tastes of food are due to acids and bases, respectively, present in them. These are called acid-base indicators or sometimes simply indicators. 2021–22
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Acids, Basesand Salts2CHAPTERYou have learnt in your previous classes that the sour and bittertastes of food are due to acids and bases, respectively, present in them.If someone in the family is suffering from a problem of acidity afterovereating, which of the following would you suggest as a remedyŒ lemonjuice, vinegar or baking soda solution?nWhich property did you think of while choosing the remedy?Surely you must have used your knowledge about the ability ofacids and bases to nullify each other™s effect.nRecall how we tested sour and bitter substances without tasting them.You already know that acids are sour in taste and change the colourof blue litmus to red, whereas, bases are bitter and change the colour ofthe red litmus to blue. Litmus is a natural indicator, turmeric is anothersuch indicator. Have you noticed that a stain of curry on a white clothbecomes reddish-brown when soap, which is basic in nature, is scrubbedon it? It turns yellow again when the cloth is washed with plenty of water. You can also use synthetic indicators such as methyl orange andphenolphthalein to test for acids and bases.In this Chapter, we will study the reactions of acids and bases, howacids and bases cancel out each other™s effects and many more interesting things that we use and see in our day-to-day life.Do You Know?Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta, and is commonly used as an indicator. When the litmussolution is neither acidic nor basic, its colour is purple. There are many other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in asolution. These are called acid-base indicators or sometimes simply indicators.
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Science182.12.1 2.12.1 2.1UNDERSTUNDERSTUNDERSTUNDERSTUNDERSTANDING THE CHEMICANDING THE CHEMICANDING THE CHEMICANDING THE CHEMICANDING THE CHEMICAL PROPERTIES OFAL PROPERTIES OFAL PROPERTIES OFAL PROPERTIES OFAL PROPERTIES OFACIDS AND BASESACIDS AND BASESACIDS AND BASESACIDS AND BASESACIDS AND BASES2.1.1 Acids and Bases in the LaboratoryActivity 2.1Activity 2.1Activity 2.1Activity 2.1Activity 2.1These indicators tell us whether a substance is acidic or basic bychange in colour. There are some substances whose odour changes inacidic or basic media. These are called olfactory indicators. Let us tryout some of these indicators.QUESTION?1.You have been provided with three test tubes. One of them containsdistilled water and the other two contain an acidic solution and a basicsolution, respectively. If you are given only red litmus paper, how willyou identify the contents of each test tube?nCollect the following solutions from the science laboratoryΠhydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3),acetic acid (CH3COOH), sodium hydroxide (NaOH), calciumhydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesiumhydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH).nPut a drop of each of the above solutions on a watch-glass one byone and test with a drop of the indicators shown in Table 2.1.nWhat change in colour did you observe with red litmus, blue litmus, phenolphthalein and methyl orange solutions for each of thesolutions taken?nTabulate your observations in Table 2.1.Table 2.1SampleRedBluePhenolph-Methylsolutionlitmuslitmus -thalein orangesolutionsolutionsolutionsolution Activity 2.2Activity 2.2Activity 2.2Activity 2.2Activity 2.2nTake some finely chopped onions in a plastic bag along with somestrips of clean cloth. Tie up the bag tightly and leave overnight inthe fridge. The cloth strips can now be used to test for acids and bases.nTake two of these cloth strips and check their odour.nKeep them on a clean surface and put a few drops of dilute HClsolution on one strip and a few drops of dilute NaOH solution onthe other.
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Acids, Bases and Salts19Which of these Œ vanilla, onion and clove, can be used as olfactoryindicators on the basis of your observations?Let us do some more activities to understand the chemical propertiesof acids and bases.2.1.2 How do Acids and Bases React with Metals?nRinse both cloth strips with water and again check their odour.nNote your observations.nNow take some dilute vanilla essence and clove oil and check theirodour.nTake some dilute HCl solution in one test tube and dilute NaOH solution in another. Add a few drops of dilute vanilla essence toboth test tubes and shake well. Check the odour once again and record changes in odour, if any.nSimilarly, test the change in the odour of clove oil with dilute HCl and dilute NaOH solutions and record your observations.Activity 2.3Activity 2.3Activity 2.3Activity 2.3Activity 2.3CAUTION: This activity needs the teacher™s assistance.nSet the apparatus as shown in Fig. 2.1.nTake about 5 mL of dilute sulphuric acid in a test tube and add afew pieces of zinc granules to it.nWhat do you observe on the surface of zinc granules?nPass the gas being evolved through the soap solution.nWhy are bubbles formed in the soap solution?nTake a burning candle near a gas filled bubble.nWhat do you observe?nRepeat this Activity with some more acids like HCl, HNO3 andCH3COOH.nAre the observations in all the cases the same or different?Figure 2.1Figure 2.1Figure 2.1Figure 2.1Figure 2.1Reaction of zinc granules with dilute sulphuric acid and testing hydrogengas by burning
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Science20 Note that the metal in the above reactions displaces hydrogen atomsfrom the acids as hydrogen gas and forms a compound called a salt.Thus, the reaction of a metal with an acid can be summarised as ŒAcid + Metal ® Salt + Hydrogen gasCan you now write the equations for the reactions you have observed?Activity 2.4Activity 2.4Activity 2.4Activity 2.4Activity 2.4The reactions occurring in the above Activity are written as ŒTest tube A: NaCOHCl(aq)Cl(aq)HO(l)+CO 23 22(s) Na (g) +®+22Test tube B: NaHCOHCl(aq)Cl(aq)HO(l)+CO 322 (s)Na (g) +®+ On passing the carbon dioxide gas evolved through lime water,Ca(OH)CO HO(l) 222(aq)(g)CaCOs 3+® +() (Lime water)(White precipitate)nPlace a few pieces of granulated zinc metal in a test tube.nAdd 2 mL of sodium hydroxide solution and warm the contentsof the test tube.nRepeat the rest of the steps as in Activity 2.3 and record your observations.The reaction that takes place can be written as follows.2NaOH(aq) + Zn(s) ® Na2ZnO2(s) + H2(g) (Sodium zincate)You find again that hydrogen is formed in the reaction. However,such reactions are not possible with all metals.2.1.3 How do Metal Carbonates and MetalHydrogencarbonates React with Acids?Activity 2.5Activity 2.5Activity 2.5Activity 2.5Activity 2.5nTake two test tubes, label them as Aand B.nTake about 0.5 g of sodium carbonate (Na2CO3) in test tube A and about0.5 g of sodium hydrogencarbonate (NaHCO3) in test tube B.nAdd about 2 mL of dilute HCl to both the test tubes.nWhat do you observe?nPass the gas produced in each casethrough lime water (calciumhydroxide solution) as shown inFig. 2.2 and record your observations.Figure 2.2Figure 2.2Figure 2.2Figure 2.2Figure 2.2Passing carbon dioxide gas through calcium hydroxide solution
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