CHAPTER-2 Acids, Bases and Salts

Transcription

1 CHAPTER-2 Acids, Bases and Salts Key Concepts: (1) Substances are classified into acids, bases and salts based on their characteristic properties. An acid is a substance whose aqueous solution 1) Turns red litmus to blue 2) Tastes sour 3) Neutralizes alkalis 4) Reacts with active metals to liberate hydrogen gas. A base is a substance whose aqueous solution 1) Turns red litmus to blue 2) Tastes bitter 3) Neutralizes acids 4) Has a soapy touch There are operational definitions of acids and bases which stand experimental verification. (2) Acids An acid which ionizes completely when dissolved in water is known as a strong acid. Examples of Strong acids are HCl and H2SO4 An aqueous solution of a strong acid contains only ions. It does not contain molecules. An acid, which dissolves only partially when dissolved in water is known as a weak acid. Examples of weak acids are CH3COOH and H2CO3. An aqueous solution of a weak acid contains both ions and molecules. (3) Indicators for testing acids and Bases 1) An indicator is a Dye which changes colour when it is put into an acid or a base. The colour of the indicator changes to a particular colour when it is stained by an acid and changes to a particular colour when it is stained by a base. The common Indicators are 1) Litmus 2) Methyl orange 3) Phenolphthalein.

2 Natural Indicators 1) Litmus is a purple dye which is extracted from a plant named Lichen (Belongs to phylum Thallophyta). In a neutral medium- it remains purple In acid medium- it turns red In basic medium red litmus to blue 2) Turmeric- has an yellow dye In a basic medium this indicator turns red. 3) Red Cabbage In an acidic medium- it remains red In basic medium- turns green Some other natural indicators: i. Beetroot Extract ii. Flowers of Petunia, Geranium and Hydrangea iii. Hibiscus Olfactory Indicators Onion has a characteristic smell. When a basic solution is added to a cloth strip treated with onions- the smell cannot be detected. However, a acidic solution, does not destroy the smell of onion. Vanilla extracts have a characteristic pleasant smell. Its smell cannot be detected in a basic solution. However, an acidic solution does not destroy the smell of vanilla extracts. Synthetic Indicators 1) Methyl Orange- It gives red colour in acidic medium and yellow in basic medium. 2) Phenolphthalein -It is colorless. It remains colourless in an acidic soloution. It changes to pink in a basic medium.

3 (4) Reaction of Acids and Bases with metals. Reaction of acids with Metals Both Dil. HCl and H2SO4 react explosively with reactive metals like K, Na and Ca. Producing metallic salts and hydrogen in the process. Metals + Acids Salt + Hydrogen Zn + 2HCl ZnCl2 + H2 Fe + H2SO4 FeSO4 + H2 Mg + 2HCl MgCl2 + H2 2Na + 2HCl 2NaCl + H2 Mn + 2HCl MnCl2 + H2 To generate hydrogen gas nitric acid is used only in a very dilute solution. HNO3 is a strong oxidizing agent, It oxidizes Hydrogen to water, thus defeating the purpose of the reaction. Activity: 2.4 Zinc Metal reacts with Sodium Hydroxide solution to form Sodium Zincate and hydrogen gas, however such solutions are not possible with metals. Zn + 2NaOH Na2 ZnO2 + H2 All the above reactions are also the example of displacement reactions. (5) Reaction of metal carbonates and metal hydrogen carbonates with acids Acids liberate CO2 gas from metallic carbonates and bi Carbonates. Na2CO 3 (s) + 2HCl(aq) 2NaCl(aq) + CO2 (g)+ H2O(l) NaHCO3 (s) + HCl(aq) NaCl(aq) + H2O(l) + CO2 (g) CO2 thus liberated turns lime water milky. Ca(OH)2 + CO2 CaCO3 + H2O (Test For CO2 ) On passing excess CO2 the solution of calcium hydrogen carbonate is obtained. CaCO3 + H2O + CO2 Ca (HCO3)2 (Aq) (6) General uses of some acids Acid Use

