CBSE Class 10 Science Notes Chapter 2 Acids.pptx

Y
ou have learnt in your previous classes that the sour and bitter
tastes of food are due to acids and bases, respectively, present in them.
If someone in the family is suffering from a problem of acidity after
overeating, which of the following would you suggest as a remedy– lemon
juice, vinegar or baking soda solution?
n Which property did you think of while choosing the remedy?
Surely you must have used your knowledge about the ability of
acids and bases to nullify each other’s effect.
n Recall how we tested sour and bitter substances without tasting
them.

1. BY GAURAV SIR CBSE Class 10 Science Notes Chapter 2 Acids Bases And Salts

2. INTRODUCTION More About Acids Some Naturally occurring Acids Organic and Inorganic (Mineral) Acids Handling acidic food- stuff in the house hold Chemical Nature of Acids what do all acids have in common? Role of water in a Dissociation of an Acid Dilution of concentrated Acids-An exothermic Reaction Effect of Dilution on H+ ion concentric of an acid Use of Mineral Acids in Industry

3. INTRODUCTION General Characteristics Properties of Bases Chemical Nature of Bases What do all bases have in common? Difference between a base and an Alkali Neutralization Reaction Between an acid and a Base Mixing of bases with water-An Exothermic Process Effect of Dilution on OH- ion concentric of a base Uses of some common Base

4. INTRODUCTION Strength of Acids and Bases Strong and weak Acids Strong and weak Bases Express Acidity or Alkalinity of a solution in terms of pH Measurement of the strength of acidic or basic solutions Role of pH in everyday Life Salts and their formation Baking Soda (Sodium Hydrogen Carbonate, NaHCO ₃) Washing Soda (Sodium Carbonate, Na2CO3.10H2O) Are the crystalline Salts Really Dry? Anhydrous Salts Plaster of Paris ( CaSO4.  1/2H2O)

5. INDICATORS Acids are sour in taste. Bases are bitter in taste. With the help of taste buds, it becomes easy to differentiate between acidic substance and basic substance. However, the substance used in a laboratory can be harmful to one if ever tasted. Therefore, to define whether they are neutral, acidic or basic, a certain special kind of substance used. This substance is known as an indicators. There are many substances which show one colour (Or odour) in the acidic medium and a different colour(or odour) in the basic medium. Such substances are called as acid-base indicators. Depending upon the property of the indicator, we have the following two types of acid-base indicators: A. Indicators showing different colours in acidic and basic medium. B. Indicators giving different odours in acidic and basic medium (Called Olfactory indicators)

6. INDICATORS Red litmus solution is obtained by acidifying the purple litmus extract whereas blue litmus solution is obtained by making the purple litmus extract alkaline. Litmus solution itself is neither acidic nor basic. It is neutral and has a purple colour. To use it as indicator, it us made acidic to give it red colour or alkaline to give it blue colour. Litmus has a tendency of turning red in acidic and blue in bases. Litmus paper is filter paper that has been treated with a lichen-provide natural-soluble dye.

7. INDICATORS Flowers of Hydrangea, if the soil from which they grow is acidic they will appear blue and if the soil from which they grow basic, they will appear pink.

8. INDICATORS Indicators showing different colours in acidic and basic medium. These are the commonly used acid-base indicators. The three well known indicators belonging to this category are litmus , Phenolphthalein and Methyl orange . Natural indicator- Litmus (Found in nature in the plants) Synthetic indicators- Phenolphthalein ad methyl orange (synthesized in the laboratory or industry). Litmus Solution- Litmus is a purple coloured dye extracted from the lichen plant (belonging to the division thallophyta ) It is the most commonly used indicator in the laboratory. In the neutral solution, it has purple colour. In the acidic solution, it turns red whereas in basic solution, it turns blue.

9. INDICATORS B. Indicators giving different odours in acidic and basic medium (Called Olfactory indicators) These are some substance which give one type of odour in acidic medium and a different odour in the basic medium. Hence, they can be used to test whether the given substance is acidic or basic. Such indicators are called olfactory indicators.

