Acids, bases and salts class 10

 Introduction :-

Properties of acids :-

• Produce hydrogen ions [H+] ions in water.
• Sour taste
• Turn blue litmus red
• Act as electrolytes in Solution.
• React with several metals releasing hydrogen gas.
• React with carbonates releasing CO2 (g)
• Destroy body tissues.
• Corrode metal surface quickly.

On the basis of origin, acids are classified as:- 

Organic acids :- 

• Acids derived from living organisms like plants and animals.
• For example :- 
• Citric acid is present in fruits e.g. Citrus reticulate (orange)  and Citrus lemon (lemon)
• Acetic acid present in vinegar.
• Oxalic acid present in tomato.
• Tartaric acid present in tamarind.
•  Lactic acid present in sour milk and curd.

Mineral acids :- 

• Also called inorganic acids .
• They are dangerous.
• For example :- Sulphuric acid [H2SO4] , Hydrochloric acid [HCl]

On the basis of their strength, acids are classified as:- 

Strong acids and weak acids.

Strong acids :- 

• Complete dissociation into its ions in aqueous solutions.
• Example:- Nitric acid [HNO3] ,
• Sulphuric acid [H2SO4],
• Hydrochloric acid [HCl] etc.

Weak acids:- 

• Acids which do not completely dissociate into its ions in aqueous.
• Example:- Sulphurous acid [H2SO4] , carbonic acid [H2CO3], Acetic acid [CH3COOH]

On the basis of their concentration, acids are classified as :- 

Dilute acids :- have a low concentrations of acids in aqueous solution.

Concentrated acids:- have a high concentration of acids in aqueous solutions.

On the basis of number of hydrogen ion, acids can be classified as:- 

Monoprotic acids :- 

Acids which produces one mile of H+ ions per mole of acid. Example:- HCl , HNO3

Diprotic acid:- 

Acids which produces two miles of H+ ions per mole of acid. Example:- H2SO4.

Triprotic acid:- 

Acids which produces three moles of H+ ions per mole of acid. Example:- H3PO4 (phosphoric acid)

Polyprotic acid:- Acids which produces more than three moles of H+ ions per mole of acid.

Chemical properties of acids:- 

(1) Acids react with active metal to give hydrogen gas.

• Acid + metal -- > salt + hydrogen gas.
• HCl (l) + Zn (s) --- > ZnCl2 (s) + H2(g)
• H2SO4 (l) + Zn (s) --- > ZnSO4 (s) + H2 (g)

(2) Acid react with metal carbonate and metal hydrogen carbonate to give carbon-di-oxide.

• Metal carbonate / Metal hydrogen carbonate + Acid -- > salt + water + carbon dioxide
• Na2CO3 (s) + 2HCl (aq) --- > 2NaCl(aq) + H2O (l) + CO2
• NaHCO3 (s) + 2HCl (aq) --- > 2NaCl(aq) + H2O (l) + CO2

(3) Acids react with bases to give salt and water . This reaction is called as neutralization reaction.

• NaOH + HCl --> NaCl + H2O
• Base + acid --> salt + water
• (4) Acids react with metal oxides to give salt and water.
• CuO + H2SO4 -- > CuSO4 + H2O

Properties of base :- 

• Produce hydroxide ions [OH-] in water.
• Water soluble base are called alkalies.
• Bitter taste 
• Corrosive in nature.
• Turn red litmus blue.
• Act as electrolyte in solution.
• Neutralize solutions containing H+ ions.
• Have a slippery, "soapy" feel.
• Dissolve fatty material.

On the basis of their strength, bases are classified as :- 

(a) Strong bases :-

•  bases which completely dissociate into its ions in aqueous solution.
• Example:- sodium hydroxide [NaOH]
• Potassium hydroxide [KOH] ,
• Lithium hydroxide [Li(OH)],
• Calcium hydroxide [Ca(OH)2]

(b) Weak bases :- 

• Bases which donor completely dissociate into its ions in aqueous solution. 
• Example:- Ammonium hydroxide [NH4OH] , Magnesium hydroxide Mg(OH)2

On the basis of their concentration, bases are classified as :-

(a) Dilute bases :- Have a low concentration of alkali in aqueous solution.

(b) Concentrated bases:- have a high concentration of alkali in aqueous solution.

