acids, bases and salts

indicators

it is a dye that undergoes a change when it is put into an acid or a base

examples:

litmus (blue in base, red in acid)

methyl orange (red in acid, yellow in base)

phenolphthalein (colorless in base, pink in base)

turmeric (yellow in acid, red in base)

olfactory indicators like onion and vanilla extract (yes smell in acid, no smell in base)

characteristics of acids

• sour in taste

• turn blue litmus red

• conduct electricity

• corrosive in nature

• dissolve in water & ionise to produce hydrogen ions (H+), which attach themselves to hydrogen molecules to form hydronium ions (H3O+)

classification of acids based on occurance

organic acids: acids present in plant and animal materials. they are generally weak acids. examples include citric acid, ethanoic acid, tartaric acid, oxalic acid etc.

mineral acids: acids prepared from the minerals of the earth. they are generally strong acids, except carbonic acid which is weak. examples include hydrochloric acid, sulphuric acid and nitric acid

why acids are diluted by adding water to acid instead of acid to water

this is because dilution of acid is a highly exothermic reaction. when acid is added to water, then a large amount of heat is evolved at once. however, if we add water to acid, the heat is evolved gradually

reaction of acids with metals

metal + acid → salt + hydrogen

Zn + H2SO4 → ZnSO4 + H2

Al + HCl → AlCl3 + H2

reaction of acids with metal (bi)carbonates

acid + metal(bi)carbonate → salt + carbon dioxide + water

HCl + Na2CO3 → NaCl + CO2 + H2O

HCl + NaHCO3 → NaCl + CO2 + H2O

reaction of acids with bases

acid + base → salt + water

this is also called a neutralisation reaction

NaOH + HCl → NaCl + H2O

reaction of acids with metal oxides

acid + metal oxide → salt + water

this shows that metal oxides are basic in nature

CuO + HCl → CuCl2 + H2O

reaction of acids with metal oxides

acid + metal oxide → salt + water

this shows that metal oxides are basic in nature

CuO + HCl → CuCl2 + H2O

why should substances like curd, vinegar and lemon juice not be kept in metal vessels

this is because the acids present in them may react with the metal to form poisonous substances

classification of acids based on strength

strong acids: acids which completely ionise in water and produce more hydrogen ions. examples: all mineral acids except carbonic acid and sulphurous acid

weak acids: acids which partially ionise in water and produce less hydrogen ions. examples: all organic acids

why distilled water does conduct electricity while rain water does

• distilled water does not conduct electricity due to the absence of any ionic compounds

• rain water conduct electricity as while falling, it dissolves the acidic CO2 gas in air to form H2CO3, which is an ionic compound

behaviour of acids in the absence of wter

in the absence of water, substances will not form hydrogen ions and hence will not show their acidic behaviour

classification of acids based on concentration

concentrated acids: acids which contain more number of hydrogen ions per unit volume

dilute acids: acids which contain less number of hydrogen ions per unit volume

characteristics of bases

• bitter in taste

• soapy to touch

• turn red litmus blue

• conduct elecricity

• corrosive in nature

• produce OH- ions on dissolving in water

water soluble bases

most bases do not dissolve in water, however NaOH, KOH, Ca(OH)2, Mg(OH)2 and NH4OH do. they are called alkalis

classification of bases based on strength

strong bases: bases which completely ionise in water to produce more OH- ions. examples: KOH, NaOH

weak bases: bases which partially ionise in water to produce less OH- ions. examples: Ca(OH)2, Mg(OH)2, NH4OH

reaction of bases with metals

bases only react with some metals to form hydrogen gas

NaOH + Zn → Na2ZnO2 + H2

NaOH + Al → NaAlO2 + H2

why neutralisation reaction occurs

neutralisation reactions occur because the H+ ions and OH- ions react to form water

H+ + OH- → H2O

reaction of bases with non-metal oxides

non-metal oxide + base → salt + water

Ca(OH)2 + CO2 → CaCO3 + H2O

this shows that non-metal oxides are acidic in nature

pH scale

invented by sorenson 1909, it is a scale of numbers 0-14 that measures the strength of an acid or a base. the higher the number, the lesser the amount of H+ ion concentration

universal indicator

it is a mixture of many indicators which gives different colours at different pH values of the entire pH scale. it goes from dark red at 0 to violet at 14, with green being the neutral colour at 7

pH in our digestive system

the dilute hydrochloric acid (pH 1.4) produced in our stomach helps in digesting in food

sometimes, excess acid is produced causing indigestion. in order to cure indigestion, bases called antacids are taken in order to neutralise the acid

pH change as the cause of tooth decay

when we eat food containing sugar, the bacteria in our mouth break down the sugar to form acids. these acids corrode the teeth and cause tooth decay (this happens when the pH drops below 5.5)

toothpastes are basic in nature in order to neutralise the excess acid in our mouth

soil pH and plant growth

plants grow best when the pH is close to 7

if the soil is too acidic, it is treated with materials like quicklime, slaked lime or limestone

