ACIDS BASES AND SALTS
- The substances which are sour in taste are acidic in nature. The acidic nature of these substances is due to the presence of particular acids in them.
- The substances which are bitter in taste and soapy to touch are basic or alkaline in nature. Basic or alkaline nature or substances is due to the presence of particular bases or alkalis.
- Acids and bases combines together to form a neutral substance (salt). The reaction which involves formation of salt from acid and base is known as neutralization reaction.
Arrhenius concept:
1) The substances which produce hydrogen ions (H+) in water or aqueous mediums are called acids.
2) Hydrogen ions always combine with a molecule of water to form hydronium ion (H3O+) in aq. solution.
3) The substances which produce hydroxyl ions (OH–) in aq. solution are known as bases.
LIMITATIONS:
1) Restricted to compounds containing H+ and OH– ions. Wasn’t able to tell anything about SO2, CaO etc.
2) Only applicable in aq. medium.
Bronsted-Lowry’s Concept of Acids and Bases:
1) The species such as cation, anion or molecule which are capable of donating a proton or protons to other substances are called Bronsted-Lowry acids. An acid is proton donor.
2) The species such as cation, anion or molecule which are capable of accepting a proton or protons from other substances are called Bronsted-Lowry bases. Base is a proton acceptors.
limitation:– it does not explain the acidic nature of SO2, CO2 and basic nature of CaO, MgO etc.
Indicators:
Those chemical substances which give different colours in different mediums, i.e., acidic and basic medium. These are also called acid-base indicators.
Natural indicators
| S. NO. | Indicator | Acidic medium | Basic medium | Neutral medium |
| 1 | Litmus(Purple coloured dye extracted from lichen.) | Red | Blue | Purple |
| 2 | Turmeric | Yellow | Reddish | Yellow |
| 3 | Red cabbage extract | Red | Green | Red |
| 4 | Flower extract of hydrangea plant | Blue | Pink | blue |
Synthetic indicators:
| Indicator | Acidic medium | Basic medium | Neutral medium |
| Methyl orange | Red | Yellow | Orange |
| Phenolphthalein | Colourless | Pink colour | colourless |
Olfactory indicators:
1) Onion: it has a specific smell in acidic medium but no smell in basic medium.
2) Vanilla: the essence of vanilla has its specific smell in acidic medium but no smell in basic medium.
3) Clove oil: it gives different odour when added to acid and base respectively.
Universal indicator:
it is the mixture of various indicators. It is more sensitive indicator as it give different colours in different mediums. It is helpful to estimate the pH of same acid in different concentrations. Therefore, this indicator is also called pH indicator.
Classification of acids and bases:
| Acids | Bases | |
| Weak acid or base | Those acids which are not completely ionized in water and give a few hydrogen ions in aq. sol. are called weak acids | Those bases which do not ionize completely to give OH– ions in water and have weak affinity for proton are called weak bases. |
| Strong acid or base | Those acids which are completely ionized in aq. sol. and give large number of hydronium ions in water by the process of ionization are known as strong acids. | Those bases which are fully ionized in all the concentrations in aq. medium and have strong affinity for a proton are strong bases. |
| Organic acids: acids derived from plants and animals. These contain carbon atoms. Acetic acid, oxalic acid. | Mineral acids: Those acids which are obtained from minerals. Carbonic acid. | |
| Dilute acids: those acids which have low percentage of water per unit volume of the solution are called dilute acids. | Conc. Acids: those solutions which have high percentage of acid and low percentage of water per unit volume are called concentrated acids. | |
| Binary acids: those acids which have hydrogen and one other element are called binary acids. | Oxoacids: those acids which have oxygen in their structure and they can give nascent oxygen are called oxoacids. | |
| Monobasic acid / Monoacidic base | Those acids whose molecule gives one H+ ion during ionization in water are called monoprotic acids. | Those bases which can give one OH– ion on dissociation are called monoacidic bases. |
| Dibasic acid / diacidic base | Those acids which can give two H+ ions in aq. sol. are dibasic acids | Those bases which can give two OH– ions on dissociation are diacidic bases |
| Polybasic acid / Triacidic base | Those acids which can give more than two H+ ions are called polybasic acids. | Those bases which can give 3 OH– ions on dissociation are triacidic bases. |
| Physical properties: | a) Highly soluble in water.
