CBSE Class 10th Science Chapter 2 Acid Base Salt Part-3

Acid Base Salt Part 3

2.4.1 Family of Salts

  Sulphate (SO₄)^2-                               Nitrate (NO₃)^1-                              Carbonate (CO₃)^2-

                                  

 

   

Activity 2.13

􀂄 Write the formulae of the salts given below.

Potassium sulphate K₂SO₄, sodium sulphate Na2SO4, calcium sulphate CaSO4,

magnesium sulphate MgSO4, copper sulphate CuSO4, sodium chloride NaCl, sodium

nitrate NaNO3, sodium carbonate Na2CO3 and ammonium chloride NH4Cl.( All above salts are white crystal, similar to salt used at home except Copper Sulphate its blue.)

Identify the acids and bases from which the above salts may be obtained.

􀂄 Salts having the same positive or negative radicals are said to belong to a family. For example, NaCl and Na2SO4 belong to the family of sodium salts. Similarly, NaCl and KCl belong to the family of chloride salts. How many families can you identify among the salts given in this Activity?

Acid + Base → salt 

i.e. Sodium hydroxide + Nitric acid → Sodium nitrite.

NaOH  + HNO₃ → NaNO₃ + H₂O

Acids:

Sulphuric acid (H₂SO₄); Hydrochloric acid (HCl), Nitric Acid (HNO₃), Carbonic acid (H₂CO₃).

Bases:

Sodium Hydroxide (NaOH); Magnesium Hydroxide (Mg(OH)₂); Calcium Hydroxide (Ca(OH)₂).

Sodium salts: Sodium sulphate, Sodium chloride, Sodium nitrate, Sodium carbonate.

other Family salts are Potassium salts, magnesium salts, Ammonium Salts, Chloride salts, Sulphate salts, Nitrate salts etc.

2.4.2 pH of Salts

Activity 2.14

·        Collect the following salt samples – sodium chloride, potassium nitrate, aluminium chloride, zinc sulphate, copper sulphate, sodium acetate, sodium carbonate and sodium hydrogencarbonate. (some other salts available can also be taken).

·        Check their solubility in water (use distilled water only).

·        Check the action of these solutions on litmus and find the pH using a pH paper. Which of the salts are acidic, basic or neutral?

·        Identify the acid or base used to form the salt

All salts of HCl, NaOH, etc are readily soluble in water.

Salts of Carbon molecule acids and bases are mostly insoluble in water but some may be insoluble. Here Sodium acetate(C2H3NaO2) is asked which is highly soluble in water.

Salts of inorganic (non Carbon molecule) acids are polar in nature. Polar substances dissolve in polar solvents like water. Salt of Carbon molecule(organic) acids and bases are mainly non-polar in nature so dissolve in a non-polar solution like Benzene, and carbon tetra chloride.

pH Test:

Salts of strong acids and strong bases form a neutral solution with water. E.g. Sodium chloride.

weak acid or base is one which doesn't dissolve(ionize) fully when it is dissolved in water. Strong acid like HCl,H₂​SO_4​ etc.

Weak acid like CH3​COOH,HCOOH etc

Strong Base KOH   (OH-) ions

Weak Base NH₃

Salts of a strong base and weak acid form a basic solution. e.g. Sodium acetate solution. It will turn the PH paper into red or yellow.

Salts of a weak base and strong acid form acidic solution with water. E.g. Ammonium chloride. So its salts form an acidic solution with water.

Na+ & Cl- form Salt NaCl which is commonly used salt at home. Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.

Common salt is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.

Sodium hydroxide

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.

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

Chlorine gas is given off at the anode, and hydrogen gas at the cathode.

Sodium hydroxide solution is formed near the cathode. The three products produced in this process are all useful. 

Bleaching Powder: chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. Bleaching powder is represented as CaOCl₂, though the actual composition is quite complex.

Ca(OH)2 + Cl2 → CaOCl2 + H2O

Bleaching powder use–

(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;

(ii) as an oxidising agent in many chemical industries; and

(iii) for disinfecting drinking water to make it free of germs.

Baking soda:

sodium hydrogencarbonate (NaHCO₃) or baking soda is used for cooking, Bhajiya. It is produced using sodium chloride as one of the raw materials.

NaCl + H₂O + CO₂ + NH₃→ NH₄Cl + NaHCO₃ ………(1)

(Sodium chloride)               (Ammonium Cloride)(Sodium hydrogencarbonate)

it can be used to neutralise an acid? It is a mild non-corrosive base. The following reaction takes place when it is heated during cooking –

                   Heat

2NaHCO₃    →        Na₂CO₃ +H₂O+CO₂ ………..(2)

(Sodium                     (Sodium

hydrogencarbonate)   carbonate)

Baking Soda is used to make Baking Powder. Normally Tartaric acid is made to react with Baking Soda to form Backing Powder

NaHCO3 + H+        →      CO2 + H2O + Sodium salt of acid

 Baking Powder is not 1 compound but mixture of some compounds. Sodium bicarbonate (NaHCO₃) is the active leavening agent, the other main ingredients are monocalcium phosphate (Ca(H₂PO₄)₂) and sodium aluminium sulphate (NaAl(SO₄)₂ ) which during heating release CO₂, making your dough spongy for eg. Cake, Dhokla etc.

