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Chapter 2 class x
 

Chapter 2 class x

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    Chapter 2 class x Chapter 2 class x Presentation Transcript

    • You have learnt in your previous classes that the sourand bitter tastes of food are due to acids and bases,respectively, present in them.You already know that acids are sour in tasteand change the colour of blue litmus to red,whereas, bases are bitter and change the colourof the red litmus to blue. Litmus is a naturalindicator, turmeric is another such indicator.Have you noticed that a stain of curry on a whitecloth becomes reddish-brown when soap, whichis basic in nature, is scrubbed on it? It turnsyellow again when the cloth is washed withplenty of water. You can also use syntheticindicators such as methyl orange andphenolphthalein to test for acids and bases.
    • pH paperRed Litmus paperBlue Litmus paper
    • In this Chapter, we will study the reactions of acidsand bases, how acids and bases cancel out eachother’s effects and many more interesting things thatwe use and see in our day-to-day life.
    • q1. You have been provided with three test tubes. Oneof them contains distilled water and the other twocontain 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?
    • The indicators tell us whether a substance is acidic or basicby change in colour. There are some substances whoseodour changes in acidic or basic media. These are calledolfactory indicators.
    • Note that the metal in the above reactions displaceshydrogen from the acids. This is seen as hydrogengas. The metal combines with the remaining part ofthe acid and forms a compound called a salt. Thus,the reaction of a metal with an acid can besummarised as –The reaction of metal with base can be summarisedas-You find again that hydrogen is formed in thereaction. However, such reactions are not possiblewith all metals.
    • The reactions occurring in the above Activityare written as –On passing the carbon dioxide gas evolvedthrough lime water,On passing excess carbon dioxide thefollowing reaction takes place:
    • Limestone, chalk and marble are differentforms of calcium carbonate. All metalcarbonates and hydrogen carbonates react withacids to give a corresponding salt, carbondioxide and water. Thus, the reaction can besummarised as –
    • we have observed that the effect of a base isnullified by an acid and vice-versa. Thereaction taking place is written as –The reaction between an acid and a base togive a salt and water is known as aneutralisation reaction. In general, aneutralisation reactioncan be written as –
    • + +
    • The general reaction between a metaloxide and an acid can be written as –Since metallic oxides react withacids to give salts and water, similarto the reaction of a base with anacid, metallic oxides are said to bebasic oxides.
    • The general reaction between a Non-metal oxide and a base can be writtenas –Since this is similar to the reactionbetween a base and an acid, we canconclude that non -metallic oxidesare acidic in nature.
    • Q1. Why should curd and soursubstances not be kept in brassand copper vessels?
    • Q2. Which gas is usually liberated when an acidreacts with a metal? Illustrate with anexample. How will you test for the presence ofthis gas?
    • Q3. Metal compound A reacts with dilutehydrochloric acid to produce effervescence.The gas evolved extinguishes a burning candle.Write a balanced chemical equation for thereaction if one of the compounds formed iscalcium chloride.
    • Glowing of the bulbindicates that thereis a flow of electriccurrent through thesolution. The electriccurrent is carriedthrough the solutionby ions.
    • Since the cation present in acidsis H+, this suggests that acidsproduce hydrogen ions, H+(aq), insolution, which are responsible fortheir acidic properties.
    • This experimentsuggests thathydrogen ions inHCl are producedin the presence ofwater. Theseparation of H+ion from HClmolecules cannotoccur in theabsence of water.
    • Hydrogen ions cannot exist alone,but they exist after combiningwith water molecules. Thushydrogen ions must always beshown as H+(aq) or hydronium ion(H3O+).H+ + H2O → H3O +
    • We have seen that acids give H3O+ or H+ (aq) ion inwater. Let us see what happens when a base isdissolved in water..Bases generate hydroxide (OH-) ions in water.Bases which are soluble in water are called alkalis.
    • Now as we have identified that all acidsgenerate H+(aq) and all bases generate OH-(aq), we can view the neutralisation reactionas follows –
    • The process of dissolving an acid or a base in wateris a highly exothermic one. Care must be taken whilemixing concentrated nitric acid or sulphuric acidwith water. The acid must always be added slowly towater with constant stirring. If water is added to aconcentrated acid, the heat generated may cause themixture to splash out and cause burns. The glasscontainer may also break due to excessive localheating. Look out for the warning sign on the can ofconcentrated sulphuric acid and on the bottle ofsodium hydroxide pellets.Mixing an acid or base with water results indecrease in the concentration of ions (H3O+/OH-) perunit volume. Such a process is called dilution andthe acid or the base is said to be diluted.