4 Boric Acid - Eye Wash/ Antiseptic Citric Acid - Food preservative Vitamin C Oxalic acid - Ink stain remover Carbonic Acid - Flavoured drinks Tartaric acid - Baking powder Acetic acid - Table vinegar cooking Hydrochloric acid Benzoic Acid- Nitric Acid - Explosive Phosphoric acid - Fertilizers (7) Reactions of acids with Bases Cleaning metal items Preservation of food, making of perfumes and medicine An acid reacts with bases to give salt and water. This reaction is known as a neutralization Reaction Acid + Base Salt and water HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) (8) Reactions of Metal Oxides with acids CuO- Which reacts with dil. HCl to form a blue-green solution of CuCl2 CuO + 2HCl CuCl2 + H2O Reactions of Non- metallic oxides with a bases. Ca(OH)2 + CO2 CaCO3 + H2O (9) The aqueous solutions of glucose, and alcohol do not show acidic character because their hydrogen does not separate out a hydrogen ion on dissolving in water. As the acids dissociates on dissolving in water they also conduct electricity. Acids do not show acidic behavior in the absence of water. The acidic behavior of acids is due to the presence of hydrogen ions. (H + ) Acids produce hydrogen ions only in the presence of water. So in the absence of water, a substance will not form hydrogen ions and hence will not show its acidic behavior.

5 Dry HCl gas fails to respond to dry litmus paper. But it turns moist blue litmus paper to red. HCl dissolved in water H + + Cl - H + + H2 O H3O + + Cl - H + Ions cannot exist alone. Hence it combines with water molecules to form H3O HYDRONINUM IONS. ( Activity 2.9 NCERT TEXT Page 23) (10) Strong Acids An acid which gets completely ionized in water and then produces a large no. of H + ions is called a strong acid. Example - HNO3, HCl, H2SO4 Strength here refers to the degree of ionization and not concentration. (11) Weak Acids An acid which is partially ionized in water and thus produces a small amount of hydrogen ions is called a weak acid. CH3COOH CH3COO - + H + When a concentrated solution of an acid is diluted by water, then the concentration of a hydrogen ions H + or hydronium (H3O) per unit volume decreases. How strong are acidic or basic solutions? The strength of an acid or an alkali does not describe whether the solution is dilute or concentrated. It describes the ability of a substance to form ions Acids- Hydrogen ions Alkalis- Hydroxide ions Both acidic and basic solutions contain hydrogen ions. In 1909 Sorensen devised a scale using which the strength of an acidic solution as well as an basic solution could be represented by making use of the ph paper. The ph of a solution is inversely proportional to the concentration of hydrogen in it. Solutions having high concentrations of hydrogen ions have a low ph value. p- Stands for potenz meaning power- Derived from puissant de hydrogen

6 ph- pure number- No units Acids have ph values less than 7 Alkalis have ph values more than 7 The neutral substances have a ph value 7. (12) The universal indicator Mixing together a number of natural dyes, scientists have developed an indicator called universal indicator. It gives the different colours at different ph value of the entire ph scale. Generally paper impregnated with the universal indicator is used for measuring ph (13) Importance of ph in everyday life 1. ph in our stomach Excess of HCl acid production in stomach causes acidity and pain. It can be neutralised by taking antacids. Mg(OH)2 or NaHCO3 2. ph change as the cause of tooth decay 3. ph scale ph close to 7 is best for growing plants. 4. ph change plays a very important role in survival of animals. 5. Bee sting leaves an acid causes pain and irritation. 6. Wasp sting is alkaline 7. Ant sting injects methanoic acid burning pain (14) Salts A salt is a compound formed by a partial or total displacement of the ionisable hydrogen atoms of an acid by a metallic ion or an ammonium ion. NaOH + H2SO4 NaHSO4 + H2O (Partial replacement) 2NaOH + H2SO4 Na2SO4 + 2H2O (Complete) Activity 2.14 NCERT textbook 1.Common salt Salt used in preparing food. NaCl sodium chloride is a neutral salt Present in nature in abundance obtained from sea water by the process of evaporation.

