CHEMISTRY STUDY NOTES
Acids Bases and Salts
4. Indicators - Indicators are substances which indicate the acidic or basic nature of the solution by
their colour change.
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5. Olfactory indicators: Substances which exhibit different odour in acidic and basic solution. Example:
Onion has characteristic smell in acidic solution which completely gets destroyed inbasic medium.
Vanilla has a characteristic smell in acidic solution and loses its smell in bases.
6. Chemical properties of acids
(i) Acids react with active metals to give hydrogen gas.
2Na + 2HCl → 2NaCl + H2
Fe + 2HCl → FeCl2 + H2 Zn +
H2SO4 → ZnSO4 + H2
Test for hydrogen gas: The gas evolved after reaction of acid with metal can be tested by bringing a
lighted candle near it. If the gas burns with pop sound, then it confirms the evolution of hydrogen gas.
Burning with pop sound is the characteristic test for hydrogen gas.
(ii) Acids react with metal carbonate and metals hydrogen carbonate to give carbon dioxide.
Na2CO3 + HCl → NaCl + H2O+CO2
MgCO3 + 2HCl → MgCl2 + CO2 + H2O
CaCO3 + 2HCl → CaCl2 + CO2 + H2O
CaCO3 + H2SO4 → CaSO4 + CO2 + H2O
2HNO3 + Na2CO3 → NaNO3 + 2H2O + CO2
Acid + Metal hydrogen carbonate → Salt + Carbon dioxide + Water
NaHCO3 + HCl → NaCl + CO2 + H2O
2NaHCO3 + H2SO4 → Na2SO4 + 2CO2 + 2H2O
The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate
turns limewater milky. This shows that the gas is carbon dioxide gas. This happens because of
formation ofwhite precipitate of calcium carbonate.
Ca(OH)2 + CO2 → CaCO3 + H2O CaCO3 +
CO2 + H2O → Ca(HCO3)2
But when excess of carbon dioxide is passed through limewater, milkiness of lime water disappear.
This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is
soluble in water, thus the milky colour of solution mixture disappears.
(iii) Acids react with bases to give salt and water. This reaction is called neutralization reaction.
NaOH +HCl → NaCl + H2O+CO2
(iv) Acids react with metals oxides to give salt and water.
CuO + H2SO4 → CuSO4 + H2O
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7. Chemical properties of Bases
(i) Reaction with Metals – Certain metals such as Zinc, Aluminum, Tin react with Alkalisolutions
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 – Non-metallic oxides are generally acidic in nature.They
react with bases to form salt and water.
2NaOH + CO2 → Na2CO3 + H2O
8. Strong Acids are those acids which completely dissociates into ions in aqueous solution iscalled
strong acid such as HCl, H2SO4, and HNO3 etc.
Weak acids are those which are partially dissociated in its aqueous solution such as
CH3COOH, H2CO3, and HCN etc.
9. Strong Bases are bases such as NaOH or KOH which completely dissociate in aqueoussolution
is called a strong base.
Weak Bases are those bases which partially dissociated in aqueous solution such as NH4OH.
Common in Acid and Base
Acids give hydrogen gas when they react with metal. This shows that all acids contain
hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3),
etc.
When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in
aqueous solution is the common property in all acids. Because of dissociation of hydrogen ion in
aqueous solution, an acid shows acidic behavior.
Example: Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl−) when it isdissolved in
water.
HCl (aq) ⇨ H+ + Cl−
Sulphuric acid (H2SO4) gives hydrogen ion (H+) and sulphate ion (SO4− −) in water.
H2SO4 (aq) ⇨ H+ + SO4− −
Nitric acid (HNO3) gives hydrogen ion (H+) and nitrate ion (NO3−) in water.
HNO3 (aq) ⇨ H+ + NO3−
Acetic acid (CH3COOH) gives acetate ion (CH3COO−) and hydrogen ion (H+).
CH3COOH (aq) ⇨ CH3COO− H+
Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the
form of hydronium ion (H3O−) in aqueous solution. That’s why hydrogen ion isalways written with
suffix (aq), such as H+ (aq).