10. General Characteristics Properties of Acids Sour taste: Almost all acidic substance have a sour taste. Action on litmus paper: Acids turn blue litmus solution red Action on methyl orange : Methyl orange turns red when 1-2 drops of its solution are added to the solution of an acidic substance. Corrosive nature : Most of the acids corrosive in nature. They produce a burning sensation on the skin and holes in the clothes on which they fall. They also attack metal structure and stonework. Hence, they are never stored in metal containers. They are always stored in containers made of glass or ceramics as they are not attacked by the acids. Further, acids have very harmful effects on the living as they can destroy the cells of living system. Concentrated acids are particular dangerous to the skin and delicate parts like eyes etc. hence, acids must be handled with utmost care in the laboratory. You would often see a danger mark on the bottle containing concentrated acids.

11. General Characteristics Properties of Acids Acids Reaction with metals- Acids react with active metals like zinc, magnesium etc. generally with the evolution of hydrogen gas. Metal + Acid Metal Salt + Hydrogen gas The reactions may be represented as follows: Zn + H 2 SO 4  → ZnSO 4  + H 2 Zn (s)+2HCl(aq)→ ZnCl2(aq)+H2(g ) The compounds formed such as ZnSO 4, ZnCl2 etc. Are called salts. Reactivity Series: K , Na, Ca, Mg, Al, Zn, Fe, Pb , H, Cu, Hg, Ag, Au, Pt

12. General Characteristics Properties of Acids Observe the rate of evolution of hydrogen gas from each of the test tubes. It observed that at room temperature. Magnesium Reacts most vigorously Zinc reacts less vigorously than magnesium. Iron reacts slowly. Copper does not react at all. Hence, the order of reactivity is Mg>Zn>Fe>Cu. Thus, all metals do not react with the same acid with the same vigour .

13. General Characteristics Properties of Acids Reaction with metal carbonate and metal hydrogen carbonates (Bicarbonate): Acids react with metal carbonates and bicarbonates to give out carbon dioxide with effervescence abd forming the corresponding salt and water. Na2CO3+H2SO4 → Na2SO4+H2O+CO2 NaHCO3+HCl→ NaCl+H2O+CO2 Thus, all metal carbonates and hydrogen carbonates react with the acids to give the corresponding salt, carbon dioxide gas and water. Hence we can write the reaction in general as Metal Carbonates or hydrogen carbonates + Acid Salt + CO2 + H2O Importance of reaction of Acids with the sodium bicarbonate. If a person is suffering from the problem of acidity due to overeating, He/she is advised to take a pinch of baking soda. This is because baking soda is sodium bicarbonate which is a base and neutralizes the acid in the stomach and the person gets relief from burning due to acidity. Similarly, limestone, chalk and marble (which are all different forms of calcium carbonate) react with the acids as follows: CaCO3(s)+2HCl(aq)→CaCl2(aq)+CO2(g)+H2O(l ) Remember that egg-shells also contain calcium carbonate as the main constituent. Hence they react with the acid as above.

14. General Characteristics Properties of Acids Conduction of electricity:- The solutions of acids in water conduct electricity. Experiment: take a rubber cork. Fix two nails into it. Place this cork in a 100ml beaker. Connect the nails to a 6 volt battery with the help of wires. Also connect a small torch bulb or an ammeter in the circuit. Now, add dilute acid into beaker so that the nails are completely immersed. Switch on the battery. It is observed that the bulb starts Glowing or the needle of the ammeter Shows deflection. Repeat the experiment With dilute hydrochloric acid, nitric acid Or acetic acid. The bulb is found to glow in each case This is shows that the solutions of acids in Water conduct electricity.

15. General Characteristics Properties of Acids Reaction with bases: when an acid reacts with a base, it forms salt and water. As a result, the acidic properties of the acid disappear. The process is called neutralization. For example when HCl (Hydrochloric acid), a strong acid, reacts with NaOH , a strong base, the resulting salt is sodium chloride and water . HCl + NaOH →  NaCl + H 2 O Ca(OH) 2  + H 2 SO 4  → CaSO 4  + H 2 O Reaction with metallic oxides : Metal oxides like quick lime, i.e., calcium oxide ( CaO ), Copper (II) oxide ( CuO ), Magnesium oxide ( MgO ) etc , react with acids to form salt and water, for example Metal oxide   + Acids → Salt   + water CaO+2HCl→CaCl2+H2O CuO + 2HCl → CuCl 2  + H 2 O Mg(OH) 2  + HCl → MgCl 2  + H 2 O Milk of magnesia is given as an antacid medicine to neutralize the hydrochloric acid .