7 Rock salt obtained from underground deposits. It is an important raw material used in obtaining many useful chemicals like: NaOH, Sodium, soap, baking soda,washing soda. 2. Sodium hydroxide-- Common name: caustic soda (NaOH) Preparation: When electricity is passed through a concentrated solution of sodium chloride called Brine. 2NaCl(aq) + 2H2O(l) electricity 2NaOH(aq) + Cl2(g) + H2(g) (Brine) electrolysis The above process is known as chloro alkali process as it produces chlorine and alkali. (NaOH) Chlorine gas is produced at anode Hydrogen gas is produced at cathode. NaOH solution is formed near cathode All the 3 products are very useful in industry. NaOH is used for making Soaps and detergents. Artificial textile fibres.(rayon) Manufacture of paper Purifying bauxite ore in extraction of aluminium metal. De-greasing metals, oil refining Making dyes and bleaches Chlorine is used 1. To sterilize drinking water supply, swimming pools as it is a disinfectant 2. In production of bleaching powder, HCl 3. In production of PVC, pesticides, CFCs, CCl4, paints, dyes Uses of hydrogen 1) Hydrogenation of oils 2) Production of hydrochloric acid, ammonia, fertilizers, methanol, etc. 3) Used as rocket propellant(liquid hydrogen) 3.Bleaching powder Common name: calcium oxy-chloride (CaOCl2) Preparation: prepared by action of chlorine on dry slaked lime.

8 Ca(OH)2 + Cl2 CaOCl2 + H2 (Dry slaked lime) (bleaching powder) It is used for Bleaching cotton and linen in the textile industry Bleaching wood pulp in paper factories and clothes in laundry As an oxidizing agent in many chemical industries Disinfecting drinking water 4.Baking soda Common name: NaHCO3 (sodium hydrogen carbonate) Preparation : Produced by reacting a cold and concentrated solution of NaCl (brine) with ammonia and carbon dioxide. NaCl + NH3 + H2O + CO2 NaHCO3 + NH4Cl (Brine) Sodium hydrogen carbonate Ammonium chloride On heating it decomposes NaHCO3 Na2CO3 + H2O + CO2 Sodium carbonate Uses (a) For making baking powder. (Mixture of NaHCO3 + tartaric acid) NaHCO3 + H + CO2 + H2O + Sodium salt of tartaric acid. (From Tartaric acid) CO2 then produced causes bread or cake to rise and become spongy. (b) Baking soda is used as an antacid. (c) Used in soda acid fire extinguishers. 5.Washing soda Common name: Sodium carbonate (Na2CO3. 10 H2O ) Anhydrous Na2CO3 soda ash

9 Preparation 1. NaCl + NH3 +H2O + CO2 NaHCO3 + NH4Cl 2. 2NaHCO3 Na2CO3 + CO2 +H2O 3. The anhydrous sodium carbonate (Soda ash) is dissolved in water and recrystallised to get washing soda crystals. Na2CO H2O Na2CO3. 10H2O Uses: 1. Used as a cleansing agent for domestic purposes. 2. Used to remove permanent hardness of water. 3. Used in manufacture of glass, soap, paper, borax. 4. Used in detergents to keep them dry. a. Water of crystallisation - hydrated salts The water molecules which form the structure of a crystal are called water of crystallisation. The salts containing water of crystallisation are called hydrated salts Refer to activity (2.15) from NCERT textbook Example Crystal hydrated CuSO4. 5H2O(blue) Na2CO3. 10H2O FeSO4. 7H2O-(Green) CaSO4. 2 H2O Amorphous anhydrous CuSO4(white) Na2CO3 (soda ash) FeSO4 CaSO4 Water of crystallisation is a part of crystal structure of a salt since; it is not free water and does not wet the salt. Water of crystallisation gives crystals of the salts their shape and in some cases their colour Plaster of Paris Preparation: It is prepared from heating gypsum to a temperature of 373 K in kiln. Heat CaSO4. 2H2O 373 K CaSO4. 1/2 H2O + H2O Plaster of paris