(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

(iii) It is also used in soda-acid fire extinguishers.

 

 Bottle contains Sulfuric acid, red cylinder is filled with Sodium bicarbonate, when put the extinguisher upside down Sulfuric acid gets mixed with Sodium bicarbonate (Baking Soda) & release CO2 when you direct this hose on fire it extinguishes.

Washing soda

Another chemical that is obtained from sodium chloride is Na2CO3.10H2O (washing soda). It is formed using equation 1 & 2. NaCl reacts with ammonia water & Carbon dioxide to give sodium hydrogen carbonate. Heating this Sodium Hydrogen carbonate gives us Sodium Carbonate(Na₂CO₃)

Na₂CO₃+10H₂O  →    Na2CO₃.10H₂O

recrystallisation of sodium carbonate gives washing soda.

Uses

·        Sodium carbonate (washing soda) is used in glass, soap and paper industries.

·        It is used in the manufacture of sodium compounds such as borax.

·        Sodium carbonate can be used as a cleaning agent for domestic purposes.

·        It is used for removing permanent hardness of water.

Activity 2.15

1.     1.     Heat a few crystals of copper sulphate in a dry boiling tube.
2.     What is the colour of the copper sulphate after heating?
3.      Do you notice water droplets in the boiling tube? Where have these come from?
4.     Add 2-3 drops of water on the sample of copper sulphate obtained after heating.
5.      What do you observe? Is the blue colour of copper sulphate restored?

 Copper sulphate is blue

1.     On heating, copper sulphate becomes white.

2.     we see few drops in the test tube. Copper sulphate which looks dry contains water of crystallisation. When we heat the crystals, this water is removed and the salt turns white.

3.     After adding water to the heated test tube, copper becomes blue again.

4.     Yes, blue color restored.

Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is Cu SO₄.5H₂O

One other salt, which possesses water of crystallisation is gypsum.

It has two water molecules as water of cyrstallisation. It has the formula

CaSO₄.2H₂O

Plaster of Paris (CaSO4.2H2O) calcium sulfate dihydrate 

On heating gypsum at 373 K(100°C), it loses water molecules and becomes calcium sulphate hemihydrate CaSO4. 1/2H2O ). This is called Plaster of Paris. Used by doctors in treating bone fracture, also used in False Ceiling at home, toys, idols etc.

CaSO₄.1/2H₂O+1 1/2H2O      →        CaSO4.2H₂O

Questions:

1.     What is the common name of the compound CaOCl2?

Ans: Bleaching powder

2. Name the substance which on treatment with chlorine yields bleaching powder.

Ans: Calcium hydroxide.

3. Name the sodium compound which is used for softening hard water.

Ans: Sodium Carbonate

4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Ans: Heating sodium hydrocarbonate yields sodium carbonate and carbon dioxide gas is Released.

2NaHCO₃    →        Na₂CO₃ +H₂O+CO₂

(Sodium                     (Sodium

hydrogencarbonate)   carbonate)

 5. Write an equation to show the reaction between Plaster of Paris and water.

 Ans: 2CaSO₄.H₂O+ 2H₂Oà2CaSO₄.2H₂O

                                                         Summary

·        Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases.

·        Acidic nature of a substance is due to the formation of H+(aq) ions in solution.

·        Formation of OH–(aq) ions in solution is responsible for the basic nature of a substance.

·        When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.

·        When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.

·        When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives the corresponding salt, carbon dioxide gas and water.

·        Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.

·        The strength of an acid or an alkali can be tested by using a scale called the pH scale (0-14) which gives the measure of hydrogen ion concentration in a solution.

·        A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution a pH more than 7.

·        Living beings carry out their metabolic activities within an optimal pH range.

·        Mixing concentrated acids or bases with water is a highly exothermic process.

·        Acids and bases neutralise each other to form corresponding salts and water.

·        Water of crystallisation is the fixed number of water molecules chemically attached to each formula unit of a salt in its crystalline form.

·        Salts have various uses in everyday life and in industries.

Exercise:

E X E R C I S E S

1. A solution turns red litmus blue, its pH is likely to be

(a) 1 (b) 4 (c) 5 (d) 10

Ans: Answer is 10 because litmus paper turns blue when reacts with basic solution (PH more than 7.

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

(a)  NaCl (b) HCl (c) LiCl (d) KCl

Ans: Answer is HCl.

Egg shells contains calcium carbonate, which on reaction with HCl liberates CO₂ gas which turn lime water to milky.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

 3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Solution: Since 10 ml of NaOH requires 8 mL of HCL, 20 ml of NaOH require 8 x 2 = 16mL of HCl Hence the answer id option d 16mL.

4. Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Solution: Indigestion is due to excess production of acid in the stomach. Medicines used to treat indigestion is called as Antacid.

5. Write word equations and then balanced equations for the reaction taking

place when

(a) Dilute sulphuric acid reacts with zinc granules.

(b) Dilute hydrochloric acid reacts with magnesium ribbon.

(c) Dilute sulphuric acid reacts with aluminium powder.

(d) Dilute hydrochloric acid reacts with iron filings.

Solution:

(a) dilute sulphuric acid reacts with zinc granules:

dilute sulphuric acid + zinc → Zinc Sulphate + Hydrogen Gas

H₂SO₄(aq) + Zn → ZnSO₄(aq) + H₂(g)

(b) dilute hydrochloric acid reacts with magnesium ribbon.

 dilute Hydrochloric + Magnesium → Magnesium Chloride + Hydrogen Gas

2HCl(aq) + Mg → MgCl₂(aq) + H₂(g)

(c) dilute sulphuric acid reacts with aluminium powder.

dilute Sulphuric Acid + Aluminium → Aluminium Sulphate + Hydrogen Gas

3H₂SO₄(aq) + 2Al(s) → Al2(SO4)₃(aq) + 3H₂(g)

(d) dilute hydrochloric acid reacts with iron filings.

dilute Hydrochloric Acid + Iron → Ferrous Chloride + Hydrogen Gas

6HCl(aq) + 3Fe(s) → 3FeCl₂(aq) + 3H₂(g)

6. Compounds such as alcohols and glucose also contain hydrogen but are not

categorised as acids. Describe an Activity to prove it

Solution: Activity 2.8 Insert two nails on the wooden or rubber cork and place them on a beaker as shown in figure. Connect iron nail to a bulb, 6 volt battery and a wire connected to switch. Pour some alcohol or glucose so as to dip the nails in glucose or alcohol. Turn the switch on and you the see the bulb not glowing despite of connection to switch. Now empty the beaker and add HCL solution. This time bulb glows. This proves acid can conduct electricity but alcohol and glucose does not conduct electricity.

7. Why does distilled water not conduct electricity, whereas rain water does?

Solution:

• Distilled water does not contain any ionic compounds in it.

• Whereas rainwater has a lot, more compounds.

• Rainwater has dissolved acidic gas such as carbon dioxide from the air and that forms carbonic acid. This means that it has hydrogen ions and carbonate ions. Therefore, with the presence of acids, rainwater can conduct electricity.

8. Why do acids not show acidic behaviour in the absence of water?

Solution: The acidic behaviour from acids is because of the presence of hydrogen ions. Hydrogen ions can only be produced in the presence of water and therefore water is definitely needed if acids are to show their acidic behaviour.

9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is

(a) neutral?

(b) Strongly alkaline?

(c) Strongly acidic?

(d) Weakly acidic?

(e) Weakly alkaline?

Solution: In increasing order of hydrogen ion concentration:

pH 11(B) -> pH 9(E) -> pH 7(A) -> pH 4(D) -> pH 1 (B)

PH11 - Strongly alkaline

pH9 - weakly alkaline

PH7 - Neutral

pH4 - Weakly acidic

pH1 - Strongly acidic

10. Equal lengths of magnesium ribbons are taken in test tubes A and B.

Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Solution: HCl is a strong acid whereas acetic is a weaker acid. Fizzing occurs because of the production of the hydrogen gas obtained due to reaction of the acid on the magnesium ribbon.

Since HCl is a very strong acid there is a lot of liberation of hydrogen gas from test tube A. therefore, more fizzing take place in test tube A.

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Solution: Fresh milk is turned to curd due to production of lactic acid. Lactic acid reduces the pH of the milk.

12. A milkman adds a very small amount of baking soda to fresh milk.

 (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

(b) Why does this milk take a long time to set as curd?

Solution: (a) He shifted the pH of the fresh milk from 6 to slightly alkaline to prevent milk from getting sour due to production of lactic acid.

(b) This milk takes long time to set into curd because the lactic acid produced here first neutralises the pH then the pH is reduced to turn milk to curd.

13. Plaster of Paris should be stored in a moisture-proof container. Explain why?

Solution: Plaster of Paris should be stored in moisture-proof container because moisture can affect plaster of Paris by slowing down the setting of the plaster because of hydration. This will turn plaster useless.

 14. What is a neutralisation reaction? Give two examples.

Solution: The reaction of the acid + base gives a product of salt + water, which is considered as neutralization reaction.

Examples:

NaOH + HCl → NaCl + H2O

Mg(OH)₂ + H2CO₃ → MgCO₃ + 2H₂O

15. Give two important uses of washing soda and baking soda.

Solution:

Washing Soda	Baking Soda
Used in manufacturing of glass, soap etc.	Used in bakery items.
It can be used as water softner for laundry	Used in Soda Acid Fire extinguisher