Ncert Class 7 Science - Acid, Base and Salt PDF

4 Acids, Bases and Salts W e use in our daily life a large You find that some of these number of substances such substances taste sour, some taste bitter, as lemon, tamarind, common some taste sweet and some taste salty. sal...

4 Acids, Bases and Salts W e use in our daily life a large You find that some of these number of substances such substances taste sour, some taste bitter, as lemon, tamarind, common some taste sweet and some taste salty. salt, sugar and vinegar. Do they have 4.1 ACIDS AND BASES the same taste? Let us recall tastes of Curd, lemon juice, orange juice and some edible substances listed in vinegar taste sour. These substances Table 4.1. If you have not tasted any of taste sour because they contain acids. these substances taste it now and enter The chemical nature of such substances the result in Table 4.1. is acidic. The word acid comes from the CAUTION Latin word acere which means sour. 1. Do not taste anything unless asked The acids in these substances are to do so. natural acids. 2. Do not touch anything unless asked What about baking soda? Does it also to do so. taste sour? If not, what is its taste? Since, it does not taste sour it means, that it Table 4.1 has no acids in it. It is bitter in taste. If you rub its solution between fingers, it Substance Taste (sour/bitter/ feels soapy. Generally, substances like any other) these which are bitter in taste and feel Lemon juice soapy on touching are known as bases. Orange juice The nature of such substances is said to Vinegar be basic. Curd If we cannot taste every substance, how do we find its nature? Tamarind (imli) Special type of substances are used Sugar to test whether a substance is acidic or Common salt basic. These substances are known as Amla indicators. The indicators change their Baking soda colour when added to a solution containing an acidic or a basic Grapes substance. Turmeric, litmus, China rose Unripe mango petals (Gudhal), etc., are some of the Cucumber naturally occurring indicators. Reprint 2024-25  Do you know? 4.2 NATURAL INDICATORS AROUND US Name of acid Found in Acetic acid Vinegar Litmus: A natural dye Formic acid Ant’s sting The most commonly used natural Citric acid Citrus fruits such indicator is litmus. It is extracted as oranges, from lichens (Fig. 4.1a). It has a lemons, etc. mauve (purple) colour in distilled Lactic acid Curd water. When added to an acidic solution, it turns red and when Oxalic acid Spinach added to a basic solution, it turns Ascorbic acid Amla, Citrus fruits blue. It is available in the form of a (Vitamin C) solution, or in the form of strips of Tartaric acid Tamarind, grapes, paper, known as litmus paper. unripe mangoes, etc. Generally, it is available as red and blue litmus paper (Fig. 4.1b). All the acids mentioned above occur in nature Name of base Found in Calcium hydroxide Lime water 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. 4.1 (a) Lichens, and (b) Red and blue litmus paper ACIDS, BASES AND SALTS 39 Reprint 2024-25 Activity 4.1  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 Fig. 4.2 Children performing litmus test drink, soap solution, shampoo, common Turmeric is another natural salt solution, sugar solution, vinegar, indicator baking soda solution, milk of magnesia, Activity 4.2 washing soda solution, lime water. If possible make solutions in distilled water.  Take a tablespoonful of turmeric Record your observations as in powder. Add a little water and make Table. 4.2. a paste. In your Table, are there any  Make turmeric paper by depositing substances on which litmus had no turmeric paste on blotting paper/filter effect? Name those substances. paper and drying it. Cut thin strips The solutions which do not change of the yellow paper obtained. the colour of either red or blue litmus  Put a drop of soap solution on the are known as neutral solutions. These strip of turmeric paper. substances are neither acidic nor basic. 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 4.2 S. No. Test solution Effect on red Effect on blue Inference litmus paper litmus paper 40 SCIENCE Reprint 2024-25  You can prepare a greeting card for your mother on her birthday. Apply turmeric paste on a sheet of plane Now I understand why a white paper and dry it. Draw a turmeric stain on my white shirt beautiful flower with soap solution is turned to red when it is with the help of a cotton bud. You will washed with soap. It is because get a beautiful greeting card. the soap solution is basic. Acid Indicator Base Soap Turmeric paste solution Similarly test the solutions listed in Table 4.3 and note down your observations. You may try solutions of other substances also. China Rose as Indicator China Activity 4.3 rose Collect some China rose (Gudhal ) petals Fig. 4.3 China rose flower and indicator and place them in a beaker. Add some prepared from it Table 4.3 S. No. Test solution Effect on turmeric solution Remarks 1. Lemon juice 2. Orange juice 3. Vinegar 4. Milk of magnesia 5. Baking soda 6. Lime water 7. Sugar 8. Common salt ACIDS, BASES AND SALTS 41 Reprint 2024-25  Table 4.4 S. No. Test solution Initial colour Final colour 1. Shampoo (dilute solution) 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 Paheli brought the following paheli time till water becomes coloured. Use (riddle) for you. the coloured water as an indicator. Add Coffee is brown five drops