10 Plaster of Paris has a very remarkable property of setting into hard mass on wetting with water. The setting of Plaster of Paris accompanied by slight expansion in volume due to which is used in making casts, statues, toys etc, used by doctors and dentists. (group activity Pg NCERT) Acids, Bases and Salts.(Questions For Practice) (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) (q) (r) (s) (t) (u) (v) (w) What is the colour of the following in acidic medium: litmus, methyl orange and phenolphthalein What happens when dilute HCl is added to sodium carbonate, sodium hydrogen carbonate and zinc oxide salts? Define: ph, Neutralization, Alkali Name the acid present in Lemon juice, Rancid butter, Gastric juice and sting of bees Two acids A and B have ph values 2 and 4 respectively, which is the stronger acid; A or B? What will be the colour of the solution when phenolphthalein is added to a colourless solution of ph =3? If a solution changes colour of litmus from red to blue, what can you say about its ph? What can you say about the ph of a solution that liberates CO2 from sodium carbonate? What is an alkali? Name two caustic alkalies Explain the statement: all alkalies are bases but all bases are not alkalies. List any three properties of an acidic solution What is a hydronium ion? You have been provided with 3 test tubes. They contain distilled water, an acidic solution and a basic solution respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? A is a soluble acidic oxide; B is a soluble base. Compared to the ph of pure water, what will be the ph of a solution of A and a solution of B? Define ph? Give the chemical name and formula of washing soda and baking soda. What is the chemical composition of plaster of Paris? Give name and formula of a compound of sodium, which exhibits efflorescence. Give chemical equation in support of your answer. A white substance having a strong smell of chlorine is used to clean water storage tanks. Identify the substance, give its chemical name and write the chemical reaction for its preparation. A lady found that the cake prepared by her is hard and small in size. Which ingredient has she forgotten to add that would have made the cake soft and fluffy? Give reasons Name the constituents of baking powder A compound which is prepared from gypsum has the property of hardening when mixed with adequate quantity of water. Identify the compound; write the chemical equation for its preparation. Mention one important use of the compound. Name a sodium compound used for softening of hard water.

11 (x) (y) (z) What is the common name of the compound CaOCl2? What happens when the solution of sodium hydrogencarbonate is heated? Name the substance which on treating with chlorine yields bleaching powder Short Answer Questions. 1. A solution of acetic acid in water is highly concentrated. Will you call it a strong acid? Explain. Ans. No, acetic acid will remain a weak acid even if it is highly concentrated because its degree of dissociation (α) is much less than one. This means that in water, only a small amount the acid exists, as ions whatever may be its concentration or amount dissolved in a given volume of water. 1. Why is not proper to call HCl (g) as hydrochloric acid? Ans. HCl (g) is known as hydrogen chloride gas and not hydrochloric acid because in a gaseous state, it does not ionize to give H + ions. These are released only when the gas dissolved in water. 3. An acidic solution always contains some OH - ions in it. Comment. Ans. The statement is correct. Actually, acids are always dissolved in water to form the solution. Now, water (H2O) will also ionize to small extent, as it is a weak electrolyte. The OH - is released by water will remain in solution. Therefore, an acidic solution will always contain some OH - ions. 4. Why should curd and sour substances not be kept in containers made up of brass (or) copper? Ans. Both curd and sour substances contain some acids in them. They react with copper to certain salts, which are poisonous nature. Therefore, it is not advisable to keep them in these containers. 5. A metal compound A reacts with dilute hydrochloric acid effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Ans. Since the gas is evolved with effervescence and extinguishes fire, it is expected to be CO2 gas. As calcium chloride is formed as one of the products, this means that the substance A must be calcium carbonate. It reacts with dilute hydrochloric acid as: CaCO3(s) + 2HCl (aq) CaCl2 (aq) + CO2 (g) + H2O(l) (A) 6.Aqueous solutions of HCl, HNO3 and H2SO4 etc. show acidic character while those of the compounds like ethyl alcohol (C2H5OH) and glucose (C6H12O6) fail to do so. Explain.