16. General Characteristics Properties of Acids We observed that all acids contain hydrogen which they liberate when they react with active metals. However, another important characteristics observed about acids is that their solution in water conducts electricity. Now, conduction of electricity through a solution of a substance can take place only if ions are present in the solution. This show that the acids when dissolved in water produce hydrogen ion H+. This H+ ions do not exist as such in the solution. Acids are the substance which contain hydrogen and which when dissolve in water give hydrogen ions in the solution. Rain water conducts electricity but distilled water does not. Distilled water does not contain any acid, base or salts which may dissociate to produce ions. Hence, it does not conduct electricity. Rain water contains dissolved gases of the air like co2, so2, NO2 etc. These gases dissolve in water to form acids like carbonic acid, nitric acids etc .

17. General Characteristics Properties of Bases: Bitter taste – Bases possess a bitter taste. Slippery or soppy touch – They possess a slippery touch like soaps. Action on litmus paper – They turn red litmus in blue. Action on phenolphthalein – They turn phenolphthalein solution pink. Corrosive nature – some bases like caustic soda (Sodium hydroxide), caustic potash (potash hydroxide) etc. Produce a burning sensation on skin. Therefore, advised never to touch and taste any base. Conduction of electricity – like acids, the solutions of bases also conduct electricity. Reaction with acids – Bases react with acids to form salt and water. The process is called neutralization reaction. As a result, the bases loses its properties. H2SO4 + 2NaOH Na2SO4 + 2H2O Reaction with metals – Some bases like sodium hydroxide and potassium hydroxide react with active metals like zinc and aluminium to liberate hydrogen gas along with the formation of a salt. Thus, 2NaoH + Zn Na2ZnO2 + H2 (Sodium Zincate) NaOH + Al + 2H2O 2NaAlO2 + 3H2 (Sodium Aluminate)

18. General Characteristics Properties of Bases: Sodium zincate and sodium aluminate are the salts formed in these reaction. However, these salts are different from those formed by action of acids on metals or neutralization of a acid with a base. Base react with non-metal oxides – Bases react with non-metal oxides (like CO2, SO2, SO3, P2O5 etc ) to form salt and water. CO2+2NaOH ⟶ Na2CO3+H2O Non-metal oxides are acidic in character. Like acids, bases also possess some general characteristics properties, this shows that chemically, they must have something in common. It is observed that, bases when dissolved in water produce hydroxide ions in the solution. Thus, this is the common characteristics of all bases.

19. General Characteristics Properties of Bases: A base is a substance which when dissolved in water gives hydroxide ions in the solution. This is called Arrhenius definition of bases as it was put forward by Arrhenius in 1884. Thus, just as the properties of acids are due to hydrogen ions which they give in the aqueous solution. Properties of the bases are dye to the hydroxide ions which they give in the aqueous solution. Difference between a Base and Alkali We have discussed that all metal hydroxide are bases. Also, we have already discussed that metal oxides are basic in nature. Hence, we conclude that bases are oxides and hydroxides of metals. Besides these, we have ammonium hydroxide, which is though not the hydroxide of a metal, yet it is a base because like other bases, it reacts with acids to form salt and water. NH 4 OH aq + HCl aq → NH 4 Cl aq + H 2 O l NH4OH+HNO3→ NH4NO3+H2O A few example of substance which act as bases are given below: CaO , CuO , NaOH , KOH etc .

20. Organic and Inorganic Acids: Acids present in plants and animals i.e., living organism are called organic acids. They are generally weak acids. A few naturally occurring sources of acids and the acids present in them are given in table below:

21. Organic and Inorganic Acids: Acids obtained from minerals of the earth are called mineral acids. They are not naturally occurring acids but are prepared in the laboratory or manufactured in the industries. The three most commonly used mineral acids are hydrochloric acid, Sulphuric acid and Nitric acids. These are strong acids. STRONG AND WEAK BASES: A base is substance which when dissolved in water and gives hydroxide ions in the solution. Greater the amount of OH- ions produced stronger is the base. Bases like sodium hydroxide and potassium hydroxide when dissolved in water dissociate completely into ions. As a result, they give large amounts of OH-ions in the solution. Such as strong base. On the other hand, bases like ammonium hydroxide (NH4OH) and Mg(OH)2 etc., when dissolved in water dissociate only partially to give hydroxide ions. As a result, the amount of OH- ions produced in the solution is small. Such bases are called weak bases.

