CBSE Class 10 Chemistry Acids Bases And Salts Notes Set 04

QUICK RECAP

Indicator : The chemical substance which is added to the solutions in very small amount to detect their acidic or basic nature is known as indicator.

Depending upon the property of the indicators, they are classified into two types :

• Acid – base indicators : The indicators which show different colours in acidic and basic medium are called acid base indicators e.g., litmus, phenolphthalein, methyl orange, etc.
• Olfactory indicators : The substances which give one type of odour in acidic medium and a different odour in basic medium are called olfactory indicators, e.g., vanilla essence, onion, clove oil, etc.

Acid-base indicators showing different colours are of two types :

• Natural indicators : Litmus is a natural indicator which is a purple coloured dye. Turmeric and red cabbage juice are other examples of natural indicators.

Natural indicators with their characteristic colours

• Litmus : Colour in neutral medium: Purple; Colour in acidic medium: Red; Colour in basic medium: Blue
• Turmeric : Colour in neutral medium: Yellow; Colour in acidic medium: Yellow; Colour in basic medium: Reddish brown
• Red cabbage leaves extract : Colour in neutral medium: Red; Colour in acidic medium: Red; Colour in basic medium: Green

Synthetic indicators

Phenolphthalein and methyl orange are synthetic indicators.

• Phenolphthalein : Colour in neutral medium: Colourless; Colour in acidic medium: Colourless; Colour in basic medium: Pink
• Methyl orange : Colour in neutral medium: Orange; Colour in acidic medium: Red; Colour in basic medium: Yellow

Acids

The substances which are sour in taste and change the colour of blue litmus to red are acids. According to Arrhenius concept, acids are substances which dissociate in aqueous solution to furnish hydrogen or hydronium ions.

Classification of acids :

• Based on the strength of the acids :
  • Strong acids : Acids which undergo complete dissociation in aqueous solution producing a high concentration of \( H^+ \) ions, are called strong acids. e.g., \( HCl \), \( H_2SO_4 \), \( HNO_3 \), etc.
  • Weak acids : Acids which undergo partial dissociation in aqueous solution producing a low concentration of \( H^+ \) ions, are called weak acids. e.g., Carbonic acid; \( H_2CO_3 \), acetic acid; \( CH_3COOH \), formic acid; \( HCOOH \), etc.
• Based on the basicity of acids : On the basis of basicity (number of replaceable \( H^+ \) ions present in an acid), acids can be classified as :
  • Monobasic acids : \( HCl \), \( HNO_3 \), \( CH_3COOH \), etc.
  • Dibasic acids : \( H_2SO_4 \), \( H_2CO_3 \), \( H_2SO_3 \), etc.
  • Tribasic acids : \( H_3PO_4 \)

General properties of acids

Physical properties :

• Almost all acidic substances have sour taste.
• Acids turn blue litmus solution to red.
• Most of the acids are corrosive in nature.
• The solutions of acids in water conduct electricity.

Chemical properties :

• Reaction with metals : Acids react with active metals such as zinc, magnesium, etc. and hydrogen gas is evolved. e.g.,
  \( Zn_{(s)} + dil. H_2SO_{4(aq)} \rightarrow ZnSO_{4(aq)} + H_{2(g)} \)
• Reaction with metal carbonates and metal hydrogen carbonates : Acids react with metal carbonates and bicarbonates to give carbon dioxide with brisk effervescence. e.g.,
  \( Na_2CO_{3(s)} + H_2SO_{4(aq)} \rightarrow Na_2SO_{4(aq)} + H_2O_{(l)} + CO_{2(g)} \)
  \( NaHCO_{3(s)} + HCl_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)} + CO_{2(g)} \)
• Reaction with bases : When an acid reacts with a base, it forms salt and water.
  \( HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)} \) (Neutralisation reaction)
• Reaction with metallic oxides : Metal oxide reacts with acid forming salt and water.
  \( CaO_{(s)} + 2HCl_{(aq)} \rightarrow CaCl_{2(aq)} + H_2O_{(l)} \)
• Reaction with water : Acids when dissolve in water give \( H_3O^+ \) or \( H^+ \) ions.
  \( HCl + H_2O \rightarrow H_3O^+ + Cl^- \)