12 Ans. All the acids that have been listed have replaceable hydrogen atoms which they release in aqueous solution. Therefore, they show acidic character. However, both ethyl alcohol (C2H5OH) and glucose (C6H12O6) do not have replaceable hydrogen atoms. They fail to show any acidic character. 7.While diluting an acid, why is it not recommended that acid should be added to water and not water to the acid? Ans. Acids, particularly the mineral acids like H2SO4, HNO3 and HCl etc., have strong affinity for water. The dilution process is highly exothermic in nature. The heat evolved may crack or break the container and also convert the acid into fog which is likely to pollute the atmosphere. In order to control the heat evolved, it is advisable to add acid drop by drop to water. In case water is added to acid, then the entire acid will get itself involved in the exothermic process. It may not be possible to control the heat evolved. 8.Write the word equation and the balanced equation for the reactions when: a. Dilute sulphuric acid reacts with zinc granules b. Dilute hydrochloric acid reacts with magnesium ribbon c. Dilute phosphoric acid reacts with aluminum powder d. Dilute hydrochloric acid reacts with iron filings. Ans. (a) Word equation: Zinc + Sulphuric acid---- Zinc sulphate +Hydrogen Balanced equation: Zn(s)+H2SO4(dil.) ZnSO4(aq) + H2(g) (b)word equation: Magnesium+Hydrochloric acid Magnesium chloride + Hydrogen Balanced equation: Mg(s) + HCl(dil.) ---- MgCl2 (aq) + H2(g) (c)word equation: Aluminium+Sulphuric acid--- Aluminum sulphate +Hydrogen. Balanced equation: 2Al(s) + 3H2SO4(dil.)--- Al2(SO4)3(aq) + 3H2(g) (d)word equation: Iron + Hydrochloric acid --- Iron chloride +Hydrogen Balanced equation: Fe(s) + 2HCl(dil.)--- FeCl2(aq) + H2(g). 9.Why does distilled water not conduct electricity whereas rain water does? Ans. Pure water (or distilled water) is a very weak electrolyte and does not dissociate into ions. However, rainwater contains some dissolved acids. As a result, water becomes acidulated and gets ionized easily. Therefore, it conducts electricity. 10.Why does acid not show any acidic behavior in the absence of water?

13 Ans. An acid gets ionized only in aqueous solution i.e. in the presence of water. In other words an acid releases H + ions or shows acidic behavior only in the presence of water. 11.Five solutions A, B, C, D and E when tested with universal indicator show ph as 4, 2, 12,7 and 9 respectively. Which solution is: 2. neutral (b)strongly alkaline (c) strongly acidic (d) weakly alkaline (e)weakly acidic (f)arrange the ph in increasing order of H + ion concentration. Ans. (a) Neutral:D (b)strongly alkaline :C (c) Strongly acidic:b (d) Weakly alkaline:b (e) Weakly acidic:a (f) C < E < D < A < B. 12. Equal lengths of magnesium ribbons are taken in test tubes A and B Hydrochloric acid (HCl) is added to test tube A while acetic aid (CH3COOH) is added to test tube B. In casea, fizzing occurs more vigorously. Why? Ans. Fizzing in the reaction is due to the evolution of hydrogen gas by the action of metal on the acid Mg(s) + HCl (aq) ---- MgCl2 (aq) + H2 (g) (In A) Mg(s) + 2CH3COOH (aq) ---- (CH3COO)2Mg (aq)+h2 (g) (In B) Since hydrochloric acid is a stronger acid than acetic acid, fizzing occurs more readily in tube A than in tube B. 13.Fresh milk has ph of 6. How do you think the ph will change as it turns into curd. Explain your answer. Ans. When milk changes into curd, the ph decreases. Actually, lactose (carbohydrate) present in milk gets converted into lactic acid. As more of acid is formed, ph of the medium decreases. 14. A milkman adds very small amount of baking soda to fresh milk. What happens to its ph? Ans. The ph of fresh milk is nearly 6. Baking soda is sodium hydrogen carbonate (NaHCO3). On adding it to fresh milk, the medium, becomes alkaline and its ph therefore, increases. 15.Why should Plaster of Paris be stored in a moisture-proof container? Ans. In the presence of moisture, Plaster of Paris gets hydrated and changes of Gypsum which is a hard mass CaSO4½ H2O + 3/2 H2O --- CaSO4.2H2O

