Class - X
Chapter : 2 Acids , Bases And Salts
Notes
ACIDS :
• These are the substances which have sour taste.
• They turn blue litmus solution red.
• They give H+ ions in aqueous solution.
• The term ‘acid’ has been derived from the Latin word, acidus, which means sour.
Strong Acids : HCl, H2SO4, HNO3
Weak Acids : CH3COOH, Oxalic acid, Lactic acid
Concentrated Acid : Having more amount of acid + less amount of water
Dilute Acid : Having more amount of water + less amount of acid
BASES :
• These are the substances which are bitter in taste and soapy in touch.
• They turn red litmus solution blue.
• They give OH− ions in aqueous solution.
Strong Bases : NaOH, KOH, Ca(OH)2
Weak Bases : NH4OH
Alkalis : These are bases which are soluble in water. e.g. [NaOH, KOH, Ca(OH)2].
SALTS :
These are the compounds formed from reaction of acid and base.
Example : NaCl, KCl.
INDICATORS :
These are the substances which change their colour/smell in different types of substances.
TYPES OF INDICATORS
Natural indicators Synthetic indicators Olfactory indicators
— Found in nature — These are chemical — These substances
in plants. substances. have different odour
in acid and bases.
—
CHEMICAL PROPERTIES OF ACIDS AND BASES
Reaction of Metals with Acids Bases
Acid + Metal → Salt + Hydrogen gas Base + Metal → Salt + Hydrogen gas
E.g., 2HCl + Zn → ZnCl2+ H2
E.g., 2NaOH + Zn → Na2ZnO2 + H2 ↑(Sodium zincate)
* Hydrogen gas released can be tested by bringing burning candle near gas bubbles, it burst with pop sound.
Reaction of Metal Carbonates/Metal Hydrogen Carbonates with
Acids Bases
Acid + Metal Carbonate/ Metal Hydrogen Carbonate → Salt + CO2 + H2O
Base + Metal Carbonate/Metal Hydrogen Carbonate
E.g., 2HCl + Na2CO3 → 2NaCl + CO2 + H2O → No Reaction
HCl + NaHCO3 → NaCl + CO2 + H2O
* CO2 can be tested by passing it through lime water.
Ca(OH)2 + CO2 → CaCO3 + H2O (Lime water turns milky.)
* When excess CO2 is passed, CaCO3 + CO2 + H2O → Ca(HCO)3 (Milkiness disappears.)
Reaction of Acids and Bases With Each Other
Acid + Base → Salt + H2O
Neutralisation Reaction : Reaction of acid with base is called as neutralization reaction.
E.g., HCl + NaOH→ NaCl + H2O
IF :
Strong Acid + Weak Base → Acidic salt + H2O
Weak Acid + Strong Base → Basic salt + H2O
Strong Acid + Strong Base → Neutral salt + H2O
Weak Acid + Weak Base → Neutral salt + H2O + H2
Reaction of Metallic Oxides with Acids
Metallic oxides are basic in nature.
E.g., CaO, MgO are basic oxides.
Metallic Oxide + Acid → Salt + H2O
CaO + 2HCl → CaCl2 + H2O
Reaction of Non-metallic Oxides with Bases
Non-metallic oxides are acidic in nature.
Non-metallic Oxide + Base → Salt + H2O
CO2 + Ca(OH)2 → CaCO3 + H2O
What do all Acids and Bases have in common
• All acids have H+ ions in common.
• Acids produce H+ ions in solution which are responsible for their acidic properties.
• All bases have OH− (hydroxyl ions) in common.
Acid or Base in Water Solution
• Acids produce H+ ions in presence of water.
• H+ ions cannot exist alone, they exist as H3O+ (hydronium ions).
H+ + H2O → H3O+
HCl + H3O → H3O+ + Cl−
Bases when dissolved in water gives OH− ions.
NaOH →H O2 Na+ + OH-
Mg(OH)2→H O2 Mg2+ + 2OH-
• Bases which are soluble in water are called alkali.
• While diluting acids, it is recommended that the acid should be added to water and not water to acid because the process of dissolving an acid or a base in water is highly exothermic. If water is added to acid, the heat generated may cause the mixture to splash out and cause burns and the glass container may also break due to excessive local heating.
Adding water to acid may cause mixture to splash out Break the glass container. Mixing an acid or a base with H2O results in decrease of concentration of ions (H3O+/OH−) per unit volume. Such a process is called as dilution.
Strength of Acid and Base
Strength of acid or base can be estimated using universal indicator.