Uses of acids

• Sulphuric acid is used in the manufacture of fertilizers, paints, explosives, synthetic fibres, dyes, drugs, etc. and is also used in petroleum refining.
• Nitric acid is used in the manufacture of fertilizers like ammonium nitrate, explosives like TNT, artificial silk, dyes and plastics.
• Hydrochloric acid is used in textile, food, leather and dye industries.

Bases

The substances which have bitter taste, soapy touch and turn red litmus to blue are bases. According to Arrhenius concept, bases are substances which when dissolved in water furnish hydroxyl (\( OH^- \)) ions.

Classification of bases :

• Based on the strength of bases :
  • Strong bases : Bases which completely ionise in aqueous solution to furnish high concentration of \( OH^- \) ions, are called strong bases. e.g., Sodium hydroxide (\( NaOH \)), potassium hydroxide (\( KOH \)), etc.
  • Weak bases : Bases which partially ionise in aqueous solution to furnish low concentration of \( OH^- \) ions, are called weak bases. e.g., Ammonium hydroxide (\( NH_4OH \)), magnesium hydroxide [\( Mg(OH)_2 \)], etc.
• Based on the acidity of bases : On the basis of acidity (number of replaceable \( OH^- \) ions present in a base), bases can be classified as :
  • Monoacidic bases : \( NH_4OH \), \( NaOH \), etc.
  • Diacidic bases : \( Ca(OH)_2 \), \( Mg(OH)_2 \), etc.

General properties of bases

Physical properties :

• They are bitter in taste.
• They are soapy to touch.
• Bases turn the colour of red litmus to blue, methyl orange from orange to yellow and phenolphthalein from colourless to pink.
• Like acids, the solutions of bases in water also conduct electricity.

Chemical properties :

• Reaction with acids : Bases react with acids to form salts and water.
  \( 2NaOH_{(aq)} + H_2SO_{4(aq)} \rightarrow Na_2SO_{4(aq)} + 2H_2O_{(l)} \)
• Reaction with metals : Some bases such as \( NaOH \), \( KOH \) react with active metals to liberate hydrogen gas along with the formation of salts.
  \( 2NaOH_{(aq)} + Zn_{(s)} \rightarrow Na_2ZnO_{2(aq)} + H_{2(g)} \)
• Reaction with non-metal oxides : Bases react with non-metallic oxides to produce salts and water.
  \( 2NaOH_{(aq)} + CO_{2(g)} \rightarrow Na_2CO_{3(aq)} + H_2O_{(l)} \)
• Reaction with heavy metal salts : Metal salts react with aqueous solution of bases to produce precipitates of insoluble metallic hydroxides.
  \( ZnSO_{4(aq)} + 2NaOH_{(aq)} \rightarrow Na_2SO_{4(aq)} + Zn(OH)_{2 \downarrow} \) (White ppt.)
• Reaction with water : Bases when dissolved in water produce \( OH^- \) ions.
  \( NaOH_{(s)} \xrightarrow{H_2O} Na^+_{(aq)} + OH^-_{(aq)} \)

Note : A base which dissolves in water is called an alkali. Thus, all alkalies are bases but all bases are not alkalies.

Concept of pH

The concentration of \( H^+ \) ion i.e., \( [H^+] \) in aqueous solution is very small, so it is very difficult to express the acidity or alkalinity of an aqueous solution. So, pH scale is used to measure the strength of acids and bases.