14 Plaster of Paris with Gypsum can be no longer used for making moulds and statues. Therefore, Plaster of Paris is kept in moist proof containers or bags. 16. Give two important uses of washing soda and baking soda? Ans. For answer consult text part. 17. Dry ammonia has no action on litmus paper but a solution of ammonia in water turns red litmus paper blue. Why is it so? Ans. Ammonia in the dry state has no hydroxyl ions. On dissolving in water, it forms ammonium hydroxide which dissociates to give OH - ions. As a result, the solution becomes basis and turns red litmus paper blue. NH3(g) + H2O (l) ---- NH4OH (aq) NH4OH (aq) NH4 + (aq) + OH - (aq) 18. You have two solutions A and B. The ph of solution A is 6 and that of solution B is 8. Which solution has more hydrogen ion concentration? Which out of these is acidic and which one is basic? Ans. The solution A with ph 6 has more hydrogen ion concentration than the solution B with ph 8. The solution with ph 6 is acidic while the other is of basic nature. 19. What effect does concentration of H + (aq) have on acidic nature of the solution? Ans. The acidic nature of solution is directly related to the concentration of H + ions. As the concentration of H + ions increases, the acidic nature of solution also increases. 20. Do basic solution also have H + (aq) ions? If yes, then why are these basic? Ans. Yes Basic solutions have also H + (aq) ions present in them. Actually, these solutions are prepared in water. Being a weak electrolyte, it dissociates to give H + and OH - ions. However, the number of H + ions is very small as compared to the number of OH - ions which are released by the base also. Therefore, the solutions as a whole are of basic nature. 21.Under what soil conditions do you think a farmer would spread or treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)? Ans. A soil usually becomes acidic when there are either high peat content, iron minerals or there is some rotting vegetable. In order to reduce the acidic strength, liming of soil is

15 usually done. For this, any of the substances that have been mentioned are added to the soil. 22. How is concentration of hydronium ions (H3O + ) affected when the solution of an acid is diluted? Ans. Upon dilution, more of acid dissociates into ions. As a result, more of H + ions or H3O + ions are released in solution. Therefore, their concentration will increase upon dilution. 23. How is concentration of hydroxyl ions (OH - ) affected when excess of base is dissolved in solution of sodium hydroxide? Ans. On adding an excess of base to the solution of sodium hydroxide, the concentration of OH - ions in solution will increase. LONG ANSWER QUESTIONS. 1. Which gas is usually liberated when an acid reacts with metal? Illustrate with an example? How will you test the presence of the gas? Ans. Hydrogen gas is usually liberated when an acid reacts with metal. For further detail consult text-part. 2. What will you observe when: i) Red litmus paper is introduced into a solution of sodium sulphate. ii) Methyl orange is added to dilute hydrochloric acid. iii) A drop of phenolphthalein is added to solution of lime water. iv) Blue litmus is introduced into a solution of ferric chloride Ans. (i). It will not undergo any colour change because the solution of sodium sulphate (Na2 SO4) in water is almost neutral. Both NaOH and H2SO4 expected to be formed in solution have nearly same strength. (ii) In the acidic solution, the colour of methyl orange will change to reddish. (iii) Lime water contains traces of calcium hydroxide, Ca(OH)2. It is therefore, basic in nature. The colour of phenolphthalein will become pink. (iv) Ferric chloride (FeCl3), solution on reacting with water will form ferric hydroxide and hydrochloric acid. FeCl3 (aq) + 3H2O(l) - Fe(OH)3(s)+3HCI(aq) (weak) (strong)