22. pH of a Solution: Expressing acidity or alkalinity of a solution in term of pH. It is found that in water or any neutral solution which is neither acid nor basic, the H+ ion concentration OH- ion concentration are equal. Concentration of H+ ions = concentration of OH- ions = mole per liter. pH of a solution is the magnitude of the negative power to which 10 must be raised to express the hydrogen ion concentration of the solution in moles per litre . For a b asic solution, OH- in concentration > mol /lit therefore, H+ ion concentration < mol /lit. Thus, solution may have pH from 0 to 14. this is called pH scale.  

23. Salts and their formation: Salts are the ionic compounds consisting of two parts, one part carrying positive charge and the other part carrying a negative charge. The number of positive and negative ions present is such that total positive charge is equal to total negative charge so that the salts as a whole is the electrically neutral.

24. Salts and their formation: By neutralization of acids and bases: Acid reacts with bases to form salt and water. for ex. NaOH + HCl NaCl + H2O KOH + HNO3 KNO3 + H2O The positive part of the salt which comes from the base is called basic radical whereas the negative part of the salt which comes from the acid is called acid radical. By action of metals on acids : Active metals react with the acids forming salt and hydrogen gas. Zn + H2SO4 ZnSO4 + H2 The reaction are, in fact displacement reactions in which hydrogen of the acid is replaced by metal atom. By actions of acids as metal carbonate and bicarbonate : CaCO3 + 2HCl CaCl2 + H2O + CO2

25. Salts and their formation: By action of metals on alkalies : Metals like zinc and aluminum react with alkalies like sodium hydroxide and potassium hydroxide on heating to form salt and give out hydrogen gas. 2NaOH + Zn heat Na2ZnO2 + H2. Sodium zincate formed is a salt consisting of the positive sodium ion and negative zincate ion. Acids which contain two or more ionizable hydrogen ions are called polyprotic acids. MORE ABOUT SALTS: Salts of strong acid and strong base : A few examples are given below NaCl , NaNO3, Na2SO4, K2SO4, KNO3 These salts on dissolving in water produce strong acid and strong base. NaCl + H2O NaOH + HCl Hence, the acid and the base produced neutralize each other completely. As a result, the solution is neutral with pH = 7.

26. MORE ABOUT SALTS: 2. Salts of strong acid and weak base: NH4Cl, CuSO4, AlCL3, ZnSO4, BaCl2. These salts on dissolving in water produce strong acid and weak base. NH4Cl + H2O NH4OH + HCl The strong acid produced is not completed neutralized by the weak base. Hence, the solution is acidic with pH<7. 3. Salts of weak acid and strong base: Na2CO3, NaHCO3, CH3COONA These salts on dissolving in water produce a strong base and weak acid. Na@CO3 + 2H2O 2NaOH + H2CO3 carbonic acid The strong base produced is not completely neutralized by the weak acid. Hence, the solution is basic with pH > 1. 4. Salts of weak acid and weak base: CH3COONH4 (Ammonium acetate) When this salt is dissolved in water, it produce weak acid and weak base. Hence, the solution is almost neutral with pH nearly 7. Now, we shall take up the study of some very common and useful salts (compounds).

27. COMMON SALT (SODIUM CHLORIDE NaCl ): Chemical Name and Formula Chemically, common salt is sodium chloride with the formula, NaCl . It is also called “table salt” because it is used as important food material. However, the table salt used as food material is not pure sodium chloride but it contains small amounts of potassium iodide. For example, Tata salt claims iodide content is more than 15 ppm (parts per million parts). For this reason, table salt sold in the market is labelled as “iodized salt”. The presence of iodide is essential because it protects us from thyroid disorders. Occurrence and extraction of common salt: Sea Water: sea water contains a number of salts dissolved in it. Most of which is sodium chloride. It is one of them main source of common salt, In India. Salt from sea water is obtained in Mumbai and Chennai. To extract the salt from sea water. It is allowed to evaporate in shallow tanks under the influence of sun and wind. The salt obtained contains impurities of MgCl2, MgSO4 etc. the salt is, therefore purified by removing these impurities by suitable methods before it is sold in the market. Gandhiji’s Dandi march for the recovery of salt from sea water is well known which played an important role in the struggle for freedom.