    • Q1. Why do HCl, HNO3, etc., show acidic characters inaqueous solutions while solutions of compounds likealcohol and glucose do not show acidic character?
    • Q2. Why does an aqueous solution of an acidconduct electricity?3. Why does dry HCl gas not change thecolour of the dry litmus paper?
    • Q4. While diluting an acid, why is it recommended thatthe acid should be added to water and not water tothe acid?q5. How is the concentration of hydronium ions (H3O+)affected when a solution of an acid is diluted ?
    • 6. How is the concentration of hydroxide ions (OH-)affected when excess base is dissolved in a solutionof sodium hydroxide?
    • We Can quantitatively find the amount of Hor OH ions present in a solution, We Canalso judge how strong a given acid or base.By making use of a universal indicator,which is a mixture of several indicators. Theuniversal indicator shows different coloursat different concentrations of hydrogen ionsin a solution.
    • A scale for measuring hydrogen ion concentrationin a solution, called pH scale has been developed.The p in pH stands for ‘potenz’ in German,meaning power. On the pH scale we can measurepH from 0 (very acidic) to 14 (very alkaline). pHshould be thought of simply as a number whichindicates the acidic or basic nature of a solution.Higher the hydronium ion concentration, lower isthe pH value.The pH of a neutral solution is 7. Values less than7 on the pH scale represent an acidic solution. Asthe pH value increases from 7 to 14, it representsan increase in OH– ion concentration in thesolution, that is, increase in the strength of alkali.
    • Generally paper impregnated with the universalindicator is used for measuring pH. One suchpaper is shown below
    • 36 Topic 10: ACIDS, BASES & SALTS
    • The strength of acids and basesdepends on the number of H ions andOH ions produced, respectively. If wetake hydrochloric acid and acetic acidof the same concentration, say onemolar, then these produce differentamounts of hydrogen ions. Acids thatgive rise to more H ions are said to bestrong acids, and acids that give lessH ions are said to be weak acids.
    • Our body works within the pHrange of 7.0 to 7.8. Livingorganisms can survive only in anarrow range of pH change. WhenpH of rain water is less than 5.6,it is called acid rain. When acidrain flows into the rivers, it lowersthe pH of the river water. The
    • Plants require a specific pH range fortheir healthy growth. To find out the pHrequired for the healthy growth of aplant, you can collect the soil fromvarious places and check the pH, Also,you can note down which plants are
    • It is very interesting to note that ourstomach produces hydrochloric acid. Ithelps in the digestion of food withoutharming the stomach.During indigestion the stomach producestoo much acid and this causes pain andirritation. To get rid of this pain, peopleuse bases called antacids. One suchremedy must have been suggested by youat the beginning of this Chapter. Theseantacids neutralise the excess acid.
    • Digestive system mouth esophagusstomach small intestine large intestine
    • Tooth decay starts when the pH of themouth is lower than 5.5. Tooth enamel, madeup of calcium phosphate is the hardestsubstance in the body. It does not dissolvein water, but is corroded when the pH in themouth is below 5.5. Bacteria present in themouth produce acids by degradation ofsugar and food particles remaining in themouth after eating. The best way toprevent this is to clean the mouth aftereating food. Using toothpastes, which are
    • Have you ever been stung by ahoney-bee? Bee-sting leaves anacid which causes pain andirritation. Use of a mild base likebaking soda on the stung areagives relief. Stinging hair ofnettle leaves inject methanoic
    • Q1. You have two solutions, A and B. The pH ofsolution A is 6 and pH of solution B is 8. Whichsolution has more hydrogen ion concentration?Which of this is acidic and which one is basic?Q2. What effect does the concentration ofH+(aq) ions have on the nature of the solution?
    • q3. Do basic solutions also have H+(aq)ions? If yes, then why are these basic? .q4. Under what soil condition do you thinka farmer would treat the soil of hisfields with quick lime (calcium oxide) orslaked lime (calcium hydroxide) or chalk(calcium carbonate)?
    • Let us understand more about their preparation,properties and uses.Potassium sulphate,sodium sulphate,calcium sulphate,magnesium sulphate,copper sulphate,sodium chloride,sodium nitrate,sodium carbonate andammonium chloride.
    • USES OF SALTSS.No SALT USE .1 Ammonium Chloride In torch batteries2 Ammonium Nitrate In fertilizers3 Calcium Chloride As drying agent4 Iron Sulphate In Iron tablets5 Magnesium Sulphate In medicine6 Potassium Nitrate In gunpowder etc.7 Silver Bromide In photography8 Sodium Chloride Making NaOH9 Sodium Stearate In making soap.