b) Sour in taste. c) Good conductor of electricity. d) Highly corrosive in nature. e) May be solid or liquid. f) High melting and boiling points. g) Highly exothermic with water |
a) Bitter in taste
b) Soapy in touch c) Corrosive in nature d) Soluble in water e) Conductor of electricity f) High melting and boiling points |
| Reaction with metals | Forms salts and hydrogen gas. Displacement reaction. Redox reactions and exothermic in nature. metal + acid à metal salt + H2 gas |
Some of the strong alkalis react with few metals to give salt and hydrogen gas Zn+NaOHàNa2ZnO2+H2 |
| Reaction with metal/non-metal oxides | Metal oxides are basic in nature, so these react with acids to give salt and water. metal oxide+acidàmetal salt+H2O |
When non-metal reacts with bases then salts and water are formed. Neutralization Rx. Ca(OH)2+CO2àCaCO3+H2O |
| Reaction of acids with metal carbonates | When acids reacts with metal carbonates, they form Co2 and water along with salt. It is a decarboxylation reaction because CO2 formed during this reaction is eliminated. metalcarbonate+acidàsalt+CO2+H2O |
|
| Reactions with metal bicarbonates | On reaction with metal bicarbonates salt, water and CO2 is formed. Bicarbonate+acidàsalt+CO2+H2O |
|
| Reaction with bases/acids | When acids react with bases, salt and water are formed and this reaction is called neutralization reaction. Highly exothermic in nature | Salt and water are formed. Highly exothermic reaction. NaOH+HClàNaCl+H2O |
STRENGTH OF ACIDS AND BASES – pH VALUE
The concentration is generally measured in terms of molarity (M). it may be defined as, ‘number of moles of the solute present in one litre of the solution’. The water is neutral because it has equal number of H+ and OH– ions. Hence, for water, concentration of H+ and OH– ions can be given as:-
Conc. Of H+ ions = conc. Of OH– ions = 10-7 mole per litre.
pH= -log[H+]. pH stands for potenz-de-hydrogen which means the power of H+ ions.
the scale by which we can compare the strengths of acids and bases in terms of pH value is known as pH scale. This scale was devised by Sorenson in 1909.
For acidic solution pH < 7
For basic solution pH > 7
For neutral solution pH = 7
- pH in digestive system: The stomach releases HCl which activates enzymes like pepsin by maintaining pH < 2. Moreover, it kills the harmful germs which come in alimentary canal along with food. Sometimes when acid is released in excess quantity, it causes acidity, pain in stomach and indigestion.
- pH changes causes tooth decay: our tooth starts to decay when its pH goes below 5.5. the acid attacks on the enamel and corrodes it. When this corrosion becomes more deep, then this is called dental caries.
- Acids in muscles during heavy exercise: glucose converts into lactic acid during the anaerobic respiration which takes place along with normal respiration. This lactic acid gets accumulated in muscles and causes pain.
- Plants and animals are sensitive to pH change
- Self protection in animals and plants through chemical warfare.
- Regaining of shine of tarnished copper articles.
SALTS
In majority of salts, the cations are derived from metals and anions from non metals. The cations are called basic radicals and anions are called acidic radicals because they are obtained from bases and acids respectively.
Preparation of salt: the salts are formed by neutralization reaction between acids and bases.