HCl + H2O ⇨ H3O+ + Cl− H2SO4 +
H2O ⇨ H3O+ + SO4− −
Thus, because of dissociation of hydrogen ions; acid shows its acidic behavior.
Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion.
Hydrogen ion in aqueous solution conducts electricity.
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Note: A dry acid, such as dry hydrochloric acid does not change the colour of blue litmuspaper to
red because a dry acid does not dissociate hydrogen ion. This is the cause that a moist litmus
paper is used to check the acidic or basic character of a gas.
Acidic behavior of carbon dioxide gas: Carbon dioxide gas produces carbonic acid whendissolved in
water. This carbonic acid dissociates hydrogen ion and carbonate ion in the aqueous solution.
CO2 + H2O ✑ H2CO3 → H+ + CO3− −
Are all compounds which contain hydrogen necessarily acids?
No, all compounds which contain hydrogen are not acid. For example, glucose (C6H12O6),methyl
alcohol (CH3OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because
these compounds do not dissociate hydrogen ion in their aqueous solution.
Common in all Base:
A base dissociates hydroxide ion in water, which is responsible for the basic behavior of acompound.
Example:
When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.
NaOH (aq) → Na+ + OH−
Similarly, when potassium hydroxide is dissolved in water, it dissociates hydroxide ion andpotassium
ion.
KOH (aq) → K+ + OH−
Thus, base shows its basic character because of dissociation of hydroxide ion.
10. pH Scale: The concentration of hydrogen ions in solution is expressed in terms of pH. The pH of a
solution is defined as the negative logarithm of hydrogen ion concentration in molesper litre.
pH =-log [H+]
pH =-log [H3O+] where [H+] or [H3O+] represents concentrations of hydrogen ions insolution.
For water or neutral solutions, pH =7
For acidic solutions, pH< 7 For basic solutions, pH > 7
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pH in PLANTS
Plants require a specific pH of the soil for their proper growth. Plants cannot grow in too alkaline
or too acidic soils. Therefore, for proper cultivation of crops it is important to haveknowledge of
the pH of the soil. If the soil is too acidic then its pH can be raised by addinglime (or slaked lime)
which neutralizes the excess acid in the soil.
Similarly, if the soil is too alkaline then its pH can be lowered by adding gypsum or someother
substance which can neutralize the excess alkali present in the soil.
pH in our DIGESTIVE SYSTEM
The medium in our stomach is highly acidic and has pH around 1.2. Our stomach produces
hydrochloric acid which helps in digestion of food. Sometimes the stomach produces too much
hydrochloric acid. Excess acid in the stomach causes pain and irritation. It can also cause ulcers in the
digestive system. The condition of excess acid in the stomach is referred toas hyperacidity. In order
to treat hyperacidity, a patient is advised to take antacid tablets or antacid suspensions. The antacid
preparations contain some mild base such as magnesium hydroxide as the active component which
neutralizes the excess acid in the stomach and thus controls the pH in the stomach.
Mg(OH)2 + 2HCl à MgCl2 + 2H2O
pH AND TOOTH DECAY
Tooth enamel is the hardest substance in our body. It is made up of calcium phosphate. It doesnot
dissolve in water and mildly acidic solutions. However, it gets corroded slowly when pH in the mouth
is below 5.5. When we eat food, some food particles remain sticking to the teeth.Bacteria present in
the mouth produce acid by the degradation of sugar and food particles present in the mouth. This
results in lowering of pH in the mouth which promotes tooth decay. To prevent tooth decay, avoid
eating sugary foods. The mouth should be cleaned after eating food. Cleaning the teeth using
toothpastes also helps in preventing tooth decay. Toothpastes are generally basic; therefore, they
neutralize the excess acid in the mouth and thus prevent tooth decay.
The knowledge of pH also helps us in maintaining the proper acidity in many products such as
shampoos, medicines, etc. pH of rainwater can be used to detect the extent of certain pollutant
gases in the atmosphere.
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