    • Salts having the same positive or negativeradicals are said to belong to a family. Forexample, NaCl and Na2SO4 belong to thefamily of sodium salts. Similarly, NaCl andKCl belong to the family of chloride salts.Salts of a strong acid and a strong base areneutral with pH value of 7. On the otherhand, salts of a strong acid and weak baseare acidic with pH value less than 7 andthose of a strong base and weak acid arebasic in nature, with pH value more than 7.
    • The salt formed by the combination ofhydrochloric acid and sodium hydroxidesolution is called sodium chloride. This is thesalt that we use in food. it is a neutral salt.Seawater contains many salts dissolvedin it. Sodium chloride is separated from thesesalts. Deposits of solid salt are also found inseveral parts of the world. These largecrystals are often brown due to impurities.This is called rock salt. Beds of rock saltwere formed when seas of bygone ages driedup. Rock salt is mined like coal.
    • sodiumhydroxide baking soda washing sodableaching powder
    • When electricity is passed through an aqueoussolution of sodium chloride (called brine), itdecomposes to form sodium hydroxide. Theprocess is called the chlor-alkali processbecause of the products formed– chlor forchlorine and alkali for sodium hydroxide.Chlorine gas is given off at the anode, andhydrogen gas at the cathode. Sodiumhydroxide solution is formed near thecathode. The three products produced in thisprocess are all useful.
    • You have already come to know thatchlorine is produced during theelectrolysis of aqueous sodium chloride(brine). This chlorine gas is used for themanufacture of bleaching powder.Bleaching powder is produced by theaction of chlorine on dry slaked lime[Ca(OH)2]. Bleaching powder isrepresented as CaOCl2, though theactual composition is quite complex.
    • Bleaching powder is used –(i) for bleaching cotton and linen in thetextile industry, for bleaching wood pulpin paper factories and for bleachingwashed clothes in laundry;(ii) as an oxidising agent in manychemical industries; and(iii) for disinfecting drinking water tomake it free of germs.
    • The soda commonly used in the kitchen formaking tasty crispy pakoras is baking soda.Sometimes it is added for faster cooking. Thechemical name of the compound is sodiumhydrogencarbonate (NaHCO3). It is producedusing sodium chloride as one of the rawmaterials.It is a mild non-corrosive base. The followingreaction takes place when it is heated duringcooking –
    • Uses of sodium hydrogencarbonate (NaHCO3)(i) For making baking powder, which is a mixtureof baking soda (sodium hydrogencarbonate) anda mild edible acid such as tartaric acid. Whenbaking powder is heated or mixed in water, thefollowing reaction takes place –Carbon dioxide produced during the reactioncauses bread or cake to rise making them softand spongy.(ii) Sodium hydrogencarbonate is also aningredient in antacids. Being alkaline, itneutralises excess acid in the stomach andprovides relief.(iii) It is also used in soda-acid fire extinguishers.
    • Another chemical that can be obtainedfrom sodium chloride is Na2CO3.10H2O(washing soda). You have seen abovethat sodium carbonate can be obtainedby heating baking soda; recrystallisationof sodium carbonate gives washingsoda. It is also a basic salt.
    • Uses of washing soda(i) Sodium carbonate (washing soda) isused in glass, soap and paper industries.(ii) It is used in the manufacture ofsodium compounds such as borax.(iii) Sodium carbonate can be used as acleaning agent for domestic purposes.(iv) It is used for removing permanenthardness of water.
    • If you moisten the crystals again with water, you will find thatblue colour of the crystals reappears.Water of crystallisation is the fixed number of water moleculespresent in one formula unit of a salt. Five water molecules arepresent in one formula unit of copper sulphate. Chemicalformula for hydrated copper sulphate is Cu SO4. 5H2O. Nowyou would be able to answer the question whether the moleculeof Na2CO3.10H2O is wet.One other salt, which possesses water of crystallisation isgypsum. It has two water molecules as water of crystallisation.It has the formula CaSO4.2H2O.
    • On heating gypsum at 373 K, it loses water molecules andbecomes calcium sulphate hemihydrate . This is calledPlaster of Paris, the substance which doctors use as plasterfor supporting fractured bones in the right position. Plaster ofParis is a white powder and on mixing with water, it changesto gypsum once again giving a hard solid mass.It is written in this form because two formula units of CaSO4share one molecule of water. Plaster of Paris is used formaking toys, materials for decoration and for makingsurfaces smooth.
    • Q1. What is the common name of thecompound CaOCl2?Q2.Name the substance which on treatmentwith chlorine yields bleaching powder.Q3. Name the sodium compound which isused for softening hard water.
    • Q4. What will happen if a solution of sodiumhydrocarbonate is heated? Give the equationof the reaction involved.Q5. Write an equation to show the reactionbetween Plaster of Paris andwater.