Universal indicator : It is a mixture of several indicators. It shows different colours at different concentrations of H+ ions in the solution.
pH Scale : A scale for measuring H+ ion concentration in a solution . p in pH stands for ‘potenz’ a German word which means power.
pH = 7 → neutral solution
pH less than 7 → acidic solution
pH more than 7 → basic solution
On diluting an acid : pH increases ↑
On diluting a base : pH decreases ↓
Importance of pH in everyday life
1. Plants and animals are pH sensitive
• Our body works within the pH range of 7-7.8.
• When pH of rain water is less than 5.6, it is called acid rain.
2. pH of the soil • Plants require a specific pH range for their healthy growth.
3. pH in our digestive system
• Our stomach produces HCl acid which helps in digestion.
• During indigestion, stomach produces more acid and cause pain and irritation.
• To get rid of this pain, people uses antacid (mild base) like milk of magnesia [Mg(OH)2] to neutralize excess acid.
4. pH change as cause of tooth decay
• Tooth decay starts when pH of mouth is lower than 5.5.
• Tooth enamel made up of calcium phosphate (hardest substance in body) does not dissolve in water but corrodes when pH is lower than
5.5 due to acids produced by degradation of food particles by bacteria.
• Using toothpaste (generally basic) tooth decay can be prevented.
5. Self defence by animals and plants through chemical warfare
(a) Bee sting leaves an acid which cause pain and irritation. Use of a mild base like baking soda on stung area gives relief.
(b) Stinging hair of nettle leaves inject methanoic acid causing burning Sensation or pain. Rubbing with leaf of dock plant give relief.
pH of Salts :
(i) Strong Acid + Strong Base → Neutral Salt : pH = 7
(ii) Salt of strong acid + Weak base → Acidic salt : pH < 7
(iii)Salt of strong base + Weak acid → Basic salt : pH > 7
Chemicals from Common Salt (NaCl)
1. Sodium Hydroxide (NaOH) : When electricity is passed through an aqueous solution of NaCl (brine), it decompose to form NaOH.
(Chlor-alkali process)
2NaCl + 2H2O → 2NaOH + Cl2 + H2
At anode : Cl2 gas
At cathode : H2 gas
Near cathode : NaOH solution is formed.
Uses :
H2 : Fuels, margarine
Cl2 : Water treatment, PVC, CFC’s
HCl : Cleaning steels, medicines
NaOH : Degreasing metals, soaps and paper making
Cl2+ NaOH → Bleach : Household bleaches, bleaching fabrics
2. Bleaching Powder (CaOCl2) : It is produced by the action of chlorine on dry
slaked lime.
Cl2 + Ca(OH)2 → CaOCl2 + H2O
Uses :
(a) Bleaching cotton and linen in textile industry.
(b) Bleaching wood pulp in paper factories.
(c) Oxidizing agent in chemical industries.
(d) Disinfecting drinking water.
3. Baking Soda (Sodium Hydrogen Carbonate) (NaHCO3) :
NaCl + H2O + CO2 + NH3→ NH4Cl + NaHCO3
Baking soda
• It is mild non-corrosive base.
• When it is heated during cooking :
2NaHCO3→∆ Na2CO3+ H2O + CO2
Uses :
(a) For making baking powder (mixture of baking soda and tartaric acid). When baking powder is heated or mixed with water, CO2 is produced which causes bread and cake to rise making them soft and spongy.
(b) An ingredient in antacid.
(c) Used in soda acids, fire extinguishers.
4. Washing Soda (Na2CO3.10H2O) : Recrystallization of sodium carbonate gives washing soda. It is a basic salt.
Na2CO3 + 10H2O → Na2CO3.10H2O
Uses :
(a) In glass, soap and paper industry.
(b) Manufacture of borax.
(c) Cleaning agent for domestic purposes.
(d) For removing permanent hardness of water.
5. Plaster of Paris (Calcium sulphate hemihydrates) (CaSO4.½H2O) :
On heating gypsum (CaSO4.2H2O) at 373K, it loses water molecules and becomes Plaster of Paris (POP).
It is a white powder and on mixing with water it changes to gypsum.
CaSO4.½H2O + 1½H2O → CaSO4.2H2O
Uses :
(a) Doctors use POP for supporting fractured bones.
(b) For making toys, material for decoration.
(c) For making surfaces smooth.
Water of Crystallization : It is a fixed number of water molecules present in one formula unit of a salt.
E.g., CuSO4.5H2O has 5 water molecules.
Na2CO3.10H2O has 10 water molecules.
CaSO4.2H2O has 2 water molecules.