• All substances having pH values between 0 and 7 are acidic in nature.
• All substances having pH values between 7 and 14 are basic in nature.
• All substances having pH value equal to 7 are neutral.
• The strength of an acid and a base is measured by a scale called pH scale. pH value of the solution indicates the concentration of hydrogen ions in the solution. The pH values range from 1 to 14.

Importance of pH in everyday life

• In humans and plants : Most of the reactions taking place in our body are in the pH range of 7.0 to 7.8. If pH falls below 7.0 or rises above 7.8, the survival of living organisms becomes difficult. For healthy growth of plants, the soil should have a specific pH which is neither highly alkaline nor highly acidic.
• In digestive system : Hydrochloric acid is produced in our stomach which helps in the digestion of food. But if the amount of acid produced is beyond the required limit, it causes pain and irritation in the stomach. This pain can be cured by antacids containing weak bases (like magnesium hydroxide).
• Tooth decay caused by acids : If the pH in our mouth falls below 5.5, the dissolution of calcium phosphate (tooth enamel) starts i.e., tooth decay begins.
• Self defence of animals and plants through chemical warfare : Sting of honey-bee or yellow ant injects methanoic acid (or formic acid) due to which we feel pain. To get relief, a solution of mild base, such as baking soda is used. Stinging hair of nettle leaves inject methanoic acid, causing burning pain.

Salts

Classification of salts :

• Normal salts : These salts are formed when all replaceable hydrogens of an acid are replaced by metal ions thus, they do not generally contain any replaceable hydrogen atom. e.g., \( NaCl \), \( Na_2SO_4 \), etc.
• Acidic salts : These salts are formed when a polybasic acid is partially neutralised by a base and salts still have some acidic \( H^+ \) ions. e.g., \( NaHCO_3 \), \( NaHSO_3 \), etc.
• Basic salts : These salts are formed by partial neutralisation of polyacidic bases with acids and salts still have some basic \( OH^- \) ions. e.g., \( Pb(OH)NO_3 \), \( Ca(OH)NO_3 \), etc.

Sodium hydroxide (caustic soda)

Sodium hydroxide is prepared by electrolysis of an aqueous solution of sodium chloride (brine). This method is called ‘chlor-alkali’ process.
\( 2NaCl_{(aq)} + 2H_2O_{(l)} \xrightarrow{\text{On passing electricity}} 2NaOH_{(aq)} + Cl_{2(g)} + H_{2(g)} \)

Uses : It is used for making soaps and detergents, for de-greasing metals, in making of artificial fibres, in petroleum refining, and as a laboratory reagent.

Washing soda (\( Na_2CO_3 \cdot 10H_2O \))

The preparation of washing soda is carried out through following steps :

• Step-I : Manufacture of sodium hydrogen carbonate :
  \( NaCl + H_2O + NH_3 + CO_2 \rightarrow NaHCO_3 + NH_4Cl \)
• Step-II : Thermal decomposition of sodium hydrogen carbonate :
  \( 2NaHCO_{3(s)} \xrightarrow{Heat} Na_2CO_{3(s)} + CO_{2(g)} + H_2O_{(g)} \)
• Step-III : Recrystallisation of sodium carbonate :
  \( Na_2CO_{3(s)} + 10H_2O_{(l)} \rightarrow Na_2CO_3 \cdot 10H_2O_{(s)} \)

Plaster of Paris

It is prepared by heating gypsum (\( CaSO_4 \cdot 2H_2O \)) at 373 K.
\( CaSO_4 \cdot 2H_2O \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \)

Uses : It is used for making moulds for toys, pottery, ceramics, etc.; for making statues, models and other decorative materials; as fire proofing materials; in medical science as plasters for setting broken and fractured bones.

Water of crystallisation

It is the fixed number of water molecules present in one formula unit of a salt. e.g., Gypsum (\( CaSO_4 \cdot 2H_2O \)) has two molecules of water of crystallisation. In hydrated copper sulphate (\( CuSO_4 \cdot 5H_2O \)), there are five molecules of water of crystallisation.