Class 10 Science Chapter2 notes with question answer

Class 10 Science Chapter2 Chemical Reactions and Equations notes of all topics with detailed explanation. You can prepare your Board examination from here because The notes is written by our Expert Theacher with many years experience. We provide Solution of NCERT question Answer and important intext question

Acids, Bases and Salts

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 or odours 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 dierent odour in basic medium are called olfactory indicators. e.g., vanilla essence, onion, clove oil, etc.

Acid-base indicators showing dierent 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.

Synthetic indicators : Phenolphthalein and methyl orange are synthetic indicators.

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.

Classi­cation 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), (H2SO4), (HNO3), 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 H2CO3, phosphoric acid H3PO4, 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, HNO3, CH3COOH, etc.
Dibasic acids : H2SO4, H2CO3, H2SO3, etc.
Tribasic acids : H3PO4

Dilute acids : A dilute acid is obtained by mixing the concentrated acid with water.

Th­e process of mixing the concentrated acid with water is highly exothermic (or heat producing). So, when a concentrated acid and water are mixed together, a large amount of heat is evolved.

The dilution of a concentrated acid should always be done by adding concentrated acid to water gradually with continuous stirring and not by adding water to concentrated acid.

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. H2SO4(aq) ZnSO4(aq) + H2(g)
  • Reaction with metal carbonates and metal hydrogen carbonates : Acids react with metal carbonates and bicarbonates to give carbon dioxide with brisk eervescence. e.g., Na2 CO3 + H2 SO4→ Na2 SO4 + H2O + CO2 ↑
  • Reaction with bases : When an acid reacts with a base, it forms salt and water.

HCl(Hydrochloric acid) + NaOH (Sodium hydroxide) NaCl 2 (Sodium chloride ) + H2O (water)

  • Reaction with metallic oxides : Metal oxide reacts with acid forming salt and water.

CaO + 2HCl → CaCl2 + H2O

  • Reaction with water : Acids when dissolve in water give H3O+ or H+ ions.

HCl + H2O→ H3O+ + Cl

Uses of acids :

Sulphuric acid is used in the manufacture of fertilizers, paints, explosives, synthetic ‑bres, dyes, drugs, etc. and is also used in petroleum re‑ning.

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.

Classi­cation 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 (NH4OH), calcium hydroxide [Ca(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 : NH4OH, 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) + H2 SO4(aq)  → Na SO4(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 (Sodium hydroxide)  + Zn(Zinc) →Na2ZnO2(Sodium zincate) + H2 (hydrogen gas)

  • Reaction with non-metal oxides : Bases react with non-metallic oxides to produce salts and water. 2NaOH(aq) + CO2(g)→ Na2CO3(aq) + H2O(l)
  • Reaction with heavy metal salts : Metal salts react with aqueous solution of bases to produce precipitates of insoluble metallic hydroxides. ZnSO4(aq) + 2NaOH(aq)→ Na2SO4(aq) + Zn(OH)2↓ White pp
  • Reaction with water : Bases when dissolved in water produce OHions.

NaOH + H2O ( ) → Na+ + OH

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

Uses of bases :

Potassium hydroxide (caustic potash) is used in alkaline batteries and so soaps.

Calcium hydroxide is used in manufacture of bleaching powder, soening of hard water, etc.

Ammonium hydroxide is used for removing grease stains from clothes.

Salts : Salts are the ionic compounds which contain a positive ion (or cation) other than H+ ion and negative ion (or anion) other than OH– ion. e.g., K2SO4, NaCl, NaNO3, etc.

Classi­cation of salts :

Normal salts : ­ese 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, Na2SO4, etc.

Acidic salts : ­ese salts are formed when a polybasic acid is partially neutralised by a base and salts still have some acidic H+ ions. e.g., NaHCO3, NaHSO3, 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., Mg(OH)Cl, etc.

Families of salts : ­The salts are classified into different families either on the basis of the acid or on the basis of the base from which they have been obtained.

Sulphate family : Na2SO4, K2SO4, MgSO4, CaSO4, etc.

Nitrate family : NaNO3, KNO3, Cu(NO3)2, etc.

Chloride family : KCl, NaCl, CaCl2, AlCl3, etc.

Concept of pH

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

In pH scale : 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.

pH of salt : – Salt of a strong acid and a strong base is neutral with pH value equal to 7.

Salt of a strong acid and a weak base is acidic with pH value less than 7.

Salt of a strong base and a weak acid is basic with pH value more than 7.

Importance of pH in everyday life : pH plays an important role in everyday life as :

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. ­is pain can be cured by antacids containing weak bases (like magnesium hydroxide). ­is base neutralises the excess acid produced.

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.

Common salt (Sodium chloride, NaCl) : Sea water is one of the main source of common salt. To extract the salt from sea water, it is allowed to evaporate in shallow tanks under the inuence of sun and wind.

Properties of common salt :

  • It is a colourless crystalline substance with melting point 820°C.
  • At low temperature (0 to –20°C), it exists as a dihydrate, NaCl2H2O.
  • It is slightly hygroscopic in nature.
  • It is soluble in water.

Uses of common salt : – It is essential constituent of our diet.

It is used to make freezing mixture (when mixed with ice) e.g., in making ice-creams.

It is used as raw material for caustic soda, bleaching powder, baking soda and washing soda.

Sodium hydroxide (caustic soda) : Sodium hydroxide is prepared by electrolysis of an aqueous solution of sodium chloride (brine). ­is method is called ‘chlor-alkali’ process. ­e complete reaction can be represented as

2NaCl + H2O → NaOH + Cl 2 + H2

­The sodium hydroxide solution is formed near the cathode.

Uses : It is used

  • for making soaps and detergents.
  • for degreasing metals.
  • in making of artificial fibres.
  • in petroleum refining.
  • as a laboratory reagent.

Bleaching powder (CaOCl2) : It is prepared by the action of chlorine gas on dry slaked lime Ca(OH)2. Ca(OH)2 + Cl2 → CaOCl2 + H2O

­The chlorine used in the above reaction is the by-product produced during the electrolysis of brine. Manufacturing of bleaching powder, is generally carried out in ‘Hasenclever plant’.

Uses : It is used in textile industry for bleaching cotton and linen

  • in paper industry for bleaching wood pulp.
  • in laundry for bleaching washed clothes.
  • for disinfecting drinking water.

Baking soda (NaHCO3) :

When an aqueous solution of sodium chloride (brine) saturated with ammonia is allowed to react with carbon dioxide, baking soda is produced along with ammonium chloride.

NaCl + H O + CO + NH3 →NH4Cl + NaHCO3

­is process is known as ‘Solvay process’.

Uses : It is used

  • as an antacid.
  • as an additive in food and drinks.
  • in fire extinguishers.

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