Chemical properties of bases:- 

(a) Reaction with metals :- 

• Certain reactive metals such as zinc, aluminium and tin react with alkali solution on heating and hydrogen gas is evolved
• Example:- Zn + NaOH -- > Na2ZnO2 + H2

(b) Reaction with acids :-

• Base react with acids to form salt and water. 

• Example :- NaOH + HCl -- > NaCl + H2O
• KOH + HCl -- > KCl + H2O

(c) Reaction with non-metallic oxides:- 

• Non metallic oxides are generally acidic in nature .
• Non metal  oxide+ base -- > salt + water
• CO2 + NaOH -- > Na2CO3 + H2O

Strength of acid or base solutions :- 

• A. Scale for measuring hydrogen ion concentration in a solution , called pH scale has been developed.
• The p in PH stands for "potenz " in German, meaning power . So p = potential or power H = hydrogen.

pH = 7 i.e. neutral solution i.e H3O+ = OH-

pH > 7 i.e basic solution i.e H3O+<OH-

pH >7 i.e Acidic solution i.e H3O+ >OH-

pH of some common substances.

• Gastric juice = 1.2pH
• Lemon juice = 2.2 pH
• Wine = 4.0 pH
• Rain = 5.5pH
• Pure water, human blood = 7.5 pH
• Baking soda = 8.5 pH
• Detergent , Milk of magnesia = 10.0 pH
• Bleach = 12.5pH
• Sodium hydroxide solution= 14pH

Importance of pH in every day life:-

(1) pH sensitivity of plants and animals:- 

• Human body works in a narrow range of pH 7.0 to 7.8 
• When acid rain [pH<5.6] flows into rivers, it lower the pH of river water.
• Thus survival of aquatic life in such river becomes difficult.
• So acidity can be lethal for plants and animals.

pH in human digestive system:-

• Stomach secretes HCl to kill bacteria in food.
• Mucus prevent the inner lining of stomach from corrosive nature of HCl.
• During indigestion the stomach produces too much acid and cause pain and irritation.
• To get rid of this pain , people use antacids such as magnesium hydroxide (milk of magnesia) ; a base to neutralize excess acid.

pH and tooth decay:- 

• Tooth enamel is the hardest substance in body; made up of calcium phosphate.
• It does not dissolve in water but corroded when pH of mouth is lower than 5.5 
• This acidic medium is produced by degradation of sugar and food particles present in mouth.
• To prevent tooth decay we clean our mouth after eating.
• Also clean teeth by toothpaste ( basic) to neutralize excess acid.

(4) pH as self defence mechanism by plants and animals:-

• Certain animals like bee and plants like nettle secrete highly acidic substances for self defence. 
• Both secrete methanoic acid.

Addition of acids or bases to water:-

• The process of dissolving an acid , specially nitric acid [HNO3] or sulphuric acid [H2SO4] or a base in water is a highly exothermic.
• As a rule :- Always add acid to water not water to acid.
• Acid must be added to water with constant stirring.
• Water must not be added to acid because heat generated cause the mixture to splash out and cause burns.

Indicators:-

• Indicators are those chemical substances which behave differently in acidic and basic medium and help in determining the chemical nature of the substances.
• Acid Base indicators indicate the presence of an acid or a base by changes in acidic or basic medium.

Indicators can be natural or synthetic.

Olfactory indicators:- Those indicators whose odour changes in acidic or basic medium.

Onion:- smell of onion diminished in a base and remain as it is in an acid.

Vanilla :- odour of vanilla essence disappears when it is added to a base.

Odour of vanilla essence persists when it is added to an acid.

Turmeric:- In acids , yellow colour of turmeric remains yellow.

In bases, yellow colour of turmeric turns red.

Phenolphthalein:- Phenolphthalein remains colourless in acids but turn pink in bases 

Methyl orange:- methyl orange turns pink in acids . It becomes yellow in bases.

Litmus:- 

• Litmus is a natural indicator. It's solution is a purple dye which is extracted from lichen.
• Acids turn blue litmus red.
• Bases turn red litmus blue.
• Water is essential for acids and bases to change colour of litmus paper.

Note:- litmus paper will act as an indicator only if either the litmus paper is moist or the acid or base is in the form of aqueous solution

Reason:- Bases and acids release OH- and H+ ions respectively in aqueous solutions.