28. COMMON SALT (SODIUM CHLORIDE NaCl ): Occurrence and extraction of common salt: Rock salt: common salt is also found in the form of solid deposits in several parts of the world e.g., Mandi (Himachal Parades) in India and khewra of Punjab (in Pakistan). Due to presence of impurities, it is often brown in colour and is called rock salt. Beds of rock salt are delivered to have been formed as a result of drying up of seas of very old times. Properties of common salts: Colour and state : it is a colourless crystalline substance with melting point 820°C. Solubility : its solubility at a room temperature (20°c) is about 35.8 per 100 g of water. The solubility does not vary much with temperature. Hygroscopic nature : It is slightly hygroscopic in nature. i.e., takes up moisture from the air and becomes wet. This is due to the presence of magnesium and calcium chloride in it. Action of conc. Sulphuric acid: on heating with conc. Sulphuric acid, it gives out fumes of HCl gas. Nacl + H2SO4 NaHSO4 + HCl

29. COMMON SALT (SODIUM CHLORIDE NaCl ): Uses of common salts: It is an essential constituent of our diet. It is used as a preservative for a number of food materials, e.g., in packed meat and fish. Mixed with ice, it is used to make freezing mixture (in making of ice creams) It has a number of industrial application for ex. (a) in the manufacture of soap, for salting out. (b) in the preparation of pottery glaze. Caustic Soda (Sodium Hydroxide, NaOH ): Caustic soda is chemically sodium hydroxide with the formula, NaOH . It is a strong base. Manufacture of Caustic Soda (Sodium Hydroxide, NaOH ): Sodium hydroxide is manufactured by electrolysis of an aqueous s0olution of sodium chloride (called brine) The method is called chlor -alkali process because the products of electrolysis are chlorine and sodium hydroxide ( chlor for chlorine and alkali for sodium hydroxide).

30. Caustic Soda (Sodium Hydroxide, NaOH ): Manufacture of Caustic Soda (Sodium Hydroxide, NaOH ): Sodium hydroxide is manufactured by electrolysis of an aqueous solution of sodium chloride (called brine) The method is called chlor -alkali process because the products of electrolysis are chlorine and sodium hydroxide ( chlor for chlorine and alkali for sodium hydroxide).

31. Caustic Soda (Sodium Hydroxide, NaOH ): Manufacture of Caustic Soda (Sodium Hydroxide, NaOH ): Sodium chloride in aqueous solution ionizes to give sodium (Na+) ions and chloride (cl-) ions. Nacl + water Na+( aq ) + Cl-( aq ) Water also ionizes to a small extent to produce H+ ions and OH- ions H2O H+ + OH- On passing electric current through the solution of sodium chloride, H+ ions are discharged more easily at the cathode (negative electrode) than Na+ ions. Here, they gain an electron to form hydrogen atoms which combine to form hydrogen atoms which combine to form hydrogen gas (H2). Thus At Cathode H+ + e- H H + H H2 same as for chlorine The complete reaction, when electricity is passed through a concentrated aqueous solution of sodium chloride, may be represented as follows: 2NaCl(aq)+2H2O(l)→Cl2(g)+2NaOH(aq)+H2(g ) The sodium hydroxide solution is formed near the cathode. completely ionizes Ionizes to A small extent

32. Caustic Soda (Sodium Hydroxide, NaOH ): Manufacture of Caustic Soda (Sodium Hydroxide, NaOH ): Thus, the three useful products obtained by the electrolysis of an aqueous solution of sodium chloride are: Hydrogen, Chlorine and Sodium hydroxide. Hydrogen and chlorine can be further combined to form hydrogen chloride which on dissolution in water produces hydrochloric acid, another useful compound. Uses of Hydrogen: In the manufacture of ammonia which is further used for the production of various fertilizers like urea, ammonium sulphate, calcium ammonium nitrate (CAN), etc. In the hydrogenation of vegetable oils to form solid fat, i.e., Vanaspati Ghee and margarine (artificial Butter). In metallurgy to reduce heavy metal oxide to metals. Liquid hydrogen (mixed with liquid oxygen) is used as a rocket fuel.