of the indicator to each of the And bitter in taste. solutions given in Table 4.4. Is it an acid? What is the effect of the indicator on Or a base? acidic, basic and neutral solutions? Don’t give the answer China rose indicator (Fig. 4.3) turns Without any test, acidic solutions to dark pink (magenta) You are in the dark and basic solutions to green. With its taste. Activity 4.4 I am not getting the same result The teacher is requested to get the dilute when using solid baking soda solution of the following chemicals from on dry litmus paper. Why? his/her school laboratory or from a nearby school: hydrochloric acid, Make a solution of baking sulphuric acid, nitric acid, acetic acid, soda and then try. sodium hydroxide, ammonium hydro- xide, calcium hydroxide (lime water). Table 4.5 S. Name of acid Effect on Effect on Effect on No. litmus paper turmeric paper China rose solution 1. Dilute hydrochloric acid 2. 3. 42 SCIENCE Reprint 2024-25  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 Stir the tube gently. Is there any change in the colour of the solution? Continue Great care should be taken while adding the sodium hydroxide solution handling laboratory acids and bases drop by drop while stirring till the pink because these are corrosive in nature, colour just appears. irritating and harmful to skin. Now add one more drop of dilute Demonstrate the effect of the three hydrochloric acid. What do you observe? indicators on each of these solutions. The solution again becomes colourless. Record your observations in Table 4.5. Again add one drop of sodium 4.3 NEUTRALISATION hydroxide solution. Is there any change in colour? The solution again becomes We have learnt that acids turn blue pink in colour. litmus red and bases turn red litmus It is evident that when the solution blue. Let us see what happens when an is basic, phenolphthalein gives a pink acid is mixed with a base. colour. On the other hand, when the We are going to use an indicator you solution is acidic, it remains colourless. have not used so far. It is called phenolphthalein. Activity 4.5 To be demonstrated by the teacher in Base 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 Acid + Indicator in colour of the acid? Add to the acidic solution a drop of sodium hydroxide solution by a dropper. Fig. 4.4 Process of neutralisation ACIDS, BASES AND SALTS 43 Reprint 2024-25  When an acidic solution is mixed have learnt in Chapter 2. But too with a basic solution, both the solutions much of acid in the stomach causes neutralise the effect of each other. When indigestion. Sometimes indigestion an acid solution and a base solution are is painful. To relieve indigestion, we mixed in suitable amounts, both the take an antacid such as milk of acidic nature of the acid and the basic magnesia, which contains magnesium nature of the base are destroyed. The hydroxide. It neutralises the effect of resulting solution is neither acidic nor excessive acid. basic. Touch the test tube immediately Ant bite after neutralisation. What do you observe? In neutralisation reaction, heat When an ant bites, it injects is always produced, or evolved. The the acidic liquid (formic acid) into the evolved heat raises the temperature of skin. The effect of the acid can be the reaction mixture. neutralised by rubbing moist baking In neutralisation reaction a new soda (sodium hydrogencarbonate) or substance is formed. This is called salt. calamine solution, which contains zinc Salt may be acidic, basic or neutral in carbonate. nature. Thus, neutralisation can be Soil treatment defined as follows: Excessive use of chemical fertilisers The reaction between an acid and makes the soil acidic. Plants do not a base is known as neutralisation. Salt grow well when the soil is either too and water are produced in this process acidic or too basic. When the soil is with the evolution of heat. too acidic, it is treated with bases like Acid+Base Salt+Water quick lime (calcium oxide) or slaked (Heat is evolved) lime (calcium hydroxide). If the soil The following reaction is an example: is basic, organic matter (compost) is Hydrochloric acid (HCl) + Sodium added to it. Organic matter releases hydroxide (NaOH) acids which neutralises the basic Sodium chloride (NaCl) + Water (H2O) nature of the soil. Boojho added dilute sulphuric acid to lime water. Will the reaction mixture Factory wastes become hot or cool? The wastes of many factories 4.4 NEUTRALISATION IN EVERYDAY contain acids. If they are allowed to LIFE flow into the water bodies, the acids will kill fish and other organisms. Indigestion The factory wastes are, therefore, Our stomach contains hydrochloric neutralised by adding basic acid. It helps us to digest food, as you substances. 44 SCIENCE Reprint 2024-25  Keywords Acid Basic Neutralisation Acidic Indicator Salt Base Neutral What you have learnt Acids are sour in taste. Generally, bases are bitter in taste and soapy to touch. Acid turns blue litmus red. Bases turn red litmus blue. Substances which are neither acidic nor basic are called neutral. Solutions of substances that show different colour in acidic, basic and neutral solutions are called indicators. 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, unfortu- nately, 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. ACIDS, BASES AND SALTS 45 Reprint 2024-25  (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. 46 SCIENCE Reprint 2024-25