NaOH + HCl à NaCl + H2O
| Action of acids on metals | Metals reacts with acids to form hydrogen gas and their respective salts. Mg + H2SO4 à MgSO4 + H2 |
| Acids with metal carbonates | Forms respective salt, CO2 and water. CaCO3 + HCl à CaCl2 + CO2 + H2O |
| Acids with metal bicarbonates | Forms salt, CO2 and water NaHCO3 + HCl à NaCl + CO2 + H2O |
| Alkalis with metals | Forms hydrogen gas and salt. NaOH + Zn à Na2ZnO2 +H2 |
Types of salts:
acidic salts: combination of strong acids and weak bases. pH < 7
basic salts: combination of strong bases and weak acids. pH > 7
neutral salts: combination of strong acids and strong base or weak acid and weak base. pH is near to 7.
SOME IMPORTANT SALTS:
| NAME OF SALT | SOURCES | PROPERTIES | USES |
| Common salt | 1) From sea water.
2) From rock salt. |
1) White crystalline solid.
2) Highly soluble in water. 3) High melting and boiling point 4) Hygroscopic in nature. NaCl+AgNO3àAgCl+NaNO3 |
1) Important component of food
2) Preservative for pickles 3) Used in freezing mixture 4) Used in manufacturing of soap to solidify it. 5) Used in production of large quantity of Cl2, H2 etc. |
| Sodium Hydrooxide | NaCl+H2Oà NaoOH+Cl2+H2 | 1) White deliquescent comp.
2) Highly soluble in water 3) High melting and boiling point 4) Corrosive in nature 5) Conductor of electricity in aq. and molten state. 6) Emulsifies oil and fat in water. |
1) Manufacturing of soaps and detergents.
2) Manufacturing of textiles and paper. 3) Cleansing of metal surface and oil refining. 4) Refining of bauxite |
| Washing soda | NaCl+NH3+H2O+CO2à NaHCO3+NH4Cl
NaHCO3 à Na2CO3+CO2+H2O Na2CO3+10H2O à Na2CO3.10H2O |
1) Transparent crystalline solid.
2) Highly solution in water, gives alkaline solution. 3) Has cleansing properties. 4) On heating, it loses water and becomes anhydrous.
|
1) Used in manufacturing of detergent.
2) Softening of hard water. 3) Manufacturing of glass and paper. 4) Manufacturing of borax and caustic soda. |
| Baking soda | NaCl+NH3+H2O+CO2à NaHCO3+NH4Cl (baking soda) |
1) White crystalline substance
2) Sparingly soluble in water. 3) Aq. sol. of baking soda is slightly alkaline. 4) On heating, it gets decomposed to give soda ash. |
1) Common antacid to remove acidity.
2) Preparation of baking powder. 3) Preparation of soda water and cold drinks. 4) Used in fire extinguishers. |
| Bleaching powder | Ca(OH)2+Cl2à CaOCl2+H2O (bleaching powder) |
1) Yellowish-white powder having the smell of chlorine.
2) Soluble in water 3) CaOCl2+CO2àCaCO3+Cl2 4) CaOCl2+HClàCaCl2+Cl2+H2O |
1) Bleaching of paper and cotton. Cotton clothes.
2) Preparation of various organic and inorganic comp. 3) Bleaching of wood pulp and linen. 4) Disinfecting of drinking water. 5) Manufacturing of insecticides and weedicides. |
| Plaster of paris | CaSO4.2H2Oà gypsum CaSO4.1/2H2O+3/2H2O |
1) White amorphous powder.
2) Soapy to touch 3) On heating, gets converted into solid mass. 4) Sparingly soluble in water. |
1) Setting of fractured bones.
2) Making dustless chalk. 3) Used in air tight packing. 4) Casting of statues. 5) Dental designing 6) Decoration of ceiling and walls. |
Hydrated salts: those salts which have fixed number of water molecules in their crystalline structure are called hydrated salts. These molecules are called water molecules of crystallization. It means these are attached through a special bonding which is called coordinate bonding. Example: CuSO4.5H2O, Na2CO3.10H2O. Those salts which have lost their water of crystallization are called anhydrous salts.
Mathomania Welcomes you