33. Caustic Soda (Sodium Hydroxide, NaOH ): Manufacture of Caustic Soda (Sodium Hydroxide, NaOH ): Uses of Chlorine: As a germicide and disinfectant for sterilization of water in the swimming pools. In the manufacture of pesticides. In the manufacture of PVC (polyvinyl Chloride), used for making shoe soles. In the manufacture of chlorofluorocarbon (CFC’s) used in refrigerants. In the bleaching of wood pulp and cotton fabrics. Uses of Caustic Soda: In the manufacture of soaps and detergents. For degreasing metals. In the paper making. In making of artificial fibers like rayon etc. In the manufacture of dyes. In petroleum refining.

34. Bleaching Powder (Calcium oxychloride, CaOCl3): Bleaching Powder (Calcium oxychloride, CaOCl3): Chemically, bleaching powder is generally represented by the formula, CaOCl3 called calcium oxychloride, though its actual composition is quite complex. Principle of Manufacture The chlorine produced as by product during the manufacture of caustic soda is used for the manufacture of bleaching powder. It is produced by the action of chlorine gas on dry slaked lime, Ca(OH)2. the following reaction takes place, Ca(OH) 2  + Cl 2  → CaOCl 2   + H 2 O Properties of Bleaching Powder It is a yellowish white powder. When exposed to air, it gives smell of chlorine. This is because carbon dioxide present in the air reacts with the bleaching powder liberating chlorine. CaOCl2 + CO2 → CaCO3 + Cl2 ↑

35. Bleaching Powder (Calcium oxychloride, CaOCl2) Bleaching Powder (Calcium oxychloride, CaOCl2): Solubility in water: it is soluble in cold water. The milkiness in the solution is due to the fact that bleaching powder contains some unreacted lime which is insoluble in water. Action of Acids: it reacts with dilute HCl acid or dilute H2SO4 acid liberating chlorine gas. CaOCl2 + H2SO4 ----> CaSO4 + H2O + Cl2  CaOCl2 + HCl  ---->  CaCl2  + H2O + Cl2  Properties of Bleaching Powder: It is yellowish white powder. When exposed to air, it gives smell of chlorine. This is because carbon dioxide present in the air reacts with the bleaching powder liberating chlorine. CaOCl2 + CO2  ---->  CaCO3   + Cl2 

36. Bleaching Powder (Calcium oxychloride, CaOCl2): Bleaching Powder (Calcium oxychloride, CaOCl2): Uses of bleaching powder: Textile industry for bleaching cotton and linen. Paper industry for bleaching wood pulp. Laundry for bleaching washed clothes. Making wool unshrinkable. As chlorine liberated by it can kill the germs, it is used for disinfecting drinking water. In many chemical industries, it is used as an oxidizing agent. It is also used in the manufacture of chloroform (CHCl3).

37. Baking Soda (Sodium Hydrogen Carbonate, NaHCO3 Baking Soda (Sodium Hydrogen Carbonate, NaHCO3): Chemically, baking soda is sodium hydrogen carbonate with the formula, NaHCO3 it is also called sodium bicarbonate. This is the compound present as the main constituent in the baking powder. CO 2  + H 2 O + NH 3  + NaCl → NaHCO 3  + NH 4 Cl Principle of manufacture: Baking soda is manufactured from sodium chloride as one of the raw materials. The method is known as solvay process. The method is based on the principle that if a solution of sodium chloride (called Brine) saturated with ammonia is carbonate (also called sodium bicarbonate) and ammonium chloride are formed according to the following reactions, CO 2  + H 2 O + NH 3  + NaCl → NaHCO 3  + NH 4 Cl Sodium hydrogen carbonate formed in the above reactions is sparingly soluble in water whereas ammonium chloride is highly soluble. Hence, sodium hydrogen carbonate separates out as solid. It is filtered and dried. On a small scale, it can be prepared in the laboratory by passing carbon dioxide gas through aqueous sodium carbonate solution. Na2CO3 + H 2 O + CO2   → 2NaHCO 3

