Page 1 :
5, , Acids, Bases and Salts, , W, , e use in our daily life a large, number of substances such, as lemon, tamarind, common, salt, sugar and vinegar. Do they have, the same taste? Let us recall tastes of, some edible substances listed in, Table 5.1. If you have not tasted any of, these substances taste it now and enter, the result in Table 5.1., CAUTION, 1. Do not taste anything unless asked, to do so., 2. Do not touch anything unless asked, to do so., Table 5.1, Substance, Lemon juice, Orange juice, Vinegar, Curd, Tamarind (imli), Sugar, Common salt, Amla, Baking soda, Grapes, Unripe mango, Cucumber, , Taste (sour/bitter/, any other), , You find that some of these, substances taste sour, some taste bitter,, some taste sweet and some taste salty., , 5.1 ACIDS, , AND, , BASES, , Curd, lemon juice, orange juice and, vinegar taste sour. These substances, taste sour because they contain acids., The chemical natur e of such, substances is acidic. The word acid, comes from the Latin word acere which, means sour. The acids in these, substances are natural acids., What about baking soda? Does it also, taste sour? If not, what is its taste? Since,, it does not taste sour it means, that it, has no acids in it. It is bitter in taste. If, you rub its solution between fingers, it, feels soapy. Generally, substances like, these which are bitter in taste and feel, soapy on touching are known as bases., The nature of such substances is said to, be basic., If we cannot taste every substance,, how do we find its nature?, Special type of substances are used, to test whether a substance is acidic or, basic. These substances are known as, indicators. The indicators change their, colour when added to a solution, containing an acidic or a basic, substance. Turmeric, litmus, China rose, petals (Gudhal), etc., are some of the, naturally occurring indicators., , 2020-21

Page 2 :
5.2 NATURAL INDICATORS, AROUND US, , Do you know?, Name of acid, Acetic acid, , Found in, Vinegar, , Formic acid, , Ant’s sting, , Citric acid, , Citrus fruits such, as oranges,, lemons, etc., , Lactic acid, , Curd, , Oxalic acid, , Spinach, , Ascorbic acid, , Amla, Citrus fruits, , Litmus: A natural dye, , (Vitamin C), Tartaric acid, , Tamarind, grapes,, unripe mangoes, etc., , All the acids mentioned, above occur in nature, Name of base, , Found in, , Calcium hydroxide, , Lime water, , The most commonly used natural, indicator is litmus. It is extracted, from lichens (Fig. 5.1a). It has a, mauve (purple) colour in distilled, water. When added to an acidic, solution, it turns red and when, added to a basic solution, it turns, blue. It is available in the form of a, solution, or in the form of strips of, paper, known as litmus paper., Generally, it is available as red and, blue litmus paper (Fig. 5.1b)., , Ammonium hydroxide Window cleaner, Sodium hydroxide/, , Soap, , Potassium hydroxide, Magnesium hydroxide Milk of magnesia, , (a), , Can I taste all substances to, find their taste?, No. Have you not read the, caution? We should not, taste unknown, substances. They could, harm us., , (b), Fig. 5.1 (a) Lichens, and (b) Red and blue, litmus paper, , 50, , SCIENCE, , 2020-21

Page 3 :
Activity 5.1, § Mix some water with lemon juice in a, plastic cup/tumbler/test tube., § Put a drop of the above solution on a, strip of the red litmus paper with the, help of a dropper., Is there any change in colour?, § Repeat the same exercise with the, blue litmus paper., Note down if there is any change in, colour., Perform the same activity with the, following substances:, Tap water, detergent solution, aerated, drink, soap solution, shampoo, common, salt solution, sugar solution, vinegar,, baking soda solution, milk of magnesia,, washing soda solution, lime water. If, possible make solutions in distilled water., Record your observations as in, Table. 5.2., In your Table, are there any, substances on which litmus had no, effect? Name those substances., The solutions which do not change, the colour of either red or blue litmus, are known as neutral solutions. These, substances are neither acidic nor basic., , Fig. 5.2 Children performing litmus test, , Turmeric is another natural, indicator, Activity 5.2, § Take a tablespoonful of turmeric, powder. Add a little water and make, a paste., § Make turmeric paper by depositing, turmeric paste on blotting paper/filter, paper and drying it. Cut thin strips, of the yellow paper obtained., § Put a drop of soap solution on the, strip of turmeric paper., What do you observe?, , To prepare limewater, take some water in a tumbler and add some lime (chuna), into it. Stir the solution and keep it for some time. Pour a little from the top., This is lime water., Table 5.2, S. No., , Test solution, , ACIDS, BASES, , AND, , Effect on red, litmus paper, , SALTS, , Effect on blue, litmus paper, , Inference, , 51, , 2020-21

