Acids, Bases and Salts We 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 Taste (sour/bitter/ any other) Lemon juice Orange juice Vinegar Curd Tamarind (imli) Sugar Common salt Amla Baking soda Grapes Unripe mango 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 nature 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. 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. 5.2 NATURAL INDICATORS AROUND US Litmus: A natural dye The most commonly used natural indicator is litmus. It is extracted from lichens (Fig. 5.1). 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. 5.1). (b) SCIENCE Table 5.4 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. Table 5.5 ACIDS, BASES AND SALTS Demonstrate the effect of the three indicators on each of these solutions. Record your observations in Table 5.5. We have learnt that acids turn blue litmus red and bases turn red litmus blue. Let us see what happens when an We are going to use an indicator you have not used so far. It is called To be demonstrated by the teacher in 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 Add to the acidic solution a drop of sodium hydroxide solution by a dropper. 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. SCIENCE 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 → Salt+Water 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 NEUTRALISATIONS IN EVERYDAY LIFE Indigestion Our stomach contains hydrochloric acid. It helps us to digest food, as you 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 hydrogen carbonate) 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 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. ACIDS, BASES AND SALTS Keywords (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 and c (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

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