38. Baking Soda (Sodium Hydrogen Carbonate, NaHCO3 Baking Soda (Sodium Hydrogen Carbonate, NaHCO3): Properties of baking soda: It is a white crystalline solid. It is stable in air. Is is sparingly soluble in water. The solution is alkaline in nature due to salt hydrolysis. NaHCO 3  + H 2 O → NaOH  + H 2 CO3(carbonic acid). NaOH formed is a strong base whreas H2CO3 formed is a weak acid. Hence, the solution is basic . Uses of Baking Soda In medicine as antacid: the excess acid formed in the stomach due to eating by spicy food is easily nutralised by sodium of food easily neutralized by sodium hydrogen carbonate. Hence, is is used as an ingredient of acid medicines. In making certain food stuff such as bread, cake etc. where softness and fluffiness is required, baking powder is added. In soda acid fire extinguisher, a solution of sodium hydrogen carbonate is placed in an iron cylinder fitted with a nozzle.

39. Washing Soda (Sodium Carbonate, Na2CO3.10H2O Washing Soda (Sodium Carbonate, Na2CO3.10H2O): Washing soda is sodium carbonate containing 10 molecules of water of crystallization, i.e., it is sodium carbonate decahydrate. Thus, its molecular formula is Na2CO3.10H2O. Sodium carbonate also exits in two other forms. Sodium carbonate monohydrate (Na2CO3.H2O). Anhydrous sodium carbonates (Na2CO3) which contains no water of crystallization. It is commonly known as soda ash. Properties of washing soda Colour and state: when freshly prepared, it is a transparent crystalline solid containing 10 molecule of water of crystallization. Action of water: when kept open in the air, the crystal of washing soda lose nine molecules of water of crystallization to form a monohydrate which is a white powder. Na2CO3.10H2O Na2CO3.H2O + 9H2O Solubility in water: it dissolve in water to give a solution which is alkaline in nature and hence turns red litmus blue, the alkaline nature of the solution can be explained on the basis of the phenomenon called “salt hydrolysis”, i.e., reaction of salt with water to form an acid and base. Na2CO3 + 2H2O 2NaOH + H2CO3

40. Washing Soda (Sodium Carbonate, Na2CO3.10H2O Washing Soda (Sodium Carbonate, Na2CO3.10H2O): Action of heat: on heating, washing soda does not decompose. However, it loses all the molecules of water and becomes anhydrous. Na2CO3.10H2O Na2CO3 + 10H2O Reaction with hard water: hardness of water is due to presence of soluble calcium and magnesium salts. On adding Na2CO3 they are precipitated as calcium and magnesium carbonates which can be filtered off. Hence water becomes soft (i.e., starts forming lather with soap). CaCl2 + Na2CO3 CaCO3 + 2NaCl Uses of sodium carbonates As the name washing soda indicates it is used in the laundry for cleaning clothes. It is used for removing permanent hardness of water. It is used in the manufacture of a number of useful products like glass, soap, paper, borax, caustic soda etc.

41. Anhydrous and Hydrated Salts The chemical formula of washing soda is Na2CO3.10H2O. Thus, one formula unit of the salt contains 10 water molecules. Similarly, there are a number of crystalline salts, each of which contains a fixed number of water molecules for ex. Crystalline copper sulphate is CuSO4.5H2O, gypsum is CaSO4.2H2O plster of paris is CasO4.1/2H2O. The fixed number of water molecules present in one formula unit of the salt is called water of crystallization. CuSO4.5H2O CuSO4 + 5H2O CuSO4 + 5H2O CuSO4.5H2O The name, plaster of paris was given to this compound because for the first time, it was made from gypsum (CaSO4.2H2O) which was mainly found in paris . Preparation: it is prepared from gypsum which is calcium sulphate dehydrate (CaSO4.2H2O). Gypsum is heated in a kiln to a temperature of 100ºC (373 K). At this temperature, it lose three fourth of its water of crystallization forming plaster of paris . CaSO4.2H2O CaSO4.1/2H2O + 1.1/2 H2O

42. Anhydrous and Hydrated Salts Properties It is a white powder. When mixed with water and left for half an hour to one hour, it sets to a hard mass. This is due to rehydration of plaster of paris to gypsum. Uses of POP In medical science, it is used for setting fractured bones in the right position and in making casts in dentistry. In making toys, casts for statues, decorative materials, jewelry and cosmetics. In making the surface smooth e.g., that of walls ceiling etc. before paint.