Page 4 :
You can prepare a greeting card for, your mother on her birthday. Apply, turmeric paste on a sheet of plane, white paper and dry it. Draw a, beautiful flower with soap solution, with the help of a cotton bud. You will, get a beautiful greeting card., , Now I understand why a, turmeric stain on my white shirt, is turned to red when it is, washed with soap. It is because, the soap solution is basic., , Acid, , Turmeric paste, , Indicator, , Base, , Soap, solution, , Similarly test the solutions listed in, Table 5.3 and note down your, observations. You may try solutions of, other substances also., , China Rose as Indicator, China, rose, , Activity 5.3, Collect some China rose (Gudhal ) petals, and place them in a beaker. Add some, , Fig. 5.3 China rose flower and indicator, prepared from it, , Table 5.3, S. No., , Test solution, , 1., , Lemon juice, , 2., , Orange juice, , 3., , Vinegar, , 4., , Milk of magnesia, , 5., , Baking soda, , 6., 7., 8., , Lime water, Sugar, Common salt, , Effect on turmeric solution, , 52, , Remarks, , SCIENCE, , 2020-21

Page 5 :
Table 5.4, S. No., 1., , Test solution, Shampoo (dilute solution), , Initial colour, , 2., , Lemon juice, , 3., , Soda water, , 4., , Sodium hydrogencarbonate solution, , 5., , Vinegar, , 6., , Sugar solution, , 7., , Common salt solution, , warm water. Keep the mixture for some, time till water becomes coloured. Use, the coloured water as an indicator. Add, five drops of the indicator to each of the, solutions given in Table 5.4., What is the effect of the indicator on, acidic, basic and neutral solutions?, China rose indicator (Fig. 5.3) turns, acidic solutions to dark pink (magenta), and basic solutions to green., , Final colour, , Paheli brought the following paheli, (riddle) for you., Coffee is brown, And bitter in taste., Is it an acid?, Or a base?, Don’t give the answer, Without any test,, You are in the dark, With its taste., , Activity 5.4, I am not getting the same result, when using solid baking soda, on dry litmus paper. Why?, Make a solution of baking, soda and then try., , The teacher is requested to get the dilute, solution of the following chemicals from, his/her school laboratory or from a, nearby school: hydrochloric acid,, sulphuric acid, nitric acid, acetic acid,, sodium hydroxide, ammonium hydroxide, calcium hydroxide (lime water)., , Table 5.5, S., No., , Name of acid, , 1., 2., , Dilute hydrochloric acid, , Effect on, litmus paper, , Effect on, turmeric paper, , Effect on, China rose, solution, , 3., ACIDS, BASES, , AND, , SALTS, , 53, , 2020-21

Page 6 :
Are you familiar with the term acid rain? Have you ever heard about damaging, effect of acid rain? As the name indicates the rain containing excess of acids is, called an acid rain. Where do these acids come from? The rain becomes acidic, because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released, into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric, acid and nitric acid respectively. Acid rain can cause damage to buildings,, historical monuments, plants and animals., CAUTION, Great care should be taken while, handling laboratory acids and bases, because these are corrosive in nature,, irritating and harmful to skin., Demonstrate the effect of the three, indicators on each of these solutions., Record your observations in Table 5.5., , 5.3 NEUTRALISATION, We have learnt that acids turn blue, litmus red and bases turn red litmus, blue. Let us see what happens when an, acid is mixed with a base., We are going to use an indicator you, have not used so far. It is called, phenolphthalein., , Stir the tube gently. Is there any change, in the colour of the solution? Continue, adding the sodium hydroxide solution, drop by drop while stirring till the pink, colour just appears., Now add one more drop of dilute, hydrochloric acid. What do you observe?, The solution again becomes colourless., Again add one drop of sodium, hydroxide solution. Is there any change, in colour? The solution again becomes, pink in colour., It is evident that when the solution, is basic, phenolphthalein gives a pink, colour. On the other hand, when the, solution is acidic, it remains colourless., , Activity 5.5, To be demonstrated by the teacher in, the class, Fill one fourth of a test tube with dilute, hydrochloric acid. Note down its colour., Note down the colour of phenolphthalein, solution also. Add 2–3 drops of the, indicator to the acid. Now shake the test, tube gently. Do you observe any change, in colour of the acid?, Add to the acidic solution a drop of, sodium hydroxide solution by a dropper., 54, , Base, , Acid + Indicator, , Fig. 5.4 Process of neutralisation, SCIENCE, , 2020-21

