Acids Bases and Salts for class 10

In this Chapter, we will study the reactions of Acids Bases and Salts how acids and bases cancel out each other’s effects and many more interesting things that we use and see in our day-to-day life. We already know that acids are sour in taste and change the colour of blue litmus to red, whereas, bases are bitter and change the colour of the red litmus to blue where Litmus is a natural indicator.

Acids Bases and Salts

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

Bases : According to the Arrhenius concept, the substances sbstances which when dissolved in water furnish hydroxyl (OH^– ) ions, which have bitter taste, soapy touch and turn red litmus to blue are bases.

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.

Indicator : It is a chemical substance which is added to the solutions in very small amounts to detect their acidic or basic nature is known as an indicator.

Properties of acids Bases and Salts

Physical properties of Acids :

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

Physical properties of bases :

1. They are bitter in taste.
2. They are soapy to touch.
3. Bases turn the colour of red litmus to blue.
4. The solutions of bases in water also conduct electricity

Classication of acids Bases and Salts

Types of Acids

1. Based on the strength of the acids : On the basis of strength of acids some of are very Stong which are completely dissociate and some of are Weak which arenot completely dissiociate 

• 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.

2. 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.

Types of Bases

1. Based on the strength of bases : On the basis of strength of base some of are very Stong which are completely dissociate and some of are Weak which arenot completely dissiociate 

• Strong bases : Bases which are completely ionised 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 are partially ionised 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.

2. 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.

Types of Salts

1. 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, Na₂SO₄, etc.

2. 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.

3. Basic salts : ese 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.

Types of Indicator

 1. Acid-base indicators : Those which show different colours or odours in acidic and basic medium are called acid-base indicators. e.g., litmus, phenolphthalein, methyl orange, 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.

• Synthetic indicators : Phenolphthalein and methyl orange are synthetic indicators

2. 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. 

Reactions of Acids Bases and Salts

Reactions of Acids

1. 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) 

2. Reaction with metal carbonates and metal hydrogen carbonates : Acids react with metal carbonates and bicarbonates to give carbon dioxide with brisk effervescence. E.g.,

Na₂CO₃ + H₂SO₄ →Na₂SO₄ + H₂O + CO↑

3. Reaction with bases : When an acid reacts with a base, it forms salt and water. eg: Hydrochloric acid react with sodium hydroxide to form Sodium chloride as a salt

HCl + NaOH→ NaCl + H₂O

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

CaO(s) + 2HCl(aq) → CaCl₂ + H₂O

5. Reaction with water : Acids when dissolved in water give H3O+ or H+ ions.

HCl + H₂O → H₃O⁺ + Cl

Ractions of Bases

1. Reaction with acids : Bases react with acids to form salts and water.

2NaOH(aq) + H₂SO_4(aq) →Na₂SO₄(aq) +2H₂O(l)

2. 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) →Na₂ZnO₂(aq) + H₂(g)

3. Reaction with non-metal oxides : Bases react with non-metallic oxides to produce salts and water.

2NaOH(aq) + CO₂(g) →Na₂CO₃(aq) + H₂O(l)

4. 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↓

5. Reaction with water : Bases when dissolved in water produce OH^– ions.

NaOH(s)  + H₂O(l) → Na⁺ (aq) OH^– (aq)

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

Uses of Acids 

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

Uses of bases 

1. Potassium hydroxide (caustic potash) is used in alkaline batteries and soaps.
2. Calcium hydroxide is used in the manufacture of bleaching powder, softening of hard water, etc.
3. Ammonium hydroxide is used for removing grease stains from clothes.

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.

In pH scale 

1. All substances having pH values between 0 and 7 are acidic in nature.
2. All substances having pH values between 7 and 14 are basic in nature.
3. All substances having pH value equal to 7 are neutral. 

pH of salt

1. Salt of a strong acid and a strong base is neutral with pH value equal to 7.
2. Salt of a strong acid and a weak base is acidic with pH value less than 7.
3. 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 

1. 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 dicult. For healthy growth of plants, the soil should have a specific pH which is neither highly alkaline nor highly acidic.

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

3. 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.

4. 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.

Importance of Salts

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

Properties of common salt 

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

Uses of common salt 

1. It is an essential constituent of our diet.
2. It is used to make freezing mixture (when mixed with ice) e.g., in making ice-creams.
3. It is used as raw material for caustic soda, bleaching powder, baking soda and washing soda.

2. Sodium hydroxide (caustic soda) : Sodium hydroxide is prepared by electrolysis of an aqueous solution of sodium chloride (brine). This method is called the ‘chlor-alkali’ process. The complete reaction can be represented as, eg: Sodium Chloride treated with water On passing electricity sodium hydroxide solution is formed near the cathode.

2NaCl(aq) + 2H₂O(l) →NaOH(aq) Cl₂(g)  + H₂(g) 

Uses of Sodium hydroxide : 

• It is used for making soaps and detergents.
• It is used for degreasing metals.
• It is used in making artificial bres. 
• It is used in petroleum refining.
• It is used as a laboratory reagent.

3. 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 the ‘Hasenclever plant’.

Uses of Bleaching Powder : 

• It is used in the textile industry for bleaching cotton and linen.
• It is used in the paper industry for bleaching wood pulp.
• It is used in laundry for bleaching washed clothes.
• It is used for disinfecting drinking water.

4. 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, this process is known as ‘Solvay process’.

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ 

Uses of Baking Soda : 

• It is used as an antacid.
• It is used as an additive in food and drinks.
• It is usedin fire-extinguishers.

5. Washing soda (Na₂CO₃.10H₂O) : The preparation of washing soda is carried out through following steps : Step-I : Manufacture of sodium hydrogen carbonate

NaCl + H2O + NH3 + CO → NaHCO₃ + NH₄Cl

Step-II : thermal decomposition of sodium hydrogen carbonate : When dry crystals of sodium hydrogen carbonate are heated strongly, they decompose to form anhydrous sodium carbonate (soda ash).

2NaHCO_3(s) + Heat →Na₂CO₃(s) + CO₂ + H₂O(l)

Step-III : Recrystallisation of sodium carbonate : Sodium carbonate thus obtained is recrystallised to form crystals of washing soda.

Na₂CO₃(s) 10H₂O(l) → Na₂CO₃.10H₂O

Uses of Washing Soda :

• It is used for softening of hard water.
• It is used for washing purposes in laundry and as cleaning agent for domestic purposes.
• It is used in textile industries and also in petroleum rening.
• It is used in the manufacturing of borax.

6. Water of crystallisation : It is the fixed number of water molecules present in one formula unit of a salt.

e.g., Gypsum (CaSO₄.2H₂O) has two molecules of water of crystallisation. In hydrated copper sulphate (CuSO₄.5H₂O), there are ve molecules of water of crystallisation.