if the soil is too basic, it is treated by adding manure and compost which are acidic in nature

pH in the survival of aquatic animals

aquatic animals can survive in lake or river water with neutral pH

however, due to acid rain, the pH of the water can drop to 5.6, due to which the aquatic animals die

hence, CaCO3 is added to the water to neutralise the acid that comes from acid rain

the role of pH in the self defence by animals and plants

when a honey-bee stings a person, it injects an acidic liquid into the skin

when a wasp stings a person, it injects a basic liquid into the skin

when an ant stings a person, it injects methanoic acid into the skin

nettle leaves inject methanoic acid into the skin, which can be neutralised by using leaves of dock plant which are basic

characteristics of salts

• formed by the replacement of hydrogen in an acid by a metal

• mostly solids

• have high melting and boiling points

• soluble in water

• conduct electricity

pH of salt solutions

• the salts are mostly neutral, or slightly basic or alkaline depending on the parent acid and base

• strong acid + strong base = neutral

• strong acid + weak base = acidic

• weak acid + strong base = basic

preparation of common salt (sodium chloride)

in laboratories: NaOH + HCl → NaCl + H2O

however, it occurs naturally and is hence obtained from seawater through the process of evaporation, or from underground deposits

uses of common salt (sodium chloride)

• used as raw material to make NaOH (caustic soda), Na2CO3 (washing soda), NaHCO3 (baking soda) etc.

• used in cooking food

• used as preservative in pickles

• used in the manufacturing of soap

• used to melt ice

preparation of caustic soda (NaOH)

NaCl (brine) + H2O →(electrolysis) NaOH + Cl2 + H2 [Cl2 is formed at anode, H2 is formed at cathode and NaOH is formed near the cathode]

this process is called chlor-alkali process

uses of caustic soda (NaOH)

• used for making soaps and detergents

• used for making rayon

• used in the manufacture of paper

• used in purifying bauxite ore

• used in degreasing metals

uses of chlorine

• used to sterilise water

• used in the production of bleaching powder

• used to make PVCs and CFCs

uses of hydrogen

• used in the hydrogenation of oils

• used to make ammonia

• used as fuel for rockets

uses of hydrochloric acid

• used for cleaning iron sheets

• used in medicines and cosmetics

• used in textile, dyeing industries

preparation of washing soda (Na2CO3.10H2O)

NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4Cl

NaHCO3 → Na2CO3 + CO2 + H2O

Na2CO3 + 10H2O → Na2CO3.10H2O

note: washing soda is a transparent crystalline solvent which is one of the few metal carbonates which are soluble in water

uses of washing soda (Na2CO3.10H2O)

• used as detergent

• used for removing permanent hardness of water

• used in the manufacture of glass and paper

• used in the manufacture of borax

preparation of baking soda (NaHCO3)

NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4Cl

note: it is a white crystal which is soluble in water, and is non-corrosive

preparation of baking powder

baking powder is a mixture of baking soda and an edible acid like tartaric acid

NaHCO3 + H+ (from acid) → Na+ + CO2 + H2O

advantages of using baking powder while cooking

• the CO2 gas produced gets trapped in the wet dough and makes it rise, and makes it soft and spongy

• the acid in it neutralises the baking soda in order to get rid of the bitter taste

uses of baking soda (NaHCO3)

• used as an antacid to remove acidity of the stomach

• used in soda-acid fire extinguishers [the baking soda combines with sulphuric acid inside the tank to form CO2, which is released into the air and extinguishes the fire]

• used in cooking

preparation of bleaching powder (CaOCl2)

Ca(OH)2 + Cl2 → CaOCl2 + H2O

note: it is a white powder with strong smell of chlorine, and is soluble in cold water

uses of bleaching powder (CaOCl2)

• used for bleaching cotton, linen and wood pulp. the bleaching action is done by the chlorine released by it

• used for disinfecting water

• used to make chloroform

• used to prepare chlorine (by reacting with acids)

preparation of plaster of paris (CaSO4.1/2H2O)

CaSO4.2H2O (gypsum) →(100C) CaSO4.1/2H2O + 1 1/2H2O

notes:

• the formula CaSO4.1/2H2O actually means that 2 molecules of CaSO4 share one molecule of water

• POP sets into a hard mass on wetting with water (it turns back into gypsum). POP is formed in order to mould it to form casts, statues and toys

  • CaSO4.1/2H2O + 1 ½ H2O → CaSO4.2H2O

uses of plaster of paris (CaSO4.1/2H2O)

• used for setting fractured bones

• used in making toys and decorative materials

• used for fire-proofing

• used for making false ceilings

water of crystallisation

• the water molecules which form part of the structure of a crystal salt are called water of crystallisation

• the salts which contain water of crystallisation are called hydrated salts

• as it is not free water, it does not wet the salt. hence, the salt appears completely dry

• the water of crystallisation give the crystals of salts their shape and colour

• hydrated salts lose their water of crystallisation upon heating. it is a reversible process

• examples: CuSO4.5H2O, FeSO4.7H2O