Page 7 :
When an acidic solution is mixed, with a basic solution, both the solutions, neutralise the effect of each other. When, an acid solution and a base solution are, mixed in suitable amounts, both the, acidic nature of the acid and the basic, nature of the base are destroyed. The, resulting solution is neither acidic nor, basic. Touch the test tube immediately, after neutralisation. What do you, observe? In neutralisation reaction, heat, is always produced, or evolved. The, evolved heat raises the temperature of, the reaction mixture., In neutralisation reaction a new, substance is formed. This is called salt., Salt may be acidic, basic or neutral in, nature. Thus, neutralisation can be, defined as follows:, The reaction between an acid and, a base is known as neutralisation. Salt, and water are produced in this process, with the evolution of heat., Acid+Base → Salt+Water, (Heat is evolved), The following reaction is an example:, Hydrochloric acid (HCl) + Sodium, hydroxide (NaOH) →, Sodium chloride (NaCl) + Water (H2O), Boojho added dilute sulphuric acid, to lime water. Will the reaction mixture, become hot or cool?, , 5.4 NEUTRALISATION IN EVERYDAY, LIFE, Indigestion, Our stomach contains hydrochloric, acid. It helps us to digest food, as you, ACIDS, BASES, , AND, , have learnt in Chapter 2. But too, much of acid in the stomach causes, indigestion. Sometimes indigestion, is painful. To relieve indigestion, we, take an antacid such as milk of, magnesia, which contains magnesium, hydroxide. It neutralises the effect of, excessive acid., , Ant bite, When an ant bites, it injects, the acidic liquid (formic acid) into the, skin. The effect of the acid can be, neutralised by rubbing moist baking, soda (sodium hydrogencarbonate) or, calamine solution, which contains zinc, carbonate., , Soil treatment, Excessive use of chemical fertilisers, makes the soil acidic. Plants do not, grow well when the soil is either too, acidic or too basic. When the soil is, too acidic, it is treated with bases like, quick lime (calcium oxide) or slaked, lime (calcium hydroxide). If the soil, is basic, organic matter (compost) is, added to it. Organic matter releases, acids which neutralises the basic, nature of the soil., , Factory wastes, The wastes of many factories, contain acids. If they are allowed to, flow into the water bodies, the acids, will kill fish and other organisms., The factory wastes are, therefore,, neutralised, by, adding, basic, substances., , SALTS, , 55, , 2020-21

Page 8 :
Keywords, Basic, Indicator, Neutral, , Acid, Acidic, Base, , Neutralisation, Salt, , What you have learnt, n, , Acids are sour in taste. Generally, bases are bitter in taste and soapy to, touch., , n, , Acid turns blue litmus red. Bases turn red litmus blue., , n, , Substances which are neither acidic nor basic are called neutral., , n, , Solutions of substances that show different colour in acidic, basic and, neutral solutions are called indicators., , n, , An acid and a base neutralise each other and form a salt. A salt may be, acidic, basic or neutral in nature., , Exercises, 1., , State differences between acids and bases., , 2., , Ammonia is found in many household products, such as window, cleaners. It turns red litmus blue. What is its nature?, , 3., , Name the source from which litmus solution is obtained. What is the, use of this solution?, , 4., , Is the distilled water acidic/basic/neutral? How would you verify it?, , 5., , Describe the process of neutralisation with the help of an example., , 6., , Mark ‘T’ if the statement is true and ‘F’ if it is false:, (i) Nitric acid turn red litmus blue. (T/F), (ii) Sodium hydroxide turns blue litmus red. (T/F), (iii) Sodium hydroxide and hydrochloric acid neutralise each other and, form salt and water. (T/F), (iv) Indicator is a substance which shows different colours in acidic, and basic solutions. (T/F), (v) Tooth decay is caused by the presence of a base. (T/F), , 7., , Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand, of customers. One customer wants acidic drink, another wants basic, and third one wants neutral drink. How will Dorji decide which drink is, to be served to whom?, , 8., , Explain why:, (a) An antacid tablet is taken when you suffer from acidity., , 56, , SCIENCE, , 2020-21

Page 9 :
(b) Calamine solution is applied on the skin when an ant bites., (c) Factory waste is neutralised before disposing it into the water, bodies., 9. Three liquids are given to you. One is hydrochloric acid, another is, sodium hydroxide and third is a sugar solution. How will you identify, them? You have only turmeric indicator., 10. Blue litmus paper is dipped in a solution. It remains blue. What is the, nature of the solution? Explain., 11. Consider the following statements:, (a) Both acids and bases change colour of all indicators., (b) If an indicator gives a colour change with an acid, it does not give a, change with a base., (c) If an indicator changes colour with a base, it does not change colour, with an acid., (d) Change of colour in an acid and a base depends on the type of the, indicator., Which of these statements are correct?, (i) All four, , (ii) a and d, , (iii) b, c and d, , (iv) only d, , Extended Learning — Activities and Projects, 1. Using the knowledge of acids and bases, write a secret message with, the help of baking soda and beet root. Explain how it works., (Hint: Prepare baking soda solution in water. Use this solution to write, the message on a sheet of white paper with a cotton bud. Rub a slice of, fresh beet root over the message.), 2. Prepare red cabbage juice by boiling a piece of red cabbage in water., Use it as an indicator and test the acidic and basic solutions with it., Present your observations in the form of a table., 3. Bring the soil sample of your area, find out if it is acidic, basic or, neutral. Discuss with farmers if they treat the soil in any manner., 4. Visit a doctor. Find out the medicines, he prescribes to treat acidity., Ask him how acidity can be prevented., , Did you know?, Each cell in our body contains an acid, the deoxyribonucleic acid or, DNA. It controls every feature of the body such as our looks, colour of, our eyes, our height etc. Proteins that build part of our cells are also, made of amino acids. The fats in our body contain fatty acids., ACIDS, BASES, , AND, , SALTS, , 57, , 2020-21