16 Since the acid is strong, the solution will be acidic. Therefore, the colour of blue litmus will change to red. 2. Discuss the role of water in the ionization of acid and base. Ans. For details, consult text part. 3. Explain why : Common salt becomes sticky during rainy seasons. Blue vitriol changes to white upon heating. Anhydrous calcium chloride is used in desiccators If bottle full of concentrated sulphuric acid is left open in the atmosphere by accident, the acid starts flowing out of the bottle of its own. Ans. a) Common salt contains the impurity of magnesium chloride (MgCl2) which is of deliquescent nature.when exposed to atmosphere, it becomes moist. Therefore common salt becomes sticky during the rainy season. b)blue vitriol (CuSO4.5H2O) upon heating changes to anhydrous copper sulphate (CuSO4) which is white in colour. c)anhydrous calcium chloride (CaCl2) is highly hygroscopic in nature. In a desiccator, it readily absorbs moisture and is therefore, used as drying agent. d)concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from air and gets diluted. Since the volume increases, the acid starts flowing out of the bottle. 4. Give brief account of: (i) Bleaching powder (ii) Plaster of Paris Ans. For answer, consult text-part. 5. A compound which is prepared from Gypsum has a property of hardening when mixed with proper quantity of water. Identify the compound. Write chemical equation to prepare the compound. Mention one important use of the compound. (C.B.S.E.Delhi 1999) Ans. The compound is plaster of Paris (CaSO4.1/2H2O). It is formed from Gypsum (CaSO2.2H2O) upon heating to a temperature of 373K and also changes back to Gypsum on adding water. For details, chemical equation and one use, consult text part. 6. Give chemical names of the following compounds. Also state one use in each case. (i)washing soda (ii)baking soda (iii)bleaching powder. (C.B.S.E. Delhi 2000 Supp.)

17 Ans. (i) Washing soda: Sodium carbonate decahydrate (Na2CO3.10H2O) (ii) Baking soda : Sodium hydrogen carbonate (NaHCO3) (iii)bleaching powder : Calcium Oxychloride ( CaOCl2) For the uses of the compounds, consult text part. 7. A chemical compound having smell of chlorine is used to remove yellowness of white clothes in laundries. Name the compound and write the chemical equation involved in its preparation. (C.B.S.E. Delhi 2001 Supp.) Ans. The compound is bleaching powder (CaOCl2). It removes yellowness from clothes due to its bleaching action. For details, consult text part. 8. Explain giving reasons: a. Tartaric acid is a component of baking powder used in making cakes. (C.B.S.E. Sample paper 2003). b. Gypsum, CaSO4, 2H2O) is used in the manufacture of cement. (C.B.S.E. Sample paper 2003) Ans. (i) Role of tartaric acid in baking powder(mixture of tartaric acid and sodium hydrogen carbonate) is to neutralize sodium carbonate formed upon heating sodium hydrogen carbonate. 2NaHCO3 Sod.hydrogen Carbonate Na2CO3 + H2O + CO2 Sod.carbonate In case, it is not done, cake will be bitter and sodium carbonate will also have injurious side effects. (ii) The role of gypsum (CaSO4.2H2O) in the manufacture of cement is to slow down the process of setting of cement. CONTENT MAPPING

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