solutions
do not contain ions. So they do not conduct electricity.
10. Dry acid and
base can’t show their acidic and basic property - because in the presence of the water they form hydronium ion and hydroxyl ion which are responsible for
conducting electricity
11. While mixing
concentrated nitric acid or sulphuric acid with water, the acid must
always be added slowly to water with constant stirring - because it is
an exothermic reaction and a large amount of heat is generated by it, that is
absorbed by water. If water is added to a concentrated acid, the heat generated
may cause the mixture to splash out and cause burns. The glass container may
also, break due to excessive local heating.
12. Dilution - Mixing an acid or base with water
results in a decrease
in the concentration of ions (H3O+/OH– ) per unit volume. Such a
process is called dilution.
13 pH scale - The scale for measuring
hydrogen ion concentration in a solution, called pH scale. The pH scale can
measure pH from 0 (strongly acidic) to 14 (strongly alkaline). The pH of a
neutral solution is 7. Values less than 7 on the pH scale represent an acidic
solution and value more than 7 on the pH scale represents a basic solution.
14. pH value of Salt - Salts of a
strong acid and a strong base are neutral with a pH value of 7. On the other
hand, salts of a strong acid and weak base are acidic with pH value less than 7
and those of a strong base and weak acid are basic, with pH value more than 7.
15. Acid rain - When the pH of rainwater is
less than 5.6, it is called acid rain. When
acid rain flows into the rivers, it lowers the pH of the river water. The
survival of aquatic life in such rivers becomes difficult.
16. An -atmosphere
of venus is made up of thick white and yellowish clouds of sulphuric
acid.
17. Acids in our stomach-Our stomach
produces hydrochloric acid. It helps in the digestion of food
without harming the stomach. During indigestion, the stomach produces too much
acid and this causes pain and irritation. To get relief from this pain, people
use bases called antacids. These
antacids neutralize the excess acid. Magnesium hydroxide (Milk
of magnesia), a mild base, is often used for this purpose.
18. Our tooth - Tooth enamel,
made up of calcium
phosphate is the hardest substance in the body. It does not dissolve
in water but is corroded when the pH in the mouth is below 5.5. Bacteria present
in the mouth produce acids by the degradation of sugar and food particles
remaining in the mouth after eating. We should use toothpaste which is
generally basic, after eating food and neutralizes the excess acid and
prevent tooth decay.
19. Bee-sting
leaves an acid which causes pain and irritation. If we use a base like baking
soda on the stung area give relief. Stinging hair of nettle leaves injects methanoic
acid causing burning pain.
20. Acid-base
indicators - are dyes or mixtures of dyes that are used to indicate the presence
of acids and bases.-
21. Neutralization reaction- The reaction between an acid and a base to give a salt and water is known as a neutralization reaction.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) . Hydrogen seems to be common to all acids.
22. All bases are not alkali - All bases
(metallic oxides) do not dissolve in water. An alkali is a base
that dissolves in water. Ex - NaOH, Ca(OH)2, KOH etc.I
23. Acidic nature of a substance is due to the formation of H+(aq) ions in solution. Formation of
OH– (aq) ions in solution are responsible for the basic nature of a substance.
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.
24. Reactions of Acids and base -
i. When an acid
reacts with a metal, hydrogen gas is evolved and a
corresponding salt is formed.
ii. When a base
reacts with a metal, along with the evolution of hydrogen gas, a
salt is formed which a negative ion has composed of the metal and oxygen.
iii. When an acid
reacts with a metal carbonate or metal hydrogen carbonates, it gives the
corresponding salt, carbon dioxide gas, and water.
25. Acidic and
basic solutions in water conduct electricity - because they produce hydrogen and
hydroxide ions respectively.
26. Rock Salt - Large
crystals of sodium chloride salt which is brown due to impurities are called rock salt.
27. Common
salt ( The raw material for chemicals )- Sodium hydroxide, baking soda, washing soda, bleaching
powder and many more are the raw material for chemicals.
28. Chlor-alkali process - When electricity is passed through an aqueous solution of sodium chloride called brine, it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlorine and alkali sodium hydroxide.
2NaCl(aq) + 2H2O(l) → 2NaOH(aq)(formed near cathode) + Cl2(g)(formed at the anode) + H2(g)
( formed at the cathode)
Brine - Aqueous solution of sodium
chloride is called brine.
29. Baking powder- is a mixture of baking soda (sodium hydrogen carbonates) and a mild edible acid
such as tartaric acid.
30. 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
31. Copper sulfate crystals
are blue in colour, contain water of crystallization. When we heat the
crystals, this water is removed and the salt turns white. If we moisten the
crystals again with water, we will find that the blue colour of the crystals
reappears.
32. Plaster of
Paris - On heating gypsum at 373 K, it
loses water molecules and becomes calcium sulfate hemihydrate (CaSO4 . 1/2H2O ).
This is called Plaster of Paris, the substance which doctors use as plaster for
supporting fractured bones in the right position.
Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.
CaSO4 .1/ 2 H2O+1/2H2 O = CaSO4 .2H2O
(Plaster of Paris) (Gypsum)
33. Recrystallization
of sodium carbonate gives washing soda. It is also a basic salt.
Na2CO3 +
10H2O= Na2CO3 .10 H2O
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