Summary of indicators:- 

Indicator->acidic med ->basic med
Litmus solution --> Red --> Blue
Methyl orange--> pink --> yellow
Phenolphthalein-> colourless-> pink
Turmeric-->yellow-->red

Properties of salts:-

• Salts forms by combination of acid and base through neutralization reaction.
• The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction.
• The most common salt is sodium chloride NaCl or table salt which is forms by combination of sodium hydroxide and hydrochloric acid.
• NaOH + HCl -- > NaCl + H2O
• Other examples include Epsom salts [MgSO4] used in both salts , ammonium nitrate [NH4NO3] used as fertilizers and baking soda [NaHCO3] used in cooking.
• The pH of salts solution depends on strength of acid and base combined in neutralization reaction.

Common salt :- A raw material for chemicals such as 

• Sodium hydroxide
• Baking soda
• Washing soda
• Bleaching powder etc.

Sodium hydroxide:-

• When electricity is passed through an aqueous solution of sodium hydroxide (brine) ,it decompose to form sodium hydroxide.
• 2NaCl (aq) + 2H2O (l)---> 2NaOH (aq) + Cl2(g) + H2(g)
• The method is called chlor alkali process.
• Chlorine gas is given off at anode.
• Hydrogen gas at cathode.
• Sodium hydroxide solution is formed near cathode.

Uses:-

H2 GAS:- 

• used as fuel.
• Ammonia is used for fertilizer.

Cl2 GAS:- 

• used in water treatment,in swimming pool.
• Pesticides, CFCs , disinfectants etc. 
• H2 + Cl2 --> HCl.

Use of HCl :- 

• Used in cleaning steel
• In manufacturing of NH4Cl
• In medicine and cosmetics

Uses of NaOH:-

•  In de-graeasing metals, soaps and detergents.
• In paper making
• In artificial fibres.

Bleaching powder:- CaOCl2

• Produced by the action of Chlorine on dry slaked lime [Ca(OH)2]
• Ca(OH)2 + Cl2 -- > CaOCl2 + H2O

Uses:- 

(1) For bleaching

• Cotton and linen in textile industry.
• Wood pulp in paper factories.
• Washed clothes in laundry.

(2) as an oxidizing agent in many chemical industries.
(3) for disinfecting drinking water to make it free of germs.

Baking soda:- NaHCO3

• It is produced using NaCl , H2O , CO2 and NH3 as raw material.
• NaCl + H2O + CO2 + NH3 -- > NH4Cl + NaHCO3
• When heated
• 2NaHCO3 + heat --> Na2CO3 + H2O + CO2.
• It is a mild base.

Uses:- 

• Used in kitchen for making crispy pakoras and for faster cooking.
• For making baking powder (mixture of baking soda and mild edible acid such as tartaric acid)
• When baking powder is heated and mixed with water following reaction takes place:- 
• NaHCO3 + H+ (from any acid) --> CO2 + H2O  + sodium salt of acid
• CO2 produced during reaction causes bread or cake to rise making them soft and spongy.
• Used as antacid .
• Used is soda acid fire extinguishers.
• Used in industrial processes.

Washing soda:- Na2CO3. 10H2O

• Produced by heating Baking soda and then recrystallization of sodium carbonate.
• Na2CO3 + 10H2O -- > Na2CO3.10H2O

Uses:- 

• used in glass , soap and paper industries 
• Used in manufacture of sodium compounds such as borax.
• Used as a cleaning agent for domestic purpose.
• Used for removing permanent hardness of water.

Are the crystals of salts really dry?

• No because they contain water of crystallization.
• When heated the crystals, water is removed and colour changes .
• If moisten the crystals again with water, colour of crystals reappear.
• Example:- hydrated copper sulphate -- > CuSO4.5H2O
• Hydrated sodium carbonate --> Na2CO3.10H2O
• Gypsum CaSO4.2H2O

Plaster of Paris (CaSO4.1/2H2O)

• On heating gypsum at 100°C  ,it loses water molecules and become calcium sulphate hemihydrate (CaSO4.1/2H2O)
• This is called plaster of Paris.
• Uses :- used by doctors for supporting fractured bones.
• Used for making toys , materials for decoration for making surface smooth.

Characteristics:-

• white powder 
• On mixing with water, it changes to gypsum once again giving a hard solid mass.
• Ca SO4.1/2H2O + 3/2 H2O